Abstract
Background:
Lower extremity torque steadiness has been shown to be an independent predictor of functional performance in older women. Hip muscle function is crucial for many types of activities of daily living, yet existing studies investigating torque steadiness for lower extremities are limited to assessing steadiness at the knee and ankle.
Objective:
The purpose of this study was to compare age and gender differences in hip extension (HE) and flexion (HF) strength, torque steadiness, and torque accuracy (TA).
Methods:
Twenty young adults (10 men, 10 women; age 24.0 ± 2.2 years) and 21 older adults (11 men, 10 women; age 65.4 ± 4.5 years) matched across age for height and body mass participated. Dominant leg HE and HF isometric strength was assessed by maximal voluntary contractions (MVC); relative (5, 25 and 50% MVC) and absolute (25 Nm) torque steadiness were assessed as standard deviation and coefficient of variation of torque fluctuations, and TA was determined as the mean deviation from target torque levels.
Results:
MVC was lower for HF than HE (p = 0.007), but HE had greater torque fluctuations (p < 0.05). For HE, the coefficient of variation of 5% MVC was greater for older than young adults (p < 0.05) and greater for women than men (p < 0.05). For HF torque steadiness there were no age or gender differences (p > 0.05). For both HE and HF, older adults were less accurate (higher TA) than their young counterparts at 25 Nm (p < 0.022).
Conclusions:
Our results indicate older as compared to young adults, and women as compared to men are less steady (greater torque fluctuations) in HE at 5% MVC target torque levels, but not at higher torque levels. For HF, torque steadiness is similar across low to high target torque levels in both genders and across younger and older adults. For both HE and HF, TA is impaired in older compared to young adults at absolute target torque levels, but not at relative torque levels.
Key Words: Elderly, Force control, Maximal voluntary contraction, Strength
Introduction
Aging is commonly associated with sarcopenia (muscle atrophy), which is characterized as a decline in muscle strength and performance [1, 2]. Such reduction in muscular function may account for age-related decrements in postural stability and functional tasks like walking and stair climbing. Torque control or steadiness during submaximal muscle contraction is another parameter of muscle function that has been implicated as an important component for maintaining balance and functional performance [3]. Previous investigations have compared torque steadiness of young and older adults for the first dorsal interosseus [4, 5], elbow flexors [6, 7], knee extensors [8, 9], and ankle plantar and dorsiflexor muscles [10, 11]. Hip muscle function is crucial for many types of activities of daily living and may be important for postural stability and dynamic balance. Examination of torque steadiness in the hip musculature could provide valuable information about the capabilities of older compared to young adults. Despite its potential importance, to our knowledge, no studies have investigated torque steadiness at the hip joint.
Torque steadiness relates to the intensity of muscle contraction, and the degree of steadiness varies with different muscle groups [12]. In previous studies the amplitude of the fluctuations has been found to differ between young and older adults [4, 5, 6, 10]; however, this is not a consistent finding [8]. When there is a difference in the amplitude of the torque fluctuations between young and old adults, it is greatest at low forces, with age difference affecting torque fluctuations at target levels smaller than 10% of maximal voluntary contraction (MVC) for the first dorsal interosseus [5], knee extensors [6] and ankle plantar and dorsiflexors [10]. In a recent study, Tracy [10] reported that women have 25% lower MVC for ankle dorsiflerxors and 24% lower for plantarflexors than men. Tracy [10] also reports greater torque fluctuations (reduced steadiness) in ankle dorsiflexor than plantarflexor muscle contractions. Since torque capacity of hip extension (HE) is typically greater than hip flexion (HF) [13], torque steadiness between HE and HF may be different during torque control challenges. The majority of previous investigations combined genders in reporting data; we are interested in investigating gender differences between young and older men and women to better understand gender and age impact on torque steadiness during HE and HF.
Most previous studies have compared torque steadiness in terms of relative strength (percent of MVC). We are interested in investigating age- and gender-related differences in steadiness during both relative and absolute loads of isometric HE and HF contractions.
