TABLE 2.

Summary of studies cited in Section “Sex- and Age-Related Differences in Hamstrings’ Anatomy and Physiology.”

Reference	Population	Anatomic part assessed	Assessment method	Main findings
Behan et al. (2018)	32 males (age: 20.6 years) and 34 females (age: 20.9 years) healthy, young, individuals with a low-moderate level of physical activity	Knee joint muscle morphology	MRI	Sex differences in muscle morphology that may predispose females to greater risk of ACL injury, primarily, a smaller knee flexors to knee extensors size ratio, but also a proportionately small sartorius and gracilis and a proportionately large vastus lateralis. Females have a larger BFlh as a proportion of the knee flexors than males, which may contribute to the higher risk of HSI in males.
Blackburn et al. (2009)	20 male (age: 20.7 years) and 20 females (age: 20.4 years) physically active individuals	Musculotendinous stiffness	Ultrasound	Musculotendinous stiffness was greater in males than in females, elastic modulus did not differ significantly across sex. Hamstring muscle size predicted 16% of the variance in hamstring musculotendinous stiffness.
Ebben et al. (2010)	12 male (age: 21.0) and 12 female (age: 19.9) university students	Magnitude and timing of hamstring activation, knee flexors-to-knee extensors ratio activation ratios, and knee flexors-to-knee extensors timing ratios of a variety of hamstring and quadriceps muscles.	EMG	In the precontact phase of jump landings and cutting: men and women are similar with respect to degree of activation of the hamstring In postcontact phase of the cut: men showed a trend toward higher knee flexors-to-knee extensors activation ratio than women
Hepple and Rice (2016)	Review	–	–	Aging conducts to changes in quantity and quality of motor unit, namely caused by motor neuron loss, neuromuscular joint instability, and repeating cycles of denervation and reinnervation leading to fiber type grouping.
Kirk et al. (2018)	11 healthy (age: 26) and 10 old (age: 80) male individuals		Intramuscular and surface EMG	Voluntary strength, evoked contractility, and MU discharge rates were diminished in old compared with young adult men No difference in relative surface EMG concurrent with significantly lower MU discharge rates may indicate that graded force generation in the hamstrings of old men is more dependent on MU recruitment. MU discharge rates of the SM and ST had a greater age-related effect compared to BF
Kong and Burns (2010)	25 males (age: 26.2) and 15 females (age: 24.2) healthy recreationally active individuals.	Hamstrings and quadriceps strength across multiple knee angles and angular velocities between the dominant and non-dominant legs	Isokinetic and isometric analysis (Biodex)	Knee flexors-to-knee extensors ratio was higher in the dominant leg than the non-dominant leg for both isometric and isokinetic measurements No difference in knee flexors-to-knee extensors ratio was found between males and females.
Lim et al. (2017)	10 young adults (24.2 ± 2.7)	Gluteus maximus, gluteus medius, vastus medialis, lateral hamstring, medial gastrocnemius, and soleus	Kinematic, force-plate and EMG	Increases in step length and frequency increases the contribution from the forces developed by gluteus maximus, gluteus medius, vastus medialis, medial gastrocnemius and soleus to both vertical support and forward progression. However, increase in step length results in greater differences in the contributions of vastus medialis and gluteus maximus and limb posture to vertical support.
Martín-San Agustín et al. (2019)	36 female (age: 21.1) and 34 male (age: 21.6) physically active individuals	Velocity contraction of BF and ST	TMG	Both male and female individuals had a similar pattern among the velocity of contraction
Narici et al. (2008)	Review	–	–	Training process apparently had no effect estimated relative length-tension properties of the muscle. Possibly, tendon stiffness and fascicle length increases canceled out each other.
Overend et al. (1992)	13 young (age: 24.5 years) and 12 old (age: 70.7 years) male individuals	Quadriceps and hamstring CSA	CT	Old male individuals had smaller quadriceps muscles and were weaker (22–32%) in knee flexion and knee extension at both angular velocities vs. young male Strength to CSA ratios were similar at 0 degree/s, but elderly had decreased ratios for both extensors and flexors at 120 degree/s. Correlations of knee extensor and flexor strength with muscle CSA were significant at both velocities in elderly men, but not at either velocity for the knee flexors in young men.
Roos et al. (1999)	13 young adults (26.2 ± 4.1) and 12 (80.0 ± 5.3) moderately active men	Quadriceps	Isometric dynamometer and EMG	Difference in age was observed on the voluntary and stimulated forces, while modest differences were found in contractible speed (slowest in older) and no change in the mean steady-state firing rates at any force level.
Ruas et al. (2019)	Review	Alternative methods of determining the knee flexors-to-knee extensors ratio as a measure of knee muscle strength balance.	–	There is not sufficient evidence to recommend any of the alternative methods of determining knee flexors-to-knee extensors ratio The higher reliability was found for rate for torque development knee flexors-to-knee extensors ratio
Smith et al. (2021)	125 football players	Association between hamstring strength, age and lower limb soft tissue injury history and subsequent hamstring injury		Increased age and previous hamstring, groin and calf injury are all associated with an elevated risk of subsequent hamstring injury in football players.
Vaughan et al. (2016)	Mice (11–13 months; and 15–21 months)	Extensor digitorum longus and soleus	Dissection	Significant increases of the number of Ia afferents in young compared to older mice. Fewer II afferents were also found in mice of middle and older age. However, intrafusal muscle fibers had no significant changes across the age. Thus, proprioceptive sensory neurons seem to degenerate prior to atrophy of intrafusal muscle fibers during aging.
Wan et al. (2017)	11 males (age: 23.6) and 10 females (age: 24.7 years) college students	Length, flexibility, and strength of hamstrings	3D modeling	Hamstring muscle optimal lengths were significantly correlated to hamstring flexibility score but not to hamstring strength The optimal knee flexion angle for maximal knee flexion moment decreased as hamstring flexibility score increased, which indicate that hamstring muscle optimal lengths may be affected by hamstring flexibility.
Webber et al. (2009)	Mathematical model	Quadriceps	Mathematical model	Changes in the Heckman-Binder motoneuron model for human data improved the frequency-current, and muscle unit force-frequency relationships. This adjustment resulted in lower firing frequencies in older and reduction in maximal force output.
Yoshiko et al. (2017)	15 young (21.0 ± 0.4 years old) and 15 old (70.7 ± 3.8)	Quadriceps femoris, hamstring and adductor	MRI	Age-related increase the intramuscular fat content, namely in the thigh areas, possibly explained by the loss of skeletal muscle cross sectional area in older.

HT, hamstrings; ST, semitendinosus; BFlh, biceps femoris long head; BF, biceps femoris; MRI, magnetic resonance imaging; ACL, anterior cruciate ligament; BFsh, biceps femoris short head; SM, semimembranosus; TMG, tensiomyography; EMG, electromyography; CSA, cross-sectional area.