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. Author manuscript; available in PMC: 2011 May 1.
Published in final edited form as: Int J Ind Ergon. 2010 May 1;40(3):315–320. doi: 10.1016/j.ergon.2009.11.003

Leg Strength Comparison between Younger and Middle-age Adults

Sukwon Kim 1, Thurmon Lockhart 2, Chang S Nam 3
PMCID: PMC2861367  NIHMSID: NIHMS165084  PMID: 20436934

Abstract

Although a risk of occupational musculoskeletal diseases has been identified with age-related strength degradation, strength measures from working group are somewhat sparse. This is especially true for the lower extremity strength measures in dynamic conditions (i.e., isokinetic). The objective of this study was to quantify the lower extremity muscle strength characteristics of three age groups (young, middle, and the elderly). Total of 42 subjects participated in the study: 14 subjects for each age group. A commercial dynamometer was used to evaluate isokinetic and isometric strength at ankle and knee joints. 2 × 2 (Age group (younger, middle-age, and older adult groups) × Gender (male and female)) between-subject design and Post-hoc analysis were performed to evaluate strength differences among three age groups. Post-hoc analysis indicated that, overall, middle-age workers’ leg strengths (i.e. ankle and knee muscles) were significantly different from younger adults while middle-age workers’ leg strengths were virtually identical to older adults’ leg strengths. These results suggested that, overall, 14 middle-age workers in the present study could be at a higher risk of musculoskeletal injuries. Future studies looking at the likelihood of musculoskeletal injuries at different work places and from different working postures at various age levels should be required to validate the current findings. The future study would be a valuable asset in finding intervention strategies such that middle-age workers could stay healthier longer.

Keywords: middle age workers, dynamic leg strength, risk of musculoskeletal disorder

1. Introduction

Age-related strength degradation has been identified as a risk factor for occupational musculoskeletal disorders (MSDs) because its dreadful impacts on a fundamental role of muscle strength in functional capabilities or performances in many work-related settings (Hoogendorn et al., 1999; Johnson and Nussbaum, 2003; Kallman et al., 1990; Kim and Lockhart, 2008; Kumar et al., 1994; National Research Council, 2001; Sjøgaard and Sjøgaard, 1998; Sprince et al., 2007; Yip, 2004).

Overexertion, rapid work pace and repetitive motion patterns, intensive static effort or forceful exertions, and insufficient recovery time significantly contributed to the increases in the likelihood of MSDs (Butler and Kozey, 2003; Jensen et al., 1996; Jensen 2008; Kee and Seo, 2007; Malchaire et al., 2001; McMillan and Nichols, 2005; Newell and Kuman, 2004; Sjøgaard and Sjøgaard, 1998; Thorn et al, 2002; Viikari-Juntura and Silverstein, 1999; Water, 2004). Additionally, risk of work-related musculoskeletal disorders (MSDs) increased as the occupational tasks required a larger fraction of a worker’s physical capability (Cooper et al., 1994; Felson et al., 1991; Jenson 2008; Johnson and Nussbaum, 2003; Kumar, 1994). In that regard, simple strength measures have been used to predict risk of the injuries at various work-settings (Frievalds, 1996; Pheasant and Scriven, 1983; Seth et all, 1999).