The purpose of this study is to compare torque steadiness in terms of relative (5, 25 and 50% MVC), and absolute strength (25 Nm) of the HE and HF in young and older men and women. Based on previous research, the first hypothesis is that older adults exhibit greater torque fluctuations (less steady) during constant voluntary isometric contractions while attempting to control submaximal absolute and relative torque of HE and HF muscles. Second, we predict lack of significant gender differences for contractions at the same relative strength, but expect females to be less steady at the absolute strength of 25 Nm. Thirdly, we hypothesize that HE muscles will exhibit less age and gender related torque fluctuations than HF.
Methods
Subjects
Twenty healthy young adults (10 men, 10 women; mean age 24.0 years, range 22–32) and 21 healthy older adults (11 men, 10 women; mean age 65.4 years, range 60–76) were recruited from the Birmingham, Ala., area. Subject characteristics are presented in table 1. Subjects with cardiorespiratory, metabolic, musculoskeletal or neurological disorders were excluded. All subjects were habitually active in exercise and/or sport activities (defined as participation of at least once per week for at least the year prior to study participation), but had not participated regularly (1 or more sessions per week) in a lower body strength training program within the last year.
Table 1.
Subject characteristics
| n | Age, years | Body mass, kg | Height, cm | BMI | Body fat, % | |
|---|---|---|---|---|---|---|
| Young | 20 | 24.0 ± 2.2 | 68.2 ± 11.5 | 171.2 ± 8.1 | 23.2 ± 2.1 | 16.7 ± 6.7b |
| Men | 10 | 24.7 ± 3.0 | 75.9 ± 9.0a | 176.5 ± 9.0a | 24.3 ± 1.5 | 11.5 ± 4.7a |
| Women | 10 | 23.3 ± 0.7 | 60.5 ± 8.1 | 165.9 ± 5.5 | 21.9 ± 2.0 | 21.9 ± 3.5 |
| Older | 21 | 65.4 ± 4.5 | 72.1 ± 8.2 | 168.9 ± 7.2 | 25.2 ± 1.9 | 25.6 ± 6.2 |
| Men | 11 | 67.3 ± 5.1 | 78.0 ± 5.4a | 175.0 ± 2.4a | 25.5 ± 2.0 | 20.9 ± 2.8a |
| Women | 10 | 63.7 ± 3.3 | 67.0 ± 6.8 | 164.0 ± 5.2 | 24.9 ± 1.9 | 30.4 ± 4.8 |
Values are group means ± SD.
Main gender effect, p < 0.05.
Main age effect, p < 0.05.
All subjects provided signed informed consent and all elderly subjects also provided signed physician's authorization forms prior to participation in this study. The institutional review board of the University of Alabama at Birmingham approved the procedures used in this study.
Experimental Design
Subjects reported to the Human Performance Laboratory, University of Alabama at Birmingham, for a familiarization and a testing session that were separated by 2–7 days. During the familiarization session subjects were introduced and practiced performing HF and HE MVC and steadiness testing for all target torque levels. For the testing session body composition was estimated from the sum of 3 skinfold measurements using gender- and age-specific equations [14, 15]. Prior to testing, subjects performed a warm-up on a cycle ergometer at 50 rpm at a self-selected work load between 25–50 W for 5 min. After warm-up subjects were tested for HF MVC and then HF torque steadiness. This was followed by testing for HE MVC and HE torque steadiness. Subjects refrained from caffeine and alcohol consumption for at least 12 h and exercise for 24 h prior to the testing.
Testing Set-Up
Subjects were tested to perform dominant side only HF and HE on a Biodex Multi-Joint System 3 Pro-Dynamometer (Biodex Medical Systems, Shirley, N.Y., USA) in the functional position of standing. Both hip and knee angles were at 0° (both extended) with the axis of rotation on the dynamometer aligned with the greater trochanter of the dominant leg and the non-dominant (non-test) leg supported over a 1-cm high non-slip support. The thigh cuff was attached with its lower border 2.5 cm above the superior border of the patella. Subjects stood erect with arms out laterally to each side and held on with both hands to secure supports to stabilize body position in an upright posture throughout the testing.
