From molecular to physical function: The aging trajectory

Abstract

Aging is accompanied by a decline in muscle mass, strength, and physical function, a condition known as sarcopenia. Muscle disuse attributed to decreased physical activity, hospitalization, or illness (e.g. sarcopenia) results in a rapid decline in muscle mass in aging individuals and effectively accelerates sarcopenia. Consuming protein at levels above (at least 50–100% higher) the current recommended intakes of ∼0.8 g protein/kg bodyweight/d, along with participating in both resistance and aerobic exercise, will aid in the preservation of muscle mass. Physiological muscle adaptations often accompany the observable changes in physical independence an older adult undergoes. Muscle fibre adaptations include a reduction in type 2 fibre size and number, a loss of motor units, reduced sensitivity to calcium, reduced elasticity, and weak cross-bridges. Mitochondrial function and structure are impaired in relation to aging and are worsened with inactivity and disease states but could be overcome by engaging in exercise. Intramuscular connective tissue adaptations with age are evident in animal models; however, the adaptations in collagenous tissue within human aging are less clear. We know that the satellite muscle cell pool decreases with age, and there is a reduced capacity for muscle repair/regeneration. Finally, a pro-inflammatory state associated with age has detrimental impacts on the muscle. The purpose of this review is to highlight the physiological adaptations driving muscle aging and their potential mitigation with exercise/physical activity and nutrition.

Highlights

• •Changes in skeletal muscle function with age are inevitable but are accelerated by disease and disuse.
• •With age, there is a “natural” decline in muscle mass, strength, and physical function: sarcopenia.
• •Increased inflammation, impaired mitochondrial function, and stiffer connective tissue exacerbate sarcopenia.
• •Exercise and protein intake are strategies to preserve muscle mass and/or mitigate physiological detriments with age.
• •Periods of disuse accelerate muscle aging resulting in incomplete recovery, and a permanent loss of muscle/function.

Abbreviations

MPS

muscle protein synthesis

MPB

muscle protein breakdown

RDA

recommended dietary allowance

1RM

one repetition maximum

CSA

cross-sectional area

COPD

chronic obstructive pulmonary disease

EAA

essential amino acids

RET

resistance exercise training

ROS

reactive oxygen species

AGEs

advanced glycation end-products

ECM

extracellular matrix

1. Introduction

The confluence of an aging population and a growing incidence of chronic disease as individuals age places an increasing demand on healthcare systems. By the year 2050, a quarter of the Canadian population will be above the age of 65 (Nauenberg et al., 2023) and similar demographic trends are seen in the US (Zoe Caplan, 2023) and EU (Affairs D-GfEaF, 2024 Ageing Report, 2024). Several factors influence health with age; however, an inactive lifestyle, along with excess energy intake and poor nutrition, can accelerate the natural aging process. Sedentary behaviour and poor nutrition can contribute to a reduction in muscle mass and a gain in fat mass, contributing to numerous metabolic diseases.

Skeletal muscle is a highly plastic tissue in which proteins turnover at a rate of ∼1.5% per day, and overall skeletal muscle makes up ∼40 ± 10% of our body mass (Koopman and van Loon, 2009). Muscle mass is dictated by the balance between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). When MPS and MPB are in equilibrium (i.e., net protein balance), an individual experiences neither loss nor gain in muscle mass. However, throughout the day, there are times when MPS is greater than MPB and vice versa (McKendry et al., 2021). Exercise, particularly that employing higher loads, and the ingestion of dietary protein can cause rates of MPS to increase resulting in a net positive protein balance (McKendry et al., 2021). In the long term, this constant elevation in MPS may translate to an increase in muscle mass if the stimulus is frequent and triggering enough. However, there are also conditions where MPB is chronically greater than MPS, such as during inactivity, especially when in combination with being in a fasted state. With reduced daily physical activity levels, complete muscle disuse, and inadequate protein and energy intake, muscle protein balance would be in a net negative state, contributing to a loss of muscle. As we age, MPS does not increase as robustly in response to the ingestion of protein and the ensuing hyperaminoacidemia (Moore et al., 2015). The changes affecting the natural sway of protein balance make gaining/maintaining muscle mass when aging more difficult. Besides alterations in the fluctuations in MPS and MPB, many physiological adaptations occur with age that underlie the decline in muscle mass observed. This review focuses on the molecular and physiological changes of the muscle with age and highlights the main reasons why maintaining a healthy amount of muscle when aging is so challenging. We do, however, offer some practical strategies to combat age-related sarcopenia.

2. Implications of declining muscle on health and strategies to mitigate muscle loss with age

Muscle loss is a hallmark of aging; in fact, it has been said that no loss of tissue with aging is more remarkable than that of skeletal muscle. As a tissue, its decline is seen by many as inevitable, but there is the ability to change the downward trajectory of muscle mass (and function/strength) with aging.

