Abstract
Background and aims
This study assessed the reliability of evaluating asymmetrical strength and power deficits in the lower limbs of healthy middle-aged, healthy older and mobility-limited elders. We also explored the relationship between limb asymmetry and physical functioning.
Methods
We evaluated baseline knee extension strength and power asymmetry data from a cohort of older adults (n=57; 74.2±0.9 yrs, 26 male) who participated in a lower body strength training study (TS) and from a cross-sectional study of community dwelling volunteers. Cross-sectional participants were recruited into: healthy middle-aged (MH) (n=31; 47.4±4.8 yrs, 14 male), healthy older (OH) (n=28; 74.0±3.6 yrs, 16 male) and mobility-limited older (OML) (n=34; 77.8±4.5 yrs, 16 male) groups. Knee extensor strength (1RM) and power asymmetry at 40% and 70% of 1RM were evaluated for test-retest reliability using intraclass correlation coefficients (ICCs).
Results
Knee extension 1RM, and peak power at 40% and 70% asymmetry ICCs exhibited excellent to good reliability in the TS and OML groups (TS= 0.8, 0.7 and 0.7, respectively; OML= 0.7, 0.7, and 0.9, respectively) but not in the MH and OH groups. No consistent association between lower limb asymmetry and measures of physical functioning was observed.
Conclusions
Assessment of lower limb strength and power asymmetry is more reliable in mobility-limited elders when compared to healthy middle-aged and older subjects. The relationship of lower limb asymmetry to physical functioning is poor, in contrast to associations between the absolute strength and power of the individual limbs and physical functioning.
Keywords: Asymmetry, mobility-limited, power, reliability, strength
INTRODUCTION
Skeletal muscle strength and power have been shown to decrease with advancing age (1–3). The cause of the deficits in strength and power can be attributed in part to the cumulative effects of pain, disease and injury (4–7). These changes may be more exaggerated in only one of the lower limbs, resulting in lower limb strength and power asymmetry (8–11).
Currently, there is limited information in regards to the amount of lower limb asymmetry that exists in older populations. Though previous studies have shown a relationship between lower limb asymmetry and measures of physical functioning such as gait velocity, balance, and frequency of falling, the reliability of the methods used to determine asymmetry is unknown (12, 13).
The purpose of the present study was to assess the reliability of the evaluation of lower limb strength and power asymmetry in healthy middle-aged, healthy older and mobility-limited elders. In addition, we explored the relationship between lower limb strength and power asymmetry and several additional tests of physical functioning within these distinct populations.
METHODS
Study population
For the reliability analysis, we evaluated baseline knee extension strength and power data from a group of volunteers who participated in a lower body strength training study. The cohort from this study consisted of community-dwelling older adults who had scored less than 10 on the Short Physical Performance Battery (SPPB) (14, 15). Written informed consent was provided by all subjects prior to enrollment.
To strengthen the reliability portion of the analysis and to evaluate the relationship between lower limb asymmetry and physical functioning, data were taken from a separate cross-sectional study of community-dwelling volunteers. Participants for this study were recruited into three distinct groups: healthy middle-aged adults (MH) (40–55 yrs), healthy older adults (OH) (70–85 yrs) and older adults with mobility limitations (OML) (70–85 yrs). Older adults were separated into the healthy older and mobility-limited older groups depending on their SPPB score at a pre-study screening visit with those scoring less than 10 on the SPPB classified as mobility-limited (14, 15). Volunteers participating in this study were subjected to the same inclusion/exclusion criteria and strength and power procedures as the participants in the previously mentioned lower body strength training study.
Study exclusions included: acute or terminal illness, cognitive impairment as defined by a score of 23 or below on the Folstein Mini Mental State Examination, symptomatic coronary artery disease, unstable congestive heart failure, myocardial infarction or fracture in the previous 6 months. The presence of neuromuscular disease, drugs that affect neuromuscular function, and uncontrolled hypertension (>150/90 mmHg) were also considered study exclusions. Prior to enrollment all participants who had met these pre-study qualifications underwent a physical examination by the study physician. This study was approved by the Institutional Review Board at Boston University and Tufts University.