Isometric Maximal Voluntary Contractions and Torque Steadiness
All assessments were performed on the dominant leg, first assessing HF performance then HE. The dominant leg was determined by asking subjects which leg was preferred for performing common activities such as kicking a ball and stepping up onto a platform. For HF a series of warm-up isometric contractions were performed at 25, 50 and 75% of self-estimated maximal torque followed by 4 maximal effort HF contractions. Each trial lasted 5 s, with 60 s rest between trials to allow full recovery [16]. If the MVC of the fourth trial was 5% or greater than the highest MVC of any of the previous trials, another trial was performed. The highest MVC in any of the trials was used as the criterion score and was used to calculate relative target torque levels.
Torque steadiness was assessed by asking subjects to attempt to match a horizontal target line for 15 s that was displayed on a 48-cm computer monitor 1.5 m in front of the subject. Subjects performed voluntary isometric contractions to match constant torque target levels at 5, 25 and 50% MVC (torque relative to maximal strength) and at 25 Nm (absolute torque). The order of target levels was randomized as determined by a computer randomization program for each subject. Two trials separated with 35-second rest periods were performed at each target level, with the mean of the trials used for analysis. After completion of HF testing, a 3-minute rest was given before the same procedures were performed for HE.
Muscle Performance Data Acquisition System
Isometric steadiness analysis measures were assessed utilizing the Biodex Dynamometer interfaced with a National Instruments (Austin, Tex., USA) data acquisition system, which included custom software written in LabVIEW (Laboratory Virtual Instrument Engineering Workbench, v. 7.1) running on a Pentium IV personal computer with a 16-channel data acquisition card. A direct connection to the analog signal access port on the dynamometer was utilized to sample at a 1,000 Hz analog-to-digital conversion rate. As indicated by previous investigations, a high resolution of torque signals is required to accurately assess steadiness, especially at low load target levels where very small fluctuations in torque may occur [17]. To address this, our system also utilized System 3 Research Tool Kit software (Biodex Medical Systems) to greatly enhance resolution of torque measurements. Sensitivity of torque was adjusted for target settings to maximize the resolution of the system. The lowest torque targets (28 Nm and below) had a sensitivity setting of 0.1152 V/Nm.
Data Analysis
Analysis for torque steadiness for each trial of submaximal contractions was determined over the middle 10 s of the 15-second test and was automatically calculated by the custom LabVIEW program using SD of torque fluctuations (SD of the torque about the line of best fit through the data), coefficient of variation (CV) ([SD/mean torque] ×100) of the torque fluctuations about the actual mean torque, and torque accuracy (TA) (difference between target constant torque level and actual mean torque of the data). SD indicates the torque fluctuations in absolute terms in Nm; a higher value indicates lower torque steadiness. CV indicates the torque fluctuations in relative terms (fluctuations expressed as a percentage of the average torque generated); a higher value indicates lower torque steadiness. TA indicates how far off the torque generated was from the target torque value; a higher value indicates more error in attempting to match the target torque.
Statistical Analysis
To compare torque fluctuations between HE and HF a 3 by 2 (torque level × muscle) analysis of variance (ANOVA) was run with relative torque level serving as a repeated measure. A factorial analysis of variance (ANOVA) was computed for 25 Nm and MVC with independent variables of muscle and level. To compare torque fluctuations at different torque levels within 1 muscle, ANOVA was run with torque level serving as a repeated measure and between-subjects factors of age and gender. A factorial ANOVA was computed for 25 Nm and MVC with independent variables of age groups and gender. To test whether any differences in strength between age and gender groups exist after correcting for fat free mass (FFM) an analysis of co-variance (ANCOVA) was calculated for MVC with FFM serving as the co-variant. Follow-up Tukey post hoc tests were performed as appropriate. For all analyses the significance level was set at α = 0.05.
Results
Maximal Voluntary Contractions
Hip extensors exhibited 6% greater strength (higher MVC) than HF (significant main effect for muscle; table 2). During HE males (mean 8 SD for combined young and old = 123.3 ± 27.7 Nm) demonstrated 41% greater strength (MVC) than females (87.2 8 16.5 Nm, significant main effect for gender; table 3). Young adults (combined men and women = 119.42 ± 29.08 Nm) were 29% stronger than older adults (92.6 8 22.7 Nm, significant main effect for age; table 3). When MVC was adjusted for FFM, the young were still 22% stronger than older adults (significant main effect for age; table 3).
Table 2.