2.1. Sarcopenia and muscle aging-related diseases

Sarcopenia is characterized by a progressive loss of skeletal muscle mass, strength, and function with age, which is measurable (compared to peak muscle mass at age 20–30) in the fourth-fifth decade of life (i.e. 40-50 years old) (von Haehling et al., 2010). The decline in muscle mass beyond age 30 occurs at a rate of ∼0.8% per year (Mitchell et al., 2012; Silva et al., 2010) and is dictated by the balance between MPS and MPB, as previously mentioned. Muscle strength and power decline at a faster rate of ∼3% per year, indicating that processes other than muscle mass contribute to declines in muscle function (von Haehling et al., 2010). A period of disuse, such as a hospital stay, illness, or a reduction in activity, may result in aging individuals crossing the threshold of physical dependence a lot sooner due to mobility loss (Phillips et al., 2020; Visser et al., 2005). On the contrary, exercise training and increased physical activity in older adults are related to improved mobility and function (Paterson and Warburton, 2010). Activities of daily living become more difficult for aging individuals, and so these individuals will often avoid engaging in these tasks. The result is an increasing cycle of sedentarism that can eventually lead to dependency. This vicious cycle of inactivity and increased risk for physical dependence is shown schematically in Fig. 1 (Oikawa et al., 2019).

Fig. 1.

The vicious cycle of reduced physical independence. Figure created using BioRender.

The progression of sarcopenia can affect an individual's life due to its influence on health span, quality of life, and the interplay between these factors. Firstly, a reduction in muscle mass puts individuals at risk of developing metabolic disorders such as type 2 diabetes mellitus. Muscle mass is inversely related to insulin resistance as muscle is a metabolically active ‘sink’ for the storage and metabolism (oxidation and conversion to other substrates) of glucose (Srikanthan and Karlamangla, 2011). Secondly, a decline in strength, most frequently assessed as grip strength, is correlated with a reduced health-related quality of life and an increased risk of declines in physical mobility (Sayer et al., 2006). Finally, functional decline and declines in strength and aerobic capacity play roles in the development of cardiovascular disease (Corsi et al., 2018). Interventions that positively affect aging and specifically negate the development of sarcopenia continue to be explored to improve both quality of life and health span.

The loss of muscle mass is common with advancing age; however, several disorders and conditions may be worsened or add to the development of sarcopenia, such as metabolic syndrome, obesity, type 2 diabetes mellitus, cardiovascular disease, and other chronic diseases like dementia (Santilli et al., 2014). It is difficult to determine a cause-and-effect relationship between these conditions as human systems are linked with one another and often display similar underlying molecular changes. Sarcopenic obesity, for example, is characterized by a reduction in muscle mass and accumulation of fat mass that is commonly related to age (Cauley, 2015). Mitochondrial dysfunction, increased inflammation, and unmitigated oxidative stress are all contributors to the worsening of health and are presented as a result of sarcopenic obesity (Polyzos and Margioris, 2018). The natural aging process encompasses a reduction in muscle mass, though chronological age is not the only contributor to muscle atrophy. Molecular adaptations such as mitochondrial dysfunction, increased inflammation, and unmitigated oxidative stress are not only a result of chronic disease states (e.g. sarcopenic obesity) but are also related to the natural aging process, making it difficult to isolate the effect of the illness or aging on the influence of physiological adaptations.

2.2. Muscle disuse contributes to accelerated aging

Decreased physical activity (relative sedentarism) or muscle disuse in healthy adults induces muscle atrophy. Full recovery from disuse does not commonly occur in older adults (Suetta et al., 2009, 2013), and those with chronic disease states face a more challenging time with recovery (Della Peruta et al., 2023). Not only do older adults experience increased atrophy in response to muscle disuse (Kortebein et al., 2007, 2008; Coker et al., 2015), but they also have an impaired ability (compared to younger persons) to regain lost muscle and strength (Suetta et al., 2013).

A period of disuse (i.e. bedrest or single-leg immobilization) can have profound implications for an older individual's physical health and function. A two-week-long reduction in daily total steps (a model of abrupt sedentarism but not full disuse) not only reduced postprandial rates of MPS and impaired insulin sensitivity but also significantly reduced leg fat-free mass (Breen et al., 2013). To make matters even worse, older individuals who were immobilized for two weeks not only experienced a reduction in muscle mass but also a decline in muscle strength that was the equivalent of ∼2–3 years of normal aging (Rommersbach et al., 2020). Highlighting that disuse in older individuals can lead to muscle atrophy that resembles accelerated aging.

The step-reduction model has shown the profound impact of even short-term periods of reduced activity (Breen et al., 2013; Oikawa et al., 2018; McGlory et al., 2018; Devries et al., 2015; Arentson-Lantz et al., 2019). While a period of reduced steps may seem somewhat benign, our data and those of others have shown that older individuals would be adversely affected after as little as weeks in which they stayed inside of their homes due to inclement weather, were bedridden or convalescing due to illness, or seated for the majority of the day due to an injury (Oikawa et al., 2019). Individuals who are generally healthy may believe that they are at reduced risk of muscle loss during a period of disuse; however, compared to younger individuals, older adults experience greater muscle mass loss in response to bed rest and appear to have a more difficult time returning to pre-inactivity conditions (Pišot et al., 2016).