Strength and power testing took place on two occasions, at the same time of day separated by one week. The testing protocol was completed in approximately 60 minutes. The same evaluator, equipment, and procedures were used for all study participants. Each participant was given the opportunity to familiarize themselves with the testing equipment through the use of a visual demonstration and practice at low resistances.
Knee extension strength
Knee extension strength and power were evaluated using pneumatic strength training equipment (Keiser Sports Health Equipment Inc., Fresno, CA). Training study (TS) participants were tested on the K400 series equipment, whereas the cross-sectional study (XS) participants were tested on the A420 series. Both models of the Keiser equipment used the same methods to quantify strength and power. Single limb strength and power measures have been shown to be highly reliable and valid in previous studies (16–19). The knee extension equipment utilizes pressurized air cylinders to provide variable resistance throughout the subjects’ range of motion. Participants were seated with knees in 90 degrees of flexion and an adjustable seat back was positioned so that the participant’s femoral lateral epicondyle was aligned with the axis of rotation of the machine’s lever arm. Participants extended their knee against a pad positioned one inch proximal to the medial malleolus. Participants were instructed to perform a unilateral knee extension through a full range of motion. The external resistance was gradually increased to the point where the participant was unable to complete a repetition through their full range of motion. Knee extensor strength was defined as the 1 repetition maximum (1RM) for each individual leg. A rest period of two minutes was provided between each repetition for all subjects.
Knee extension power
Following a rest period of 5 minutes, each participant was instructed to complete a repetition through their full range of motion as fast as possible at resistances equal to 40% (PP40) and 70% (PP70) of their 1RM for each leg (16). A total of 5 repetitions were performed for each external resistance and the highest measured power for each of the resistances was considered their peak power (PP). Ultrasonic devices mounted on top of the pressurized air cylinders measure distance and velocity, and the manufacturer software uses this information to calculate PP. This PP measure was calculated from the data collected between 5% and 95% of the concentric phase of the knee extension. The first and last five percent of the measured range of motion are not analyzed on this equipment in order to minimize the effect of signal noise at the beginning and end of each movement. PP was determined by obtaining the measure given by the digital display and adjusting it using a custom written statistical program (MatLab 7.0; The Mathworks, Natick, MA). For each test that is performed on the knee extension equipment, a data file is created by the manufacturer’s software. This data file was translated using the Matlab program and the variables of interest were extracted (i.e. force and PP).
Asymmetry measurement
For the reliability analysis, lower limb strength and power asymmetry was determined as (Right Leg – Left Leg). This analysis involved examining the absolute difference in strength and power between the two lower limbs, rather than using a percentage to describe the difference. For interpretation purposes, we felt this was the best way to evaluate the consistency of lower limb strength and power asymmetry over two separate trials. For the comparison of lower limb asymmetry to physical functioning measures, asymmetry was calculated as ((|Weak − Strong|) / Strong) × 100% (12). Using this analysis, the value 0% represents equal strength and power between the lower limbs.
Measures of physical functioning
The SPPB was performed as described previously (14, 15). Subjects were asked to complete assessments of balance, 4 meter-walk time, and chair-rise time. The results of these three tests were combined and graded on a 12-point summary scale that identifies levels of risk for mobility disability in the following manner: low risk (score ≥10), moderate risk (score 7–9) and severe risk (score 4–6).
Gait velocity was measured using an Ultratimer device (DCPB Electronics, Glasgow, Scotland). Subjects were instructed to walk at their normal pace over a distance of 10 m and the average of 2 trials was recorded.
Stair climb performance was assessed using a standard riser of stairs and a multifunction timer (Lafayette Instrument Co.). A pressure sensitive switchmat was placed at the bottom of a set of stairs and on the tenth step of the same rise of stairs. Subjects were instructed to ascend the stairs at a normal pace, holding on to the hand-rail or using an assistive device if necessary. The average of 2 trials was recorded.
Chair-rise time was determined using a standard chair and a stopwatch. Subjects were asked to rise from the chair ten times, as fast as possible with their arms folded across their chest. The ten-chair rise test was performed once.