In 2004, Center for Disease Control (CDC) reported that workers age between 35 and 54 accounted for 50% of MSDs whereas workers age over 54 accounted for only 10% of MSDs suggesting the importance of understanding the injury characteristics of this worker population (i.e., middle aged group age between 35–54 years). More importantly, leg strength measures from this working group are somewhat sparse. This is especially true for the lower extremity strength measures in dynamic conditions (i.e., isokinetic) although among occupational musculoskeletal injuries and disabilities, the knee is one of the most common sites of occurrence (Gagnon et al., 2002; Gallos, 2006; Jin et al., 2009; Sulsky et al., 2002). The objective of this study was to quantify the lower extremity muscle strength characteristics of two age groups (young and middle adults). Additionally, the strengths of older adults (over 65 years-old, retired) were measured to compare with that of the two age groups; authors thought that comparing the strengths between middle-age adults and older adults would accentuate the significance of understanding the risk of injury of middle-age workers. The present study hypothesized that middle-age adult’s strength would not be statistically different from younger adults’ strength. More importantly, the present study hypothesized that leg strengths in middle-age adults would not be different from that in older adults; the study expected to see leg strength differences between younger adults and middle-age adults, but, not between middle-age adults and older adults (over 65 years old). The major goal of this evaluation was to compare overall leg strengths between younger adults and middle-age adults; the evaluation in this study was not performed to identify the individual’s ability to perform certain manual material handling tasks such as lifting, pushing, pulling or carrying.

2. Methods

2.1. Subject

Fourteen young (18–34 years old) individuals (7 male and 7 female), 14 middle age (35–54 years old) individuals (7 male and 7 female) and 14 elderly (65 and older) individuals (7 male and 7 female) participated in this experiment (Table 1). The young adults were recruited from general student population at Virginia Tech, and the middle age adults and older adults were recruited from the local community. All the middle age adults were working full time and all the older adults were independently living. All participants were compensated for their time and effort. Each participant completed an inform consent procedure approved by the Virginia Tech Internal Review Board (IRB). Participants were excluded from the study if they indicated any physical problems (i.e. hip, knee, ankle problems). A questionnaire was used as an initial screening tool.

Table 1.

Descriptive statistics for height and weight of each group.

Younger Middle-age Older
Mean S.D Mean S.D Mean S.D p-value
Age (yrs) 25.34 2.98 41.01 3.24 70.20 3.46 0.00
Height (cm) 172.41 10.93 170.9 7.20 168.50 9.09 0.53
Body Mass (kg) 71.59 12.14 72.58 16.30 82.60 21.13 0.18
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2.2. Apparatus, Testing, and Recording

Participants were instructed to avoid physical activity for at least 24 hours before testing. A commercial dynamometer (Biodex System 3; Biodex Medical Systems, Shirley, NY) was used to test ankle plantar- and dorsi-flexor muscle functions and knee extensor and flexor muscle functions. This commercial dynamometer was found to be acceptable in measuring valid and reliable joint torques and positions (Drouin et al., 2004). Participants were seated in the adjustable Biodex chair and were strapped by two diagonal straps across the chest and by a seatbelt applied over the hips to minimize extraneous movements during the contractions. The commercial dynamometer (Biodex System 3) used in the present study was reported to have acceptable reliability and validity for measuring torque (Drouin et al., 2004). Visual inspections were performed to properly align the axis of rotation of the ankle and knee with the axis of rotation of the dynamometer before each trial. Isokinetic and isometric strength measures were used to test the lower limb strength in the present study although isotonic, isovelocity, etc. strength measures can be used to find their impacts on work performance and safety. The torques measured by the dynamometer were synchronously recorded and stored in a laboratory computer by LabView at a sampling rate of 1024 Hz. The data from the dynamometer were low-pass filtered using a computer program developed using LabView (Butterworth, zero phase-lag, 4th order, 6 Hz cut-off).

Ankle Strength Testing

For ankle isokinetic strength tests (Figure 1 and 2), the footplate was adjusted at 40 degrees for the total range of motion at the ankle joint axis: 10 degrees if dorsiflexion (flexion) and 30 degrees of plantarflexion (extension). Participants were instructed to push down against the footplate and pull back up “as hard as you can” continuously for 5 seconds after a spoken cue. Participants were allowed to practice two to three trials before collecting measurement data and asked to apply three dynamic maximum exertions at each of three levels randomly (30, 60, and 120 degrees per second). The rest periods of 45–60 seconds were given after each exertion. The highest plantarflexion and dorsiflexion torques of the three trials in each testing level was used for statistical testing. For isometric strength evaluation, participants were asked to apply their maximum exertion at each of three fixed angles randomly (0, 15, and 30 plantar-flexed angles, see Figure 2b) and the best of them were used for evaluation. The each individual’s ankle strengths were normalized by his/her body mass (N·m/kg)

Figure 1.