Healthy older adults experience exacerbated rates of loss of muscle mass in response to muscle disuse and bed rest (Oikawa et al., 2018; Pišot et al., 2016). In real-world settings, individuals are often placed in these conditions due to illness and injury (Quinn et al., 2019). Controlled models of disuse, inactivity, and bed rest tend to utilize healthy older adults, and so the outcome data from these studies likely represents the best-case scenario for health outcomes. We propose that the presence of an underlying chronic condition – type 2 diabetes, peripheral arterial disease, metabolic syndrome – would make older adults much more vulnerable to muscle atrophy and the development of sarcopenia. Older individuals experiencing elective hip surgery experienced substantial leg muscle atrophy after only 6 days of hospitalization (Kouw et al., 2019). The increased muscle loss experienced following hip arthroplasty in the non-operated limb puts an individual at increased risk of delayed recovery from surgery, leading to a cycle of further decreased physical activity. If an individual is in a health-compromised (pre-diabetes, type 2 diabetes, peripheral arterial disease) state prior to hospitalization, this could play an additive role in the drastic reduction in muscle mass (Barreiro and Sieck, 2013; Bruggeman et al., 2016). Current research focuses on strategies to mitigate the reduction in muscle mass seen with disuse through methods of prevention and respective recovery in both disease and non-disease states (Nunes et al., 2022).

3. Physiological adaptations that drive muscle aging

We are beginning to uncover several salient changes that occur with aging in skeletal muscle. Most of these age-related changes result in reduced muscle function, and they range from intra to extracellular in location. Here, we outline some of the more prevalent adaptations that occur with age and that are linked to declines in muscle mass and function.

3.1. Muscle fibre changes with age

Muscle fibre adaptations with age encompass not only a decrease in muscle fibre size but a loss of muscle fibres, a reduction in motor units, and a disruption of the mechanical properties of myofibrils (Fig. 2). These myofibrillar changes with age can have drastic implications for the physical function, strength, and muscle mass of older adults.

Fig. 2.

Myofibrillar changes in skeletal muscle aging. Figure created using BioRender.

It is well known that type 1 and 2 muscle fibres play distinctive roles in muscle force production and have characteristics that make their role in specific activities predominant (Linssen et al., 1991). With age, there is a reduction in fibre number and size, yet the reduction is most evident in type 2 fibres (Nilwik et al., 2013). Type 2 fibres have a greater capacity for force production, whereas type 1 or slow oxidative fibres are less fatigable (Linssen et al., 1991). Not only are muscle fibres reduced in number and size, but they also have a reduced ability for force production (lower force/fibre CSA) and a lower capacity to trigger contraction, as indicated by a decreased sensitivity to calcium (Lamboley et al., 2015), especially in type 2 fibers. Resistance exercise has been shown to increase type 2 muscle fibre size in older adults and is responsible for most of the increases observed in the quadriceps cross-sectional area in response to exercise (Nilwik et al., 2013). Since most of these changes are fiber type specific, the focus is on the type 2 fibers that show the most change with aging (Verdijk et al., 2007).

With age, there is a decline in number of motor units and decreased stability of neuromuscular junctions (Piasecki et al., 2016). The reduction in motor units may be substantial, and a loss of 40% of motor units can be seen by the age of ∼70 (Piasecki et al., 2016). The loss of motor units with age is accompanied by a reduced ability of remaining motor nerves to reinnervate muscle fibres and compensate for this loss (Hepple, 2018). An impaired capacity for reinnervation of denervated muscle fibres, along with the aforementioned anabolic resistance, correlated to increased muscle loss with age (Hepple, 2018). A study by Rowan and colleagues discovered that age-associated muscle fibre atrophy can be largely attributed to muscle denervation (Rowan et al., 2012). Long-term physical activity, at least based on cross-sectional data, has been shown to improve capacity for reinnervation and was related to better relative preservation of muscle strength (Mosole et al., 2014).

Impairments in myofibril mechanical properties can have drastic implications for optimal muscle contraction. Single skeletal muscle fibres from older men experience increased instantaneous stiffness or reduced elasticity in comparison to younger men (Ochala et al., 2007). This reduction in elasticity is also observed in the fascia, which can contribute to reduced flexibility and mobility in older adults (Marcucci and Reggiani, 2020). A reduction in myosin protein content and post-translational modifications may disrupt the mechanism of contraction at the cross-bridge level (Miljkovic et al., 2015). Post-translational modifications of myosin in aging affect its ability to bind to actin, thus reducing the number of strong cross-bridges formed during muscle contraction (Li et al., 2015). Both the reduced elasticity and poorly formed cross-bridges affect force production during muscle contraction and are deleterious with aging (Fig. 2).

3.2. Mitochondrial changes with age

On top of the myofibrillar changes in our skeletal muscle with aging, changes in the mitochondria of the muscle are also observed (Zhu et al., 2022; Burtscher et al., 2023; Distefano and Goodpaster, 2018). Mitochondrial dysfunction can be associated with age; however, a detailed causal relationship can be difficult to find since there are many variables affecting the muscle's mitochondria other than aging, with relative inactivity being a significant factor(Webb and Sideris, 2020). In addition, in 1996 it was already shown that mitochondrial MPS, oxidative capacity of the skeletal muscle, and mitochondrial function decline with age (Rooyackers et al., 1996). Whether the changes in the mitochondria are caused by or causes of sarcopenia remains unclear. Regardless of the order of events, the function of mitochondria with aging is a target for intervention, as mitochondrial dysfunction has been identified as a key theory in aging (Bratic and Larsson, 2013; Guo et al., 2023).