Statistical methodology
Data were analyzed using the SPSS software package. All values are reported as means±standard deviations. Two-way, fixed model intraclass correlation coefficients (ICC) were used to determine the test/retest reliability of the asymmetry measures using the SPSS data editor. An ICC ≥0.75 is considered excellent, those between 0.41 and 0.74 are considered fair to good and those <0.41 reflect poor reliability (20). Spearman rank correlations were calculated to examine the relationship between asymmetry, weak and strong limb strength and power and measures of physical functioning. A Bonferroni correction was applied and statistical significance was accepted at p<0.0125 to account for multiple comparisons. Analysis of strength and power measures were performed on results obtained from the second evaluation in all participants.
RESULTS
Reliability
Fifty-seven older mobility-limited subjects (26 men, 31 women) participated in the lower body strength training study (TS). Their average age, body mass index (BMI) and SPPB scores were, respectively, 74.2±6.9 yrs, 29.0±5.9 kg/m2, and 7.7±1.4. The 1RM, PP40, and PP70 asymmetry testing conditions all displayed excellent to good reliability in the TS group. The ICCs for absolute asymmetry in the TS group were 0.80, 0.70 and 0.71 for knee extension 1RM, PP40 and PP70, respectively.
Cross-sectional study (XS) participant characteristics are displayed in Table 1. Significant differences existed between the OML group and OH and MH groups in age and total SPPB score. In the MH group, 1RM, PP40, and PP70 asymmetry all exhibited poor reliability (Fig. 1a). In the OH group, PP40 and PP70 asymmetry showed moderate reliability, but 1RM asymmetry reliability was poor (Fig. 1b). However, absolute asymmetry of the 1RM, PP40, and PP70 measures exhibited excellent to good reliability in the OML group (Table 2, Fig. 1c).
Table 1.
Participant characteristics.
| MH | OH | OML | |
|---|---|---|---|
| Male/Female | 14/17 | 16/12 | 16/18 |
| Age | 47.2 (4.8) | 74.0 (3.6) | 77.8 (4.5) |
| SPPB | 11.7 (0.5) | 11.0 (0.9) | 7.9 (1.3)*^ |
| BMI | 25.7 (3.0) | 24.3 (5.9) | 26.3 (3.2)*^ |
| Medications | 2.9 (2.2) | ||
| Diagnoses | 2.1 (1.6) |
Results are reported as Mean (SD).
significant difference vs Healthy Middle-Aged;
significant difference vs Healthy Older.
Fig. 1.
Absolute Knee Extension Asymmetry at PP70 (Day 1 vs Day 2). A) MH: Middle-Aged Healthy (ICC: 0.25). B) OH: Older Healthy (ICC: 0.61). C) OML: Older Mobility-Limited (ICC: 0.87).
Table 2.
Intra-class correlation coefficients (absolute asymmetry).
| MH | OH | OML | |
|---|---|---|---|
| 1RM | 0.37 | 0.27 | 0.68 |
| PP40 | −0.11 | 0.52 | 0.70 |
| PP70 | 0.25 | 0.61 | 0.87 |
Relative asymmetry vs Physical functioning
Relative asymmetry measures are shown in Table 3. The 1RM asymmetry was similar across MH, OH, and OML. In the OML group, PP40 and PP70 asymmetry were significantly greater than OH and MH. Correlations for the OH and OML groups are shown in Table 4. No consistent association was found between relative asymmetry of 1RM, PP40, or PP70 and measures of physical functioning.
Table 3.
Relative asymmetry.
| MH | OH | OML | |
|---|---|---|---|
| 1RM (%) | 9.9 (9.1) | 14.1 (14.1) | 15.5 (14.9) |
| PP40 (%) | 10.1 (7.9) | 12.1 (10.1) | 19.7 (14.6)*^ |
| PP70 (%) | 10.0 (8.4) | 11.9 (9.0) | 22.2 (14.8)*^ |
Results are reported as Mean (SD).
significant difference vs Healthy Middle-Aged;
significant difference vs Healthy Older.
Table 4.
Correlations coefficients: Physical Function vs Relative Asymmetry Strength and Power.
Strong and weak limb vs Physical functioning
Strong and weak limb strength and power measures are shown in Figure 2a and b. Significant differences exist between the OML group and both healthy groups for all strong limb knee extension strength and power measures. In addition, significant differences were found between the OH and MH groups for all three measures. Correlations between strong limb strength and power and measures of physical functioning, for the OH and OML groups, are shown in Table 5.
Fig. 2.