Hip extension and flexion comparisons of MVC, torque steadiness and torque accuracy at relative and absolute target torque levels
| Hip extension | Hip flexion | p | ||
|---|---|---|---|---|
| Strength | ||||
| MVC (Nm) | 105.7 ± 29.1 | 100.0 ± 33.0 | M: | 0.007 |
| Torque steadiness | ||||
| Standard deviation (Nm) | ||||
| 5% MVCa | 0.42 ± 0.40 | 0.20 ± 0.07 | L: | <0.001 |
| 25% MVCb | 0.76 ± 0.36 | 0.52 ± 0.29 | M: | 0.001 |
| 50% MVCb | 1.64 ± 0.93 | 1.39 ± 0.76 | L × M: | 0.847 |
| 25 Nm | 0.72 ± 0.50 | 0.52 ± 0.18 | M: | 0.001 |
| Coefficient of variation (%) | ||||
| 5% MVCa | 9.99 ± 15.12* | 4.34 ± 1.87 | L: | 0.001 |
| 25% MVCb | 3.00 ± 1.52 | 2.12 ± 0.95 | M: | 0.008 |
| 50% MVCb | 3.25 ± 1.82 | 2.80 ± 1.07 | L × M: | 0.033 |
| 25 Nm | 2.90 ± 2.09 | 2.08 ± 0.73 | M: | 0.012 |
| Torque accuracy | ||||
| Mean deviation from target level (Nm) | ||||
| 5% MVCa | 0.20 ± 0.22 | 0.12 ± 0.07 | L: | <0.001 |
| 25% MVCa | 0.31 ± 0.21 | 0.22 ± 0.17 | M: | 0.676 |
| 50% MVCb | 1.00 ± 0.74 | 1.08 ± 1.20 | L × M: | 0.389 |
| 25 Nm | 0.38 ± 0.30 | 0.32 ± 0.31 | M: | 0.190 |
Values are group means ± SD. M = Muscle; L = level.
p < 0.05, vs. all other target level means for hip extension and hip flexion. Target torque level means with different superscript letters are significantly different (p < 0.05).
Table 3.
Hip extension age and gender differences of MVC, torque steadiness and torque accuracy at relative and absolute target torque levels
| Young |
Old |
P | ||||
|---|---|---|---|---|---|---|
| men | women | men | women | |||
| Strength | ||||||
| MVC (Nm) | 141.3 ± 21.6 | 97.6 ± 16.0 | 106.9 ± 22.2 | 76.9 ± 8.7 | A: | <0.001 |
| G: | <0.001 | |||||
| A × G: | 0.234 | |||||
| MVC adjusted for | 119.4 ± 11.26 | 112.3 ± 8.9 | 97.9 ± 7.3 | 92.6 ± 6.2 | A: | 0.001 |
| FFM (Nm) | G: | 0.575 | ||||
| A × G: | 0.234 | |||||
| Torque steadiness | ||||||
| Standard deviation (Nm) | ||||||
| 5% MVC a | 0.24 ± 0.12 | 0.33 ± 0.14 | 0.40 ± 0.38 | 0.73 ± 0.58 | A: | 0.157 |
| 25% MVC b | 0.76 ± 0.28 | 1.03 ± 0.56 | 0.73 ± 0.13 | 0.56 ± 0.26 | G: | 0.661 |
| 50% MVC c | 1.80 ± 0.88 | 2.14 ± 1.35 | 1.60 ± 0.66 | 1.1 ± 0.37 | L: | <0.001 |
| A × G: | 0.216 | |||||
| A × L: | 0.002 | |||||
| G × L: | 0.374 | |||||
| AX G × L: | 0.064 | |||||
| 25 Nm | 0.44 ± 0.14 | 0.94 ± 0.32 | 0.82 ± 0.81 | 0.68 ± 0.36 | A: | 0.676 |
| G: | 0.243 | |||||
| A × G: | 0.041 | |||||
| Coefficient of variation (%) | ||||||
| 5% MVCa,*, † | 3.44 ± 1.75 | 6.88 ± 3.35 | 7.44 ± 7.66 | 22.32 ± 25.56 | A: | 0.065 |
| 25% MVCb | 2.16 ± 0.70 | 4.13 ± 2.22 | 2.84 ± 0.88 | 2.93 ± 1.44 | G: | 0.030 |
| 50% MVCb | 2.57 ± 1.10 | 4.25 ± 2.26 | 3.28 ± 2.19 | 2.92 ± 1.19 | L: | 0.002 |
| A × G: | 0.438 | |||||
| A × L: | 0.019 | |||||
| G × L: | 0.049 | |||||
| AX G × L: | 0.110 | |||||
| 25 Nm | 1.76 ± 0.55 | 3.76 ± 1.27 | 3.33 ± 3.40 | 2.72 ± 1.43 | A: | 0.678 |