Figure 1

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Biodex dynamomter setup and position for ankle strength test

Figure 2.

Figure 2

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Range of motion for a) isokinetic and b) isometric strength tests.

Knee Strength Testing

Peak isokinetic knee strengths at 30, 60, and 120 degrees/second, and isometric knee flexion strengths at 0, 15, and 30 degree extension were evaluated. Participants were instructed to sit with their right ankle strapped to a knee attachment fixed to a commercial dynamometer. They were instructed to exert their maximum efforts as instructed similar to ankle strength test. The each individual’s knee strengths were normalized by his/her body mass (N·m/kg)

Statistical Analysis

Descriptive and inferential statistical analyses were performed by utilizing the JMP statistical packages (SAS Institute Inc. Cary, NC, USA). 2 × 2 (Age group (younger, middle-age, and older adult groups) × Gender (male and female)) between-subject design was used to test main effect (Age) and interaction effect (Age × Gender). In addition, Post-hoc analysis among three different age groups’ strength data was performed pairwise comparisons among the groups. The results were considered as statistically significant when p≤0.05.

3. Results

3.1. Ankle Strength

Overall, isokinetic and isometric ankle strengths among three age groups were statistically different although a few measures such as IK 120 RE and IM 0 LE indicated no significant age effect (Table 2). Most importantly, the results by Post-hoc analysis (Table 3) indicated that, overall, ankle strengths in middle-age adults were not different from that in older adults (Figure 3). In Table 3, the same letter such as B in both middle age group and older age group suggested that middle age group’s ankle strength was almost same as older group’s ankle strength. In other words, older adults’ strengths and middle-age adults’ strengths were significant lower than younger adults.

Table 2.

Descriptive Statistic of Isokinetic and Isometric Ankle Strength by Age and ANOVA Summary Table

Level Younger Middle-age Older Age Effect Age x Gender
Mean S.D Mean S.D Mean S.D p-value p-value
IK 30 R E 0.94 0.29 0.62 0.17 0.65 0.20 0.00009* 0.62
IK 30 R F 0.36 0.09 0.30 0.09 0.27 0.07 0.03* 0.56
IK 60 R E 0.68 0.22 0.55 0.19 0.45 0.05 0.02* 0.21
IK 60 R F 0.28 0.06 0.22 0.06 0.21 0.05 0.04* 0.99
IK 120 R E 0.46 0.13 0.40 0.14 0.37 0.17 0.20 0.56
IK 120 R F 0.22 0.06 0.18 0.04 0.18 0.04 0.02* 0.68
IK 30 L E 0.84 0.36 0.64 0.21 0.68 0.21 0.10 0.80
IK 30 L F 0.35 0.09 0.33 0.1 0.27 0.08 0.07 0.68
IK 60 L E 0.67 0.28 0.51 0.21 0.54 0.20 0.20 0.75
IK 60 L F 0.28 0.07 0.25 0.08 0.20 0.09 0.05* 0.41
IK 120 L E 0.50 0.23 0.37 0.16 0.42 0.16 0.15 0.99
IK 120 L F 0.23 0.06 0.19 0.05 0.18 0.05 0.09 0.62
IM 0 R E 1.05 0.38 0.75 0.18 0.86 0.33 0.05* 0.66
IM 15 R E 0.74 0.27 0.54 0.15 0.55 0.23 0.04* 0.50
IM 30 R E 0.53 0.17 0.34 0.11 0.33 0.15 0.001* 0.43
IM 0 L E 1.01 0.56 0.76 0.25 0.91 0.38 0.28 0.89
IM 15 L E 0.75 0.38 0.55 0.166 0.64 0.23 0.16 0.85
IM 30 L E 0.51 0.26 0.35 0.10 0.34 0.15 0.04* 0.83
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*

indicates statistical significances, p < 0.05

Strength values are displayed in N·m/kg

Level is expressed in abbreviations; IK= Isokinetic strength, IM= Isometric strength, R= right, L= left, E= plantarflexion, F= dorsiflexion

30 represent either 30°/second for IK test or 30° for IM test

Table 3.