That age affects the structure of the mitochondria, and the cristae is not new. Miquel et al. (1980) described that changes in mitochondrial structure and cristae in skeletal muscle but also liver, kidney, and the cerebral cortex have been found in mammals (e.g., mice, rats, and human subjects). It has been discovered that with age, there is an increased reactive oxygen species (ROS) production that can have deleterious effects on a number of metabolic pathways, including the phosphorylation of the Akt/mTOR pathway, see Fig. 3. There is an increase in damaged mitochondrial DNA, reduced DNA repair systems, and reduced PGC-1a protein expression thought to be consequences of increased ROS with age (Boengler et al., 2017). PGC-1a plays an important role in the regulation of mitochondrial biogenesis, meaning that when there is a reduced PGC-1a protein expression, mitochondrial biogenesis is also reduced (Liang and Ward, 2006). Additionally, ROS production can hinder the phosphorylation of proteins in anabolic pathways such as the Akt/mTOR pathway and its downstream targets 4E-BP1 and p70S6K can inhibit MPS (Gomez-Cabrera et al., 2020). In addition, the increased amount of damaged mitochondrial DNA, in combination with a reduced DNA repair system, can reduce mitochondrial biogenesis and repair. Damaged mitochondria ultimately negatively affect the mitochondrial structure and function (Joseph et al., 2012). It is also known that the mitochondrial gene (Melov et al., 2007; Phillips et al., 2013) and protein (Murgia et al., 2017; Robinson et al., 2017; Ubaida-Mohien et al., 2019) content are lower in older adults. Some of these downregulated proteins are part of respiratory chain complexes and are consistent with lowering the mitochondrial respiratory function in muscle fibres from older adults (Ubaida-Mohien et al., 2019). The reductions in mitochondrial protein abundance are likely driven by the dysregulation of mitochondrial fusion and fission processes (Murgia et al., 2017).

Fig. 3.

Phosphorylation/activation of multiple protein signalling pathways changes with aging resulting in decreases in MPS and mitochondrial biogenesis post-exercise when compared to healthy young individuals. The green circles with a white cross in the middle of it indicate an increase in phosphorylation/activation, and the amount of bubbles indicates the amount of proteins that are phosphorylated/activated. The red circles with a white line in the middle of it indicate a decrease in phosphorylation/activation, and the amount of bubbles indicates the amount of proteins that are phosphorylated/activated. Figure created using BioRender.

Short et al. (2005) analyzed muscle from 146 healthy men and women aged 18–89 years old and looked at the functional changes in mitochondria with aging and some of the mechanisms behind the aging process. They found that mtDNA and mtRNA declined with age, which resulted in a reduced mitochondrial protein. In addition, they concluded that the abundance of mtDNA was positively correlated with mitochondrial ATP production which in turn is closely correlated with oxidative capacity and glucose tolerance. Therefore, with aging, the reduced capacity of ATP production by the mitochondria is, at least partly, caused by the reduction in mtDNA and mtRNA. Collectively, the processes mentioned above are suggested to contribute to the lower muscular function and the higher insulin resistance commonly seen in an older population.

It is clear that with aging, the main loss of muscle is found in type 2 muscle fibres (Lexell, 1995), and the distribution of fibre type between different muscles within the same person can be vastly different (Johnson et al., 1973). It is suggested that with aging, differences in function within different muscles can be observed. Conley et al. (2007) showed that with aging, the type 2 muscle fibres are mainly affected, and they suggested that a reduction in oxidative capacity of the mitochondria, specifically in type 2 fibres, is causing the decline in the cross-sectional area (CSA) of type 2 fibres with aging. However, it can also be speculated that the decline in CSA of type 2 muscle fibres is caused by the reduction in engagement in higher-intensity physical activity that would mainly require recruitment of the type 2 muscle fibres. It seems logical that a combination of aging and inactivity would be the cause of the decrease in CSA in type 2 fibres; however, this remains speculative, and more research needs to be done in order to come to a clear consensus.

Some of the age-related changes in the mitochondria of skeletal muscle are partially reversible with different types of exercise, and practically all age-related changes are exacerbated with inactivity (Gram et al., 2014; Hackney and Ploutz-Snyder, 2012; Pillon, 2023), type 2 diabetes mellitus (Jornayvaz and Shulman, 2010), and other diseases such as cancer or Alzheimer's (Jornayvaz and Shulman, 2010; Palmer et al., 2021). Previous research shows that endurance exercise seems to be an effective mode of exercise to stimulate muscle mitochondrial biogenesis and function. Endurance exercise increases the phosphorylation of PGC-1a, which is a marker for mitochondrial biogenesis (Coggan et al., 1990, 1992; Spina et al., 1996). Resistance exercise is found to be very effective in preventing the decline in the CSA of type 2 fibres, mainly by stimulating the mTOR pathway (Moro et al., 2020; Kosek et al., 2006; Verdijk et al., 2009). It is also suggested that a combined exercise protocol could be beneficial for older adults when specifically looking at the decreased oxidative capacity found in an older population. Irving et al. (2015), showed that an 8-week combined training protocol was superior in increasing muscle oxidative capacity when directly compared to an 8-week resistance exercise protocol, an 8-week endurance exercise protocol, and a control group.

To summarize, with aging, there is a decline in mitochondrial volume and a breakdown in structure and function, with differences between muscle fibre types (i.e. greater loss in type 2 fibres). The observed decline in mitochondrial function can, at least partly, be prevented/reversed by different types of exercise and can be exacerbated by inactivity and disease states.