| G: | 0.280 | |||||
| A × G: | 0.045 | |||||
| Torque accuracy | ||||||
| Mean deviation from target level (Nm) | ||||||
| 5% MVCa | 0.13 ± 0.08 | 0.09 ± 0.04 | 0.26 ± 0.31 | 0.34 ± 0.27 | A: | 0.538 |
| 25% MVCa | 0.23 ± 0.22 | 0.18 ± 0.15 | 0.47 ± 0.29 | 0.42 ± 0.18 | G: | 0.906 |
| 50% MVCb | 0.99 ± 0.5 | 1.30 ± 1.00 | 1.09 ± 0.89 | 0.74 ± 0.37 | L: | <0.001 |
| A × G: | 0.382 | |||||
| A × L: | 0.043 | |||||
| G × L: | 0.850 | |||||
| AX G × L: | 0.101 | |||||
| 25 Nm | 0.20 ± 0.90 | 0.35 ± 0.19 | 0.40 ± 0.40 | 0.57 ± 0.32 | A: | 0.022 |
| G: | 0.076 | |||||
| A × G: | 0.932 | |||||
Values are group means ± SD. A = Age; G = gender; L = level.
p < 0.05, older vs. young.
p < 0.05, women vs. men. Target torque level means with different superscript letters are significantly different (p < 0.05). For standard deviation 25 Nm follow-up post hoc tests on the A×G interaction were not significant (p > 0.05). For coefficient of variation 25 Nm follow-up post hoc tests on the A×G interaction were not significant (p > 0.05). For torque accuracy across relative MVC torque levels follow-up post hoc tests on the A×L interaction were not significant (p > 0.05).
Similar MVC results were found for the HF. During HF males (118.9 8 30.9 Nm) were 49% stronger than females (80.1 8 21.4 Nm, significant effect for gender; table 4); and young adults (116.24 ± 34.10) were 38% stronger that older adults (84.5 ± 23.3 Nm) (significant effect for age; table 4). When MVC was adjusted for FFM, young were still 28% stronger than older adults (significant main effect for age; table 4).
Table 4.
Hip flexion age and gender differences of MVC, torque steadiness and torque accuracy at relative and absolute target torque levels
| Young | Old | P | ||||
|---|---|---|---|---|---|---|
| men | women | men | women | |||
| Strength | ||||||
| MVC (Nm) | 142.2 ± 22.9 | 90.4 ± 21.0 | 97.8 ± 20.3 | 69.8 ± 17.0 | A: | <0.001 |
| G: | <0.001 | |||||
| A × G: | 0.070 | |||||
| MVC adjusted for FFM (Nm) | 117.8 ± 12.5 | 106.8 ± 9.9 | 87.5 ± 8.1 | 87.3 ± 8.1 | A: | <0.001 |
| G: | 0.655 | |||||
| A × G: | 0.391 | |||||
| Torque steadiness | ||||||
| Standard deviation (Nm) | ||||||
| 5% MVCa | 0.21 ± 0.09 | 0.21 ± 0.87 | 0.19 ± 0.44 | 0.19 ± 0.62 | A: | 0.009 |
| 25% MVCb | 0.73 ± 0.46 | 0.51 ± 0.23 | 0.45 ± 0.13 | 0.36 ± 0.11 | G: | 0.064 |
| 50% MVCc,‡ | 1.85 ± 1.04 | 1.48 ± 0.75 | 1.30 ± 0.43 | 0.85 ± 0.32 | L: | <0.001 |
| A × G: | 0.924 | |||||
| A × L: | 0.012 | |||||
| G × L: | 0.080 | |||||
| A × G × L: | 0.683 | |||||
| 25 Nm | 0.47 ± 0.20 | 0.58 ± 0.17 | 0.49 ± 0.18 | 0.53 ± 0.18 | A: | 0.817 |
| G: | 0.212 | |||||
| A × G: | 0.592 | |||||
| Coefficient of variation (%) | ||||||
| 5% MVCa | 3.04 ± 1.30 | 4.56 ± 2.09 | 4.18 ± 1.36 | 5.50 ± 1.86 | A: | 0.561 |
| 25% MVCb | 2.10 ± 1.37 | 2.16 ± 0.78 | 1.94 ± 0.63 | 2.18 ± 0.98 | G: | 0.080 |