Pairwise Comparisons (Post hoc Student’s t) of Ankle Strength for Age Groups, (Age groups not connected with same letter indicate significant difference, for example in IK 60 RF, middle age group’s strength (B) was not different from older group’s strength (B) whereas both middle age and older adults’ strengths (Bs) were different from younger adults (A))

Level Younger Middle age Older p-value
IK 30 R E A B B 0.00009*
IK 30 R F A A, B B 0.03*
IK 60 R E A A, B B 0.02*
IK 60 R F A B B 0.04*
IK 120 R F A B B 0.02*
IK 60 L F A A, B B 0.05*
IM 0 R E A B B 0.05*
IM 15 R E A B B 0.04*
IM 30 R E A B B 0.001*
IM 30 L E A B B 0.04*
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Figure 3.

Figure 3

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Mean and ±1SD for ankle a) Isokinetic 60 Left Extension, and b) Isometric 30 Left Extension

3.2. Knee Strength

Overall, isokinetic and isometric knee strength was different among three age groups (Table 4, Figure 4). Post hoc analysis (Student’s t, Table 5) suggested that younger adults statistically exhibited stronger isokinetic and isometric knee strength. Additionally, Post hoc analysis suggested that, overall, middle age adults’ strengths were statistically not different from older age adult’s strength. Particularly, strengths of knee flexor muscle in middle-age groups demonstrated almost identical to older adults. In Table 5, the same letter such as B in both middle age group and older age group suggested that middle age group’s knee strength was equivalent as older age group’s knee strength.

Table 4.

Descriptive Statistic on Knee Strength by Age.

Level Younger Middle-age Older Age Effect Age x Gender
Mean S.D Mean S.D Mean S.D p-value p-value
IK 30 R E 2.02 0.48 1.66 0.43 0.35 0.34 0.0007* 0.63
IK 30 R F 1.13 0.21 0.97 0.25 0.81 0.21 0.003* 0.25
IK 60 R E 1.68 0.46 1.35 0.31 1.11 0.30 0.0008* 0.45
IK 60 R F 1.01 0.22 0.81 0.21 0.71 0.24 0.005* 0.77
IK 120 R E 1.21 0.50 1.06 0.09 0.80 0.09 0.02* 0.88
IK 120 R F 0.82 0.27 0.70 0.26 0.60 0.21 0.07 0.37
IK 30 L E 1.94 0.65 1.55 0.41 1.27 0.33 0.003* 0.80
IK 30 L F 1.15 0.22 0.98 0.23 1.01 0.31 0.18 0.91
IK 60 L E 1.58 0.52 1.21 0.37 1.03 0.28 0.004* 0.43
IK 60 L F 1.07 0.21 0.87 0.21 0.89 0.23 0.03* 0.86
IK 120 L E 1.13 0.43 0.93 0.32 0.74 0.23 0.01* 0.97
IK 120 L F 0.90 0.19 0.75 0.21 0.70 0.23 0.04* 0.73
IM 0 R E 2.15 0.72 1.73 0.46 1.52 0.55 0.02* 0.96
IM 0 R F 0.91 0.28 0.67 0.22 0.59 0.17 0.002* 0.99
IM 15 R E 2.10 0.74 1.78 0.43 1.52 0.38 0.02* 0.75
IM 15 R F 1.05 0.33 0.79 0.24 0.70 0.17 0.002* 0.99
IM 30 R E 1.80 0.61 1.48 0.39 1.32 0.24 0.02* 0.88
IM 30 R F 1.13 0.37 0.81 0.25 0.79 0.20 0.004* 0.82
IM 0 L E 2.19 0.80 1.46 0.46 1.46 0.44 0.002* 0.76
IM 0 L F 0.99 0.29 0.74 0.22 0.66 0.14 0.001* 0.91
IM 15 L E 2.02 0.74 1.59 0.51 1.43 0.35 0.02* 0.93
IM 15 L F 1.07 0.31 0.85 0.20 0.77 0.17 0.004* 0.97
IM 30 L E 1.71 0.51 1.42 0.45 1.17 0.30 0.008* 0.65
IM 30 L F 1.17 0.34 0.88 0.19 0.89 0.15 0.006* 0.95
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*