3.3. Connective tissue and extracellular matrix changes

Intramuscular connective tissue is a crucial part of skeletal muscle function and is critical for force translation. The intramuscular connective tissue consists of 3 different extracellular matrix structures (the epimysium, perimysium, and endomysium), serves as protection, and plays an important role as a supportive force-transferring lattice to the contractile portion of the muscle to the tendon to the bone resulting in contraction (Purslow, 2020). It has been suggested that the turnover rate of intramuscular connective tissue protein was lower compared to skeletal muscle turnover (Holm et al., 2010; Trommelen et al., 2020). However, a recent study showed that intramuscular connective tissue showed a relatively greater fractional synthetic rate when compared to the myofibrillar fractional synthetic rate after an acute bout of resistance exercise (Aussieker et al., 2023). Meaning, that even though intramuscular connective tissue turnover might be lower than myofibrillar turnover, intramuscular connective tissue shows characteristics of a highly plastic portion of the skeletal muscle.

Research in murine models has shown that older mice show an increase in collagen crosslinks when compared to younger mice (Wohlgemuth et al., 2023). While enzymatic collagen crosslinks can rapidly change due to external stimuli like exercise, injury, or disease, during aging, there is an increase in non-enzymatic crosslinks, also known as advanced glycation end products (AGEs), as we get older. As a result of the increase in AGEs, the extracellular matrix (ECM) can become stiffer, impair the range of biomechanical properties, and the ECM becomes more resistant to turnover (Wohlgemuth et al., 2023). The increase in difficulty in turning over ECM collagen, most likely due to a reduced degradation (Nowotny and Grune, 2014), helps to explain the accumulation of collagen, or fibrosis, in older individuals. Interestingly, periods of immobilization worsen ECM stiffness (Järvinen et al., 2002). The intramuscular connective tissue (in this case, the endomysium and perimysium specifically) undergoes both quantitative (i.e. collagen accumulation) and qualitative AGE-induced alteration (Wohlgemuth et al., 2023; Järvinen et al., 2002), further impairing the biomechanical properties of the intramuscular connective tissue with aging.

In mice, it is clear that with aging, the collagen content of the muscle increases (Kragstrup et al., 2011). However, in aging humans, it remains debatable whether the collagen content of the muscle actually increases. Even though some evidence suggests that with human aging, the collagen content of the muscle tends to increase, a clear consensus has not yet been formed (Pavan et al., 2020; Fede et al., 2022). Similarly, collagen synthesis rates, as mice data show, are reduced with aging (Mays et al., 1991; Goldspink et al., 1994), whereas human data shows an increase in collagen synthesis with aging (Babraj et al., 2005). This raises the question if collagen degradation is the main factor at play here and it seems that heavily crosslinked collagen is more difficult to break down (Nowotny and Grune, 2014). However, more research is needed to settle this issue.

It is clear that with resistance exercise, the turnover of muscle connective tissue protein can be increased (Holm et al., 2010; Miller et al., 2005); in addition, whether exercise has a positive effect on the detrimental effects in older persons on intramuscular connective tissue remains to be determined. In addition, it seems to be clear that additional supplementation of protein does not increase intramuscular connective tissue protein synthesis (Holm et al., 2010; Holwerda and van Loon, 2022; Dideriksen et al., 2011; Mikkelsen et al., 2015; Moore et al., 2009; Wilkinson et al., 2008).

More human research is needed to draw firm conclusions about the relationship between age-associated changes in the muscle ECM/intramuscular connective tissue and the decline in force production in older adults. Since the older population is generally less active, it remains difficult to associate changes in the ECM/intramuscular connective tissue with aging specifically and not with other external factors like increased inactivity.

3.4. Changes in the muscle stem cell niche

The muscle stem cell niche in skeletal muscle can be described as the microenvironment that surrounds the muscle stem cells (i.e., satellite cells). The stem cell niche provides structural and biochemical support to regulate satellite cell behaviour, including maintenance, activation, proliferation, differentiation, and self-renewal, which is crucial for muscle growth, repair, and regeneration (Mashinchian et al., 2018).

It has been demonstrated that when a younger mouse is connected to an older mouse through the use of parabiosis the decline of satellite cell activity of the old mice (common with age) can be restored (Conboy et al., 2005). Whereas when combining the vascular systems of two old mice (again through parabiosis) the progenitor cell activity is not restored. To clarify, parabiosis is a surgical procedure in which the skin and blood vessels of two mice are surgically connected, with the goal of creating a shared circulatory system allowing for the blood of both mice to mix. This study (Conboy et al., 2005) suggests that the local vascular environment of these progenitor cells negatively influences the progenitor cell activity with aging.

It is suggested that the ECM of the muscle fibres becomes thicker (i.e., fibrosis) with age and impairs the crosstalk of the muscle stem cell niche. Nederveen et al. showed that the thickness of laminin in type 2 muscle fibres was higher in older participants than in younger participants (Nederveen et al., 2020). In addition, they also showed a greater amount of satellite cells that were surrounded by laminin in the older population (they labelled such cells as incarcerated), which may inhibit the satellite cell activation in older persons following exercise (Nederveen et al., 2020). Based on mouse models, multiple pathways are responsible for this increase in fibrosis with age (Brack et al., 2007; Brack and Rando, 2007; Lukjanenko et al., 2019). However, a very small amount of research has been done on human muscle tissue, and therefore, a lot remains to be confirmed and discovered.