A) Weak and strong limb knee extension strength. Values are mean±SE. B) Weak and strong limb knee extension power. Values are mean±SE. MH: Middle Age Healthy; OH: Older Healthy; OML: Older Mobility-Limited. *significant difference vs MH. ^significant difference vs OH.
Table 5.
Correlations coefficients: Physical Function vs Strong Limb Strength and Power.
Significant differences exist between the OML group and both healthy groups for all weak limb knee extension strength and power measures. In addition, significant differences were found between the OH and MH groups for all three measures. Pearson correlations showed similar results to the strong limb vs physical functioning analysis. Significant inverse correlations were found in the OH group between weak limb 1RM, PP40, PP70 and chair-rise time (r=−0.48, −0.52, and −0.47, p<0.05, respectively) (data not shown).
DISCUSSION
This study found that in healthy middle-aged and healthy older subjects the reliability of the asymmetry measure was fair to poor. In contrast, asymmetrical strength and power measurement in mobility-limited elders was highly reliable over the course of two separate testing sessions and this observation was consistent in two cohorts of mobility-limited elders. However, despite finding the asymmetry measures to be reliable among mobility-limited elders, we found no consistent association between asymmetrical strength and power measures and physical functioning in this population.
Measures of absolute muscle strength and power have been shown to be highly reliable (16, 21, 22). However, no study has previously evaluated the reliability of limb strength and power asymmetry. In both cohorts of mobility-limited older subjects, the ICC values for test-retest reliability of lower limb asymmetry were similarly high compared to previous studies of absolute strength and power in older individuals. Callahan et al. assessed the reliability of knee extension muscle power testing in mobility-limited men and women over the course of two separate trials (16). Leg extension power exhibited excellent reliability at two external resistances with ICCs ranging from 0.85–0.93. Suzuki et al. showed ankle torque measures exhibit good to excellent reliability in a group of mobility-limited older women (21). Lastly, in an assessment of the reliability of isokinetic knee extension in independently living elderly subjects, Hartmann et al. reported ICCs ranging from 0.81 to 0.99 (22).
The consistent results for the mobility-limited participants in the asymmetry measurement may be attributed to impairments in the weak limb. Significant pain, disease, or musculoskeletal injury in the weak limb may be responsible for decreased knee extension strength and power output relative to the opposite or strong limb (23, 24). Similarly, the performance of the mobility-limited subjects on the SPPB test at screening may suggest that this group had pre-existing conditions affecting their functional ability. These conditions may have been a factor in this population producing consistent asymmetry results in the two testing sessions. In contrast, healthy middle-aged and healthy older participants exhibited poor reliability in strength and power asymmetry assessment. The mechanism for low reliability of the asymmetry measure in these groups is unclear, but it may be related to the better overall health status in the healthy middle-aged and healthy older cohorts.
Despite the fact that the study results show reliability in the asymmetry measure for the OML group, a consistent relationships between lower limb strength and power asymmetry and physical functioning was not observed in this population. Prior studies have shown an association between leg power asymmetry, gait velocity and frequent falling (12, 13). In a large sample of healthy older women, Portegijs et al. reported that lower limb asymmetry was an independent predictor of walking velocity (13). Skelton et al. reported that lower limb power asymmetry may be a better predictor of future falls than traditional measurements of strength among female elders with a history of falling (12). In the current study, we explored this relationship in several distinct populations and included both men and women. Both of these factors may have contributed to the discordant findings. However, our data demonstrates that weak and strong limb knee extension strength and power have a more pronounced association with measures of physical functioning than asymmetrical strength and power in healthy older and mobility-limited older men and women. This finding is consistent with a previous study examining the relationship between lower limb asymmetry, strength and power output and physical functioning measures in both males and females (25). A possible explanation for this finding is that lower limb asymmetry is driven by the strength and power of one of the lower limbs; therefore, it may be more useful to focus on the effects of unilateral lower limb strength and power on physical functioning.
The ability to evaluate the asymmetry measure in distinct groups that were classified based on physical functioning was one of the strengths of this study. However, the study has several limitations that include the cross-sectional study design, sample size, and participants that may not have had a degree of impairment severe enough to clearly observe if asymmetry is associated with physical functioning. In addition, generalization of the lack of association between lower limb asymmetry and physical functioning results to lower functioning populations may be complicated. The cohorts used for this study were fairly independent and able to care for themselves. Evaluation of a cohort of participants who rely on assistive devices for ambulation may show a greater relationship between asymmetry and physical functioning.