indicates statistical significances, p < 0.05

Strength values are displayed in N·m/kg

Level is expressed in abbreviations; IK= Isokinetic strength, IM= Isometric strength, R= right, L= left, E= extension (movement away from body, F= Flexion (movement toward body)

30 represent either 30°/second for IK test or 30° for IM test

Figure 4.

Figure 4

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Mean and ±1SD for Knee a) Isokinetic 60 Right Extension, and b) Isometric 30 Right Flexion

Table 5.

Pairwise Comparisons (Post hoc Student’s t) of Knee Strength for Age Groups.

Younger Middle-age Older p-value
IK 30 R E A B B 0.0007*
IK 30 R F A A, B B 0.003*
IK 60 R E A B B 0.0008*
IK 60 R F A B B 0.005*
IK 120 R E A A, B B 0.02*
IK 30 L E A B B 0.003*
IK 60 L E A B B 0.004*
IK 60 L F A B B 0.03*
IK 120 L E A A, B B 0.01*
IK 120 L F A B B 0.04*
IM 0 R E A A, B B 0.02*
IM 0 R F A B B 0.002*
IM 15 R E A A, B B 0.02*
IM 15 R F A B B 0.002*
IM 30 R E A A, B B 0.02*
IM 30 R F A B B 0.004*
IM 0 L E A B B 0.002*
IM 0 L F A B B 0.001*
IM 15 L E A A, B B 0.02*
IM 15 L F A B B 0.004*
IM 30 L E A A, B B 0.008*
IM 30 L F A B B 0.006*
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Age groups not connected with same letter indicate significant difference, for example in IK 30 LE, middle age group’s strength (B) was not different from older group’s strength (B) whereas both middle age and older adults’ strengths (Bs) were different from younger adults (A)

4. Discussion

The main objective of the study was to examine three different age groups’ leg strengths, and, particularly, the study was interested in identifying differences in leg strength between a middle age adult group and a younger adult group. Additionally, leg strength of an older adult group was evaluated in order to emphasize the values in leg strength found in the middle-age group; the author thought that comparing older adults’ leg strengths with middle-age group’s leg strength would draw attention to the risk of injuries of middle-age workers.

Overall, in disagreement with previous literature (Lindle et al., 1997), leg strengths in middle age adults were not statistically different from older adults in the present study. In the previous study (Lindle et al., 1997), muscle strength comparisons were performed on 654 women and men aged between 20–93 years old, whereas, the present study evaluated muscle strengths of 42 healthy adults; Statistical Power analysis and Sample Size Estimation were performed in the present study. The previous study suggested that isokinetic strengths degraded with advancing age. The different outcomes between the present study and the previous study may be resulted by the fact that the previous studies did not normalize the torque by individual weights. It is very important to normalize the individual torque measures when presenting data since a person with more mass is likely to produce more torque at their joints. So, it may have been the weight differences between older adults and middle-age adults that have contributed to different outcomes although the data from the Lindle et al. (1997) showed very similar average weights between two age groups. A study by Frontera et al., (1991) suggested that muscular strength was significantly lower in the 65- to 78-yr-old adults than in the 45- to 54-yr-old adults, but, when strength was normalized for muscle mass, the age-related differences were not significant in most of the muscle groups. Additionally, in the present study, it was interesting to see more degradation in flexor muscles than extensor muscles in middle-age adults when compared to younger adults.