The muscle satellite cell pool decreases with age (Shefer et al., 2006). Interestingly, the impaired regeneration of muscle with aging may be reversible with exercise (Joanisse et al., 2016a). It has been shown that with exercise, mice can increase satellite cell content and improve the systemic environment. The importance of the systemic environment is stressed due to the development of the capillary impact zone theory (Joanisse et al., 2016b). This theory suggests that each capillary within the muscle has a specific area in which it can send blood-borne signals to the muscle satellite cells in order to activate the satellite cell for muscle regeneration, repair, and remodelling. It is good to know that with exercise, the capillary impact zone can be increased. A study in older men has shown that with age, the satellite cells are located further away from capillaries, which would suggest that more satellite cells are outside or on the far end of these capillary impact zones, which could impair/delay regeneration, repair, and remodelling of the muscle (Nederveen et al., 2016). We refer the interested reader to a review by Sousa et al. (Sousa-Victor et al., 2022) and a book by Hwang and Brack (2018) for deeper reading in this area.

3.5. Changes in systemic factors – inflammaging

Inflammaging can be defined as ‘sterile’ immune dysregulation (i.e., inflammation in the absence of a pathogenic vector: bacteria or virus) and, specifically, an increase in systemic concentrations of pro-inflammatory markers with age (Franceschi et al., 2000). The pro-inflammatory state is characterized by an increase in pro-inflammatory markers, including IL-1, IL-1 receptor antagonist protein (IL-1RN), IL-6, IL-8, IL-13, IL-18, C-reactive protein (CRP), IFNα and IFNβ, transforming growth factor-β (TGFβ), tumour necrosis factor (TNF) and its soluble receptors (TNF receptor superfamily members 1A and 1B), and serum amyloid A (Ferrucci and Fabbri, 2018).

A large-scale longitudinal aging study showed that high levels of cytokines, specifically IL-6 and CRP, are associated with an increased risk of muscle loss and reductions in strength (Schaap et al., 2006). The exact mechanism of how the pro-inflammatory state seen in older adults results in the loss of muscle and strength is unknown; however, it is speculated that there could be an increase in the ubiquitin-protease pathway induced via inflammation (Mitch and Goldberg, 1996; Ferrucci et al., 1999). This ubiquitin-proteasome pathway system is mainly responsible for muscle proteolysis but not the myofibrillar lattice. In addition, it is known that the main intramuscular signalling pathways that are influenced by this inflammatory state are the NF-κB, JAK/STAT, and p38MAPK pathways that contribute to the loss of muscle and strength (Huang et al., 2020; Fang et al., 2021; Zanders et al., 2022; Ueno et al., 2021). Finally, inactivity and the development of type 2 diabetes mellitus can lead to an accelerated loss of muscle with age due to increased blood glucose levels and a pro-inflammatory state than in healthy, active older persons (Prattichizzo et al., 2018; Ji et al., 2022).

With the continued developments of (in vivo/vitro in human) measurement techniques, the intricate relationship between inflammaging and the muscle niche with aging will become clearer. Unfortunately, due to the complex nature of these studies, there is a lack of human mechanistic data, and therefore, more studies are required.

4. Mitigating muscle loss in aging

Two main strategies proposed to mitigate sarcopenia include the incorporation of exercise and sufficient dietary protein intake (McKendry et al., 2021). The common issue is that in addition to poor nutrition, or possible malnutrition, and decreased physical activity levels, aging individuals experience anabolic resistance (Burd et al., 2013). Anabolic resistance describes the observation that skeletal muscle is less sensitive to either (or both) post-absorptive hyperaminoacidemia (Wall et al., 2015) or resistance exercise (Kumar et al., 2009), leading to the reduced response of MPS. Due to this anabolic resistance, aging individuals require additional anabolic stimuli to optimally simulate MPS compared to younger individuals (Burd et al., 2013). Most nutritional and exercise recommendations are based on studies performed using younger individuals, and it has been recognized that the results observed in a younger population do not always translate to older individuals (Volpi et al., 2013).

4.1. Older individuals require specific recommendations for protein consumption

Based on data from previous studies, adults are recommended to consume (with some variability between estimates) ∼0.8g protein/kg/day (Traylor et al., 2018) from mixed protein sources. A study by McKendry and colleagues has recently highlighted that the current recommended dietary allowance (RDA) or Recommended Nutrient Intake (RNI) for protein consumption does not optimally simulate MPS rates in older adults (McKendry et al., 2024a). There are several other observations around the requirement of specific essential amino acid needs for optimizing MPS (Szwiega et al., 2021; Rafii et al., 2024), and protein needs to optimize glutathione synthesis (Paoletti et al., 2024; Jackson et al., 2004) that indicate the insufficiency of the RDA or RNI to optimize protein synthesis. In addition, McKendry et al. (2024a) utilized specific strategies to combat anabolic resistance by increasing total protein intake (Traylor et al., 2018), increasing per meal protein dose (Moore et al., 2015), evenly distributing protein throughout the day (Norton et al., 2016; Farsijani et al., 2016), and utilizing high-quality proteins (Pinckaers et al., 2021), which may be important for older persons, based on previous research. By manipulating the previously mentioned strategies of protein intake, rates of MPS were elevated in older adults compared to consuming the current RDA for protein, with more protein being consumed at dinner and less during breakfast and lunch, which are typical of older adult eating patterns.