CONCLUSIONS
The results of this study show that asymmetrical lower limb strength and power assessments are a reliable measure in mobility-limited elders, when compared to healthy older and middle-aged populations. Additionally, we found no consistent relationship between lower limb asymmetry and measures of physical functioning. Future studies should focus on determining whether the asymmetry in the lower limbs of elders is driven by the lack of strength in the weak limb. Additionally, the effect of lower limb asymmetry and weak limb strength on functional balance and postural stability should be explored.
Acknowledgments
This work was supported by the National Institute on Aging (NIA) grant number AG18844 and this work is based upon work supported by the U.S. Department of Agriculture, under agreement No. 58-1950-7-707. This research was also supported by the Boston Claude D. Pepper Older Americans Independence Center (1P30AG031679). Any opinions, findings, conclusion, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
References
- 1.Frontera WR, Hughes VA, Fielding RA, Fiatarone MA, Evans WJ, Roubenoff R. Aging of skeletal muscle: a 12-yr longitudinal study. J Appl Physiol. 2000;88:1321–6. doi: 10.1152/jappl.2000.88.4.1321. [DOI] [PubMed] [Google Scholar]
- 2.Hughes VA, Frontera WR, Wood M, et al. Longitudinal muscle strength changes in older adults: influence of muscle mass, physical activity, and health. J Gerontol. 2001;56:B209–17. doi: 10.1093/gerona/56.5.b209. [DOI] [PubMed] [Google Scholar]
- 3.Metter EJ, Conwit R, Tobin J, Fozard JL. Age-associated loss of power and strength in the upper extremities in women and men. J Gerontol. 1997;52A:B267–B76. doi: 10.1093/gerona/52a.5.b267. [DOI] [PubMed] [Google Scholar]
- 4.Rantanen T, Guralnik JM, Sakari-Rantala R, et al. Disability, physical activity, and muscle strength in older women: the Women’s Health and Aging Study. Arch Phys Med Rehabil. 1999b;80:130–5. doi: 10.1016/s0003-9993(99)90109-0. [DOI] [PubMed] [Google Scholar]
- 5.Lamb SE, Guralnik JM, Buchner DM, et al. Factors that modify the association between knee pain and mobility limitation in older women: the Women’s Health and Aging Study. Ann Rheum Dis. 2000;59:331–7. doi: 10.1136/ard.59.5.331. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Leveille SG, Ling S, Hochberg MC, et al. Widespread musculoskeletal pain and the progression of disability in older disabled women. Ann Intern Med. 2001;135:1038–46. doi: 10.7326/0003-4819-135-12-200112180-00007. [DOI] [PubMed] [Google Scholar]
- 7.Herrick C, Steger-May K, Sinacore DR, et al. Persistent pain in frail older adults after hip fracture repair. J Am Geriatr Soc. 2004;52:2062–8. doi: 10.1111/j.1532-5415.2004.52566.x. [DOI] [PubMed] [Google Scholar]
- 8.van Wilgen CP, Akkerman L, Wieringa J, Dijkstra PU. Muscle strength in patients with chronic pain. Clin Rehabil. 2003;17:885–9. doi: 10.1191/0269215503cr693oa. [DOI] [PubMed] [Google Scholar]
- 9.Robertson S, Frost H, Doll H, O’Connor JJ. Leg extensor power and quadriceps strength; an assessment of repeatability in patients with osteoarthritic knees. Clin Rehabil. 1998;12:120–6. doi: 10.1191/026921598673072472. [DOI] [PubMed] [Google Scholar]
- 10.Sicard-Rosenbaum L, Light KE, Behrman AL. Gait, lower extremity strength, and self assessed mobility after hip arthroplasty. J Gerontol A Biol Sci Med Sci. 2002;57:M47–51. doi: 10.1093/gerona/57.1.m47. [DOI] [PubMed] [Google Scholar]
- 11.Mizner RL, Stevens JE, Snyder-Mackler L. Voluntary activation and decreased force production of the quadriceps femoris muscle after total knee arthroplasty. Phys Ther. 2003;83:359–65. [PubMed] [Google Scholar]