Zhang and Buhr (2002) stated “Naturally, degradation of muscular strength leads to reduced functional capabilities and possibly elevated risk of injury…” As an intervention for reducing risks of injuries, strength improvement was proposed by researchers (Campbell et al., 1997; Hainaut and Duchateau, 1989; Kuruganti et al., 2006; Müller, 1965) because strength improvement enhanced not only strength but also muscular resistance to fatigue which was a key contributing factor to risks of injuries. In the present study, the attenuation of muscle strength in both middle age group and older adult group was almost identical when comparing to younger adult group. Studies (Keran et al., 1994; Shin et al., 2006; Vollestad, 1997; Westerblad et al., 1998) suggested that reductions in muscle strength were found to be positively related to muscle fatigue which was a key precursor for musculoskeletal injury. These results from the current study could suggest that not only older adult group was exposed to higher likelihood of MSDs, but also middle-age group was exposed to higher likelihood of MSDs such as knee injuries (Gagnon et al., 2002; Gallos, 2006; Jin et al., 2009; Sulsky et al., 2002). The previous study (Sulsky et al., 2002) suggested that the risk of disability discharge due to knee injuries among women in the US army increased following the age of 33 years.

The results in the present study indicated a significant trend in strength degradation; degradations in knee flexor muscle were more dominant in comparison to knee extensor muscle in middle-age adults. Knee flexor muscles play a dominant role in decelerating forward leg momentum while walking. Older adults were reported to be at higher risk of slip-induced falls because inability to properly decrease forward leg momentum at heel contact was suggested (Lockhart et al., 2005), suggesting that the middle-age adults could be at as much risk of slips and falls as older adults although a complex of many factors were responsible for slip induced falls for older adults. The knee flexors also function as dynamic stabilizers at the knee joint. This prevents tibial anterior displacement and sustains the function of the ligaments at the knee joint. While pushing, pulling, and carrying a load, significant lower leg strengths are required to stabilize the knee joints as much as to perform tasks. Knee joint injuries at work were common (Gagnon et al., 2002; Gallos, 2006; Jin et al., 2009; Sulsky et al., 2002) and examples were torn meniscus of the knee, anterior cruciate ligament injuries, medial and lateral collateral ligament injuries, and patella femoral pain syndrome. Knee osteoarthrosis (all degrees) was commonly found among workers with heavy physical work such as kneeling or squatting work (Jensen and Eenberg, 1996). For example, carpet fitters and floor layers were in higher risk of knee injuries because the recent popularity of fitted floor covering (Cooper et al., 1994; McMillan and Nichols, 2005). Meniscus tears were often caused by sudden rotation or abduction on the semi-flexed knee joint and the miner was opt to meniscus tears because of their working posture such as the kneeling or crowling (McMillan and Nichols, 2005). Meniscus injury could facilitate the injured knee to develop knee osteoarthrosis with advancing age. In fact, this suggested that middle-age workers who experience knee injuries would be likely to develop osteoarthrosis as they got older. Frequent or heavy lifting, pushing, pulling, or carrying heavy objects were the potential risk factors for these injuries because such activities could involve bodily reaction and exertion which accounted for 40% of total nonfatal occupational injuries and illnesses with days away from work in 2004, and could contribute to sprains, strains, and tears which accounted for 76% of total musculoskeletal disorders with days away from work in 2001 (CDC, 2004).

Overall, the study suggests that middle-age work forces’ health should be administered with elevated concerns although the present only studied 14 middle age workers who resided in Blacksburg, Virginia. Future studies looking at the likelihood of musculoskeletal injuries at different work places and from different working postures at various age levels should be required to validate the current findings. The future study would be a valuable asset in finding intervention strategies such that middle-age workers could stay healthier longer.

Footnotes

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