| 50% MVCc | 2.68 ± 1.44 | 3.29 ± 1.25 | 2.74 ± 0.63 | 2.43 ± 0.65 | L: | <0.001 |
| A × G: | 0.626 | |||||
| A × L: | 0.008 | |||||
| G × L: | 0.010 | |||||
| A × G × L: | 0.406 | |||||
| 25 Nm | 1.90 ± 0.77 | 2.30 ± 0.68 | 2.00 ± 0.72 | 2.16 ± 0.80 | A: | 0.897 |
| G: | 0.207 | |||||
| A × G: | 0.639 | |||||
| Torque accuracy | ||||||
| Mean deviation from target level (Nm) | ||||||
| 5% MVCa | 0.11 ± 0.09 | 0.13 ± 0.05 | 0.09 ± 0.07 | 0.13 ± 0.07 | A: | 0.261 |
| 25% MVCa | 0.22 ± 0.16 | 0.16 ± 0.17 | 0.19 ± 0.16 | 0.30 ± 0.20 | G: | 0.542 |
| 50% MVCb | 1.72 ± 1.70 | 0.94 ± 1.12 | 0.73 ± 0.67 | 0.91 ± 0.92 | L: | <0.001 |
| A × G: | 0.163 | |||||
| A × L: | 0.141 | |||||
| G × L: | 0.369 | |||||
| A × G × L: | 0.228 | |||||
| 25 Nm | 0.17 ± 0.12 | 0.22 ± 0.15 | 0.36 ± 0.26 | 0.52 ± 0.47 | A: | 0.010 |
| G: | 0.230 | |||||
| A × G: | 0.558 | |||||
Values are group means ± SD. A = Age; G = gender; L = level.
p < 0.05, young vs. older. Target torque level means with different superscript letters are significantly different (p < 0.05). For coefficient of variation across relative MVC torque levels follow-up post hoc tests on the A × L and G × L interactions were not significant (p > 0.05).
Torque Steadiness
Standard Deviation
Across relative target torque levels, HE had higher SD of torque than HF (significant main effect for muscle; table 2), although there was no significant interaction effect of level and muscle (L × M), indicating within each relative target torque level there were no SD differences between HE and HF. At the 25 Nm absolute target torque level, the SD for HE was 38% higher than HF (significant main effect for muscle; table 2), indicating subjects had greater torque fluctuations (less steadiness) in HE than HF.
SD of torque during HE increased with an increase in relative target torque by 81% from 5 to 25% MVC, and by 116% from 25 to 50% MVC (significant main effect of level and post hoc tests; table 3) indicating reduced HE absolute steadiness (greater torque fluctuations) as relative target torque levels increase.
Similar to HE, the SD in torque during HF increased with increases in relative target torque levels (significant main effect of level and post-hoc tests; table 4) indicating reduced HF absolute steadiness (greater torque fluctuations) as relative target torque levels increase. For HF, young adults had 53% higher SD of torque than older adults at 50% MVC (significant age and level interaction and significant [p = 0.016] follow-up post hoc test; table 4).
Coefficient of Variation
Hip extension had higher CV of torque than HF across relative target torque levels (significant main effect for muscle; table 2) and the absolute target torque level of 25 Nm (significant main effect for muscle) indicating less steadiness in HE than HF for both relative and absolute target torque levels.