- 12.Skelton DA, Kennedy J, Rutherford OM. Explosive power and asymmetry in leg muscle function in frequent fallers and non-fallers aged over 65. Age Ageing. 2002;31:119–25. doi: 10.1093/ageing/31.2.119. [DOI] [PubMed] [Google Scholar]
- 13.Portegijs E, Sipila S, Alen M, et al. Leg extension power asymmetry and mobility limitation in healthy older women. Arch Phys Med Rehabil. 2005;86:1838–42. doi: 10.1016/j.apmr.2005.03.012. [DOI] [PubMed] [Google Scholar]
- 14.Guralnik JM, Simonsick EM, Ferucci L, et al. A short physical performance battery assessing lower extremity function: association with self-reported disability and prediction of mortality in nursing home admission. J Gerontol. 1994;49:M85–94. doi: 10.1093/geronj/49.2.m85. [DOI] [PubMed] [Google Scholar]
- 15.Guralnik JM, Ferrucci L, Pieper CF, et al. Lower extremity function and subsequent disability: consistency across studies, predictive models, and value of gait speed alone compared with the short physical performance battery. J Gerontol A Biol Sci Med Sci. 2000;55:M221–31. doi: 10.1093/gerona/55.4.m221. [DOI] [PubMed] [Google Scholar]
- 16.Callahan D, Phillips E, Carabello R, Frontera WR, Fielding RA. Assessment of lower extremity power in functionally limited elders. Aging Clin Exp Res. 2007;19:194–9. doi: 10.1007/BF03324689. [DOI] [PubMed] [Google Scholar]
- 17.Fielding RA, LeBrasseur NK, Cuoco A, Bean J, Mizer K, Fiatarone Singh MA. High velocity resistance training increases skeletal muscle peak power in older women. J Am Geriatr Soc. 2002;50:655–62. doi: 10.1046/j.1532-5415.2002.50159.x. [DOI] [PubMed] [Google Scholar]
- 18.Reid KF, Callahan DM, Carabello RJ, Phillips EM, Frontera WR, Fielding RA. Lower extremity power training in elderly subjects with mobility limitations: a randomized controlled trial. Aging Clin Exp Res. 2008;20:337–43. doi: 10.1007/bf03324865. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Bean JF, Kiely DK, Herman S, et al. The relationship between leg power and physical performance in mobility-limited older people. J Am Geriatr Soc. 2002;50:461–7. doi: 10.1046/j.1532-5415.2002.50111.x. [DOI] [PubMed] [Google Scholar]
- 20.Shrout PE, Fleiss JL. Intraclass correlations: uses in assessing rater reliability. Psychol Bull. 1979;86:420–8. doi: 10.1037//0033-2909.86.2.420. [DOI] [PubMed] [Google Scholar]
- 21.Suzuki T, Bean JF, Fielding RA. Muscle power of the ankle flexors predicts functional performance in community-dwelling older women. J Am Geriatr Soc. 2001;49:1161–7. doi: 10.1046/j.1532-5415.2001.49232.x. [DOI] [PubMed] [Google Scholar]
- 22.Hartmann A, Knols R, Murer K, de Bruin ED. Reproducibility of an isokinetic strength testing protocol of the knee and ankle in older adults. Gerontology. 2008;55:259–68. doi: 10.1159/000172832. [DOI] [PubMed] [Google Scholar]
- 23.Holder-Powell HM, Rutherford OM. Unilateral lower limb injury: its long term effects on quadriceps, hamstrings, and plantar flexor muscle strength. Arch Phys Med Rehabil. 1999;80:717–20. doi: 10.1016/s0003-9993(99)90179-x. [DOI] [PubMed] [Google Scholar]
- 24.Madsen OR, Laudrisen UB, Sorenson OH. Quadriceps strength in women with a previous hip fracture: relationship to physical ability and bone mass. Scand J Rehabil Med. 2000;32:37–40. doi: 10.1080/003655000750045721. [DOI] [PubMed] [Google Scholar]
- 25.Perry MC, Carville SF, Smith IC, Rutherford OM, Newham DJ. Strength, power output and symmetry of leg muscles: effect of age and history of falling. Eur J Appl Physiol. 2007;100:553–61. doi: 10.1007/s00421-006-0247-0. [DOI] [PubMed] [Google Scholar]