A study by Moore and colleagues recommended that aging individuals should consume 0.4 g/kg of protein per meal, leading to a total dose (assuming three daily meals) of at least 1.2 g/kg/day in order to maintain muscle mass (Moore et al., 2015). Furthermore, consuming an even distribution of protein in all meals throughout the day ensures that MPS is optimally stimulated during the day and that protein requirements are met (Mamerow et al., 2014). Regarding protein sources, dairy proteins are a high-quality source as they contain a higher relative proportion of leucine (an essential amino acid that is critical for the stimulation of MPS) and is rapidly digested (Szwiega et al., 2021; Tang et al., 2009). Older individuals should consider, in priority order, total protein intake, per meal dose, protein distribution throughout the day, and quality of a protein source when making food selections to take advantage of all available tools to preserve or increase muscle mass (Murphy et al., 2016). Older adults are at risk of inadequate energy intake, which may make it difficult to achieve the aforementioned protein recommendations. By focusing on achieving adequate energy and protein intake older adults can mitigate potential losses of muscle due to poor nutrition (Miller and Wolfe, 2008; Yannakoulia et al., 2018).

4.2. Exercise provides benefits for aging muscle

Increased physical activity and exercise provide a multitude of benefits for not only healthy young adults but also for older individuals (Rebelo-Marques et al., 2018). However, specific recommendations and evidence-based training protocols for older adults are less well-researched.

Resistance exercise is a potent stimulator of MPS leading to muscle hypertrophy and strength. Older individuals should aim to increase, or at least preserve, muscle mass and strength over time. There are various modes of resistance exercise, and there are various training strategies for aging individuals (Izquierdo et al., 2021). Guidance for the development of muscle (or at least to prevent its loss), enhancement of strength, and increased function has been comprehensively summarized (Izquierdo et al., 2021). It is important to note, however, that various resistance exercise training (RET) protocols show benefits for hypertrophy and strength outcomes (Currier et al., 2023; McLeod et al., 2024). Importantly for older persons, the benefits of improving function, as assessed by tests like the short physical performance battery, time-up-and-go, and various walk tests, is that many RET protocols achieve positive outcomes with very little distinction between the different protocols as longs as the intensity is high (i.e. low repetitions with higher weights or higher repetitions with lower weights) (Currier et al., 2023; McLeod et al., 2024). Thus, lower-load (30–50% 1RM, higher volume) RET may be a viable option for RET and may target more of the physiological deficits observed in older adults (e.g., increasing mitochondria content and improving glycemic control) than traditional higher-load RET (80% 1RM) (Burd et al., 2010; Beaudry et al., 2024; Lim et al., 2019). Lower load RET provides an alternate training mode to higher loads and may be a means to combat age- and obesity-related health decrements simultaneously.

4.3. Combined lifestyle strategies are the key to muscle maintenance and preservation

A combination of both resistance and aerobic exercise, along with adequate protein consumption, appears to be a potent strategy to mitigate muscle aging detriments (Churchward-Venne et al., 2012; Burich et al., 2015; Deutz et al., 2014). Performing aerobic exercise combined with a resistance exercise training regimen demonstrated improvements in glycemic control, body composition, strength, and function in older adults with type 2 diabetes mellitus (Tan et al., 2012). Exercise is impactful for the muscles of older adults, but when combined with adequate protein intake may be utilized as a sarcopenia-preventive or at least mitigative strategy. As previously mentioned, a reduction in daily physical activity can exacerbate the muscle aging process, this process is potentially mitigated through the combined use of nutrition and exercise. Consuming a whey supplement (20g) and performing lower-load RET thrice weekly attenuated the decline in rates of MPS and preserved muscle mass during a two-week step reduction protocol (<1500 steps/day) (Devries et al., 2015). Hence, it is evident that the combination of both nutrition and exercise could be an effective target to slow down muscle aging.

4.4. Pre-habilitation: the importance of getting active now

Often in life, there are instances that we cannot prevent that place us at risk for muscle atrophy. For example, some older adults undergo hip surgery, which requires them to be bedridden for a short time. While others may, unfortunately, experience a fall that sends them to the hospital, requiring the immobilization of a limb for recovery. Pre-habilitation is a strategy that has come to light to prevent the risk of poor postoperative outcomes, such as increased length of stay and loss of function, which puts an individual at risk of greater muscle atrophy due to sedentary behaviour (Halloway et al., 2015). Pre-habilitation is a strategy utilized prior to surgery to improve outcomes postoperatively (Baimas-George et al., 2020). A study done by Smeuninx et al. (2023) had older adults perform a unilateral bout of RET prior to 5 days of bed rest (Smeuninx et al., 2023). Although it was not possible to mitigate all reductions in quadriceps CSA due to bed rest, the exercising leg experienced less muscle loss and a less drastic reduction in integrated myofibrillar protein synthesis. Various other strategies have been proposed pre-, during-, and post-disuse to aid in mitigating skeletal muscle atrophy and are discussed in a review by McKendry and colleagues (2024) (McKendry et al., 2024b). Further research is warranted to discover the optimal strategy to preserve muscle mass during disuse. However, it is evident that resistance exercise increases MPS and muscle mass, so getting active now may put you in a better position later in life.

5. Research issues in aging muscle physiology in humans

Working with older persons in muscle physiology research presents several challenges. Researchers face many difficulties when investigating age-related muscular adaptations in older adults. As much as researchers in this field acknowledge the issues with completing clinical trials and studying in this area, here we outline some of the barriers to work in this field.

5.1. Medication use

The exclusion criteria of any given study would ideally include a variety of medications that may affect muscle metabolism or the response to an exercise and/or nutritional intervention; for example, chemotherapy, use of anabolic steroids, use of hormones, etc. Older individuals are, however, prescribed a combination of medications either prophylactically or to control the symptoms of a current condition, such as hypertension, hypercholesterolemia, and various others. In 2015–2016, according to the National Center for Health Statistics, 53.3% of the older population aged 60–79 years old took 1 or more medications, of which 10.4% took 5 or more medications (Hales et al., 2019). It can be assumed that this percentage is even higher in older adults with a BMI higher than 25. The increasing popularity of GLP-1 agonists as a medication for type 2 diabetes mellitus or as a weight loss strategy highlights the difficulties in recruitment in specific older adult populations, such as those with overweight/obesity (Watanabe et al., 2024; Mahase, 2024). However, currently, studies are done on older adults who are not taking these certain medications. Numerous studies have specifically recruited older adults who are not taking any medications. However, this methodology substantially increases the time and resources required to achieve adequate participant enrollment. This raises concerns about the generalizability of such findings to the broader older population, given that it is uncommon to find individuals over the age of 60 who are not on any pharmacological treatment. In summary, it is difficult to isolate conditions within aging when individuals are on medication that affects the muscle; however, finding older adults not on medications is difficult and often limits the generalizability of the results.

5.2. Causal relationships

When looking at the causal relationships of sarcopenia, the effects of the natural aging process, inactivity, and medication use are almost impossible to separate. As mentioned before, older inactive persons usually take one or more medications that can influence the muscle environment or the muscle directly, and therefore, it becomes difficult to discern the effect of the natural aging process in an individual. In addition, since older adults typically have lower muscle mass and reduced ability to build muscle or recover from disuse than healthy adults, bed rest studies to investigate the effect of inactivity are difficult to justify ethically.

6. Future directions

Conducting research in an older population and specifically focusing on the effect of aging can be challenging. However, the outcomes of these studies will provide valuable information to help mitigate declines in muscle with aging, and thus, participation in aging studies should be promoted. Focusing on isolating the causal relationships between normal aging, (in)activity, medication use, and medical complications is crucial, and creating new models in which we can effectively isolate causal relationships of human tissue should be prioritized. Most importantly, methods that focus on getting the message across that mitigation of the decline in muscle mass is better than treating the consequences of having a low muscle mass and attempting to restore muscle loss should be promoted. Therefore, a goal for future research is to translate research to get people active from a young age until the day they physically can no longer be active in an attempt to prolong their health span. In addition, it might be a suggestion to include participants who are on any medicine. This would create a more representable image for your day-to-day 60+-year-old, even though the medication might affect the study outcomes.

There is a “natural” aging process of the muscle. There is a typical reduction in muscle mass, strength, and eventual physical function termed sarcopenia. On top of this “natural” decline in muscle and strength, several factors like disuse, inactivity, disease states, and undernutrition/malnutrition can cause the decline in muscle to be exacerbated. On the other hand, remaining physically active and eating sufficient protein can cause the declines in muscle mass, strength and function to be mitigated. When adopting a muscle-centric view, there are a lot of negative changes with aging in the muscle fibre and mitochondrial pool of the muscle (Fig. 4), the ECM, the muscle stem cell niche, and the microvascular system with age. Although there are challenges when conducting human research in an older population, like the influence of medications, activity levels, and motivation to participate, we strongly encourage researchers to keep developing new techniques and to keep investing in human studies where possible.

Fig. 4.

Several factors influence/change muscle physiology with aging, such as fibrosis, anabolic resistance, mitochondrial structure, decreased muscle mass, lower phosphorylation of several signalling pathways, mitochondrial ROS production increases, higher systemic inflammation, stiffer connective tissue, a greater chance of developing T2D, increased use of medication, and fibrosis of the ECM. The changes/consequences in muscle physiology with aging are divided in 3 subsections. ROS; Reactive oxygen species, T2DM; Type 2 diabetes mellitus, ECM; extracellular matrix. Figure created using BioRender.

Authorship contribution statement

TAHJ, CVL, and SMP conceived and designed the review. TAHJ and CVL wrote the review in an equal and collaborative manner. All authors edited, revised, and approved the final version of the paper.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The Canada Research Chairs Program supports SMP. SMP thanks Manchester Metropolitan University for their support via a Global Chair.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: S.M. Phillips reports a relationship with Exerkine that includes: equity or stocks. S.M. Phillips has patent #11504413 issued to Exerkine. SMP has received grant funding from the Canadian Institutes for Health Research, the National Science and Engineering Research Council of Canada, the US National Institutes for Health, Roquette Freres, Nestle Health Sciences, Friesland Campina, The US National Dairy Council, Dairy Farmers of Canada and Myos. SMP has received travel expenses and honoraria for speaking from Nestle Health Sciences, Optimum Nutrition, and Nutricia. SMP holds patents licensed to Exerkine Inc. but reports no financial gains. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

No data was used for the research described in the article.