During HE, CV of relative torque was 231% greater for 5% MVC compared to 25% MVC (p < 0.001) and 206% greater than 50% MVC (p < 0.001), but there was no significant difference in CV of relative torque between 25% MVC and 50% MVC (p = 0.988; table 3). These results indicate much greater torque fluctuations (less steadiness) at 5% MVC than at higher relative target torque levels. For HE relative target torque levels there was a significant interaction between age and level and between gender and level (table 3). Older adults demonstrated 182% greater CV of torque (less steadiness) during HE at 5% MVC (p = 0.023) than young adults, but not at 25% MVC (p = 1.000) or 50% MVC (p = 1.000; table 3). These results indicate that when steadiness is expressed in relative terms as CV (fluctuations expressed as a percentage of the average torque generated), older adults are less steady than young adults only at low target values. CV of torque for females was 168% greater than males at 5% MVC (p = 0.022), but not at 25% MVC (p = 0.999) or 50% MVC (p = 1.000) during HE (table 3), indicating females have less relative steadiness than males only at low target values.
During HF, CV of relative torque was 105% greater at 5% MVC compared to 25% MVC (p = 0.001) and 55% greater at 5% MVC than 50% MVC (p > 0.001), and CV of relative torque at 50% MVC was 33% greater than 25% MVC (p > 0.001; table 4). These results indicate that for HF, the greatest torque fluctuations (least steadiness) occur at 5% MVC, the second greatest torque fluctuations occur at 50% MVC, and smallest torque fluctuations occur at 25% MVC.
Torque Accuracy
There was no significant difference in TA in HE as compared to HF (table 2). During HE, TA values were higher (further from target levels or more error) with increases in relative target torque levels (significant main effect of level; table 3). Although all group mean values for TA were relatively small (1.3 Nm or less), TA at 50% MVC was 400% greater than TA at 5% MVC (p > 0.001), and TA at 50% MVC was 214% greater than TA at 25% MVC (p > 0.001). However, there was no difference between TA at 5% and 25% MVC (p = 0.369). Torque accuracy values for older adults were 76% higher during HE at 25 Nm than young adults (significant main effect of age; table 3) indicating an impairment in TA for older adults at this absolute torque level.
Similar to HE, TA values during HF were also higher with increases in relative target torque levels (significant main effect of level; table 4). Torque accuracy at 50% MVC was 832% greater than TA at 5% MVC (p < 0.001) and 392% greater than TA at 25% MVC (p < 0.001). Torque accuracy at 5 and 25% MVC were not different (p = 0.766). Also similar to HE, TA values for older adults were 126% higher during HF at 25 Nm than young adults (significant main effect of age; table 4).
Discussion
The main findings of this study were that when compared to young adults and males, older adults and females have impaired normalized relative torque steadiness (CV) at lower target torques for HE, but not HF. Amplitudes of HE torque fluctuations were consistently greater (reduced steadiness) as compared to HF for both relative and absolute torque levels.
Torque Steadiness Differences between Muscles
Based on previous research that shows increases in muscle strength with resistance exercise training lowers torque fluctuations [18], we predicted the stronger HE muscles would exhibit less torque fluctuations than hip flexors. Our prediction was contrary to our actual findings. Thus, age- and gender-related reduced torque steadiness does not always affiliate with lower muscle strength. The amplitude of HE torque fluctuations was consistently greater as compared to HF for both relative and absolute torque expressed as SD, CV and TA. Our findings are consistent with those from a recent study by Tracy [10], who reported greater torque fluctuations in stronger plantarflexor muscles as compared to the weaker dorsiflexor muscles. Thus, it appears that torque steadiness may be affected depending upon the specific muscle being tested. A consideration for the different results in the different lower extremity muscle groups is that certain HE muscles (hamstrings and adductor magnus) have substantially greater moment arms when the hip is more flexed [13]. We tested HE with the hip positioned at 0°, which places the hip extensor muscles in a shortened position. Previous research demonstrates that as the hip muscles go from a lengthened to shortened position, muscle strength decreases [13]. Thus, disadvantage in functional positioning may account for greater torque fluctuations for HE as compared to HF.
Torque Steadiness
To our knowledge this is the first study that assessed torque control or steadiness at the hip. For HE at very low target torque levels (5% MVC) older adults had greater torque fluctuations than young adults. Other studies have assessed lower extremity torque steadiness at target levels ranging from 2.5 to 80% MVC [4,6,9,11,12]. Similar to our findings, Tracy and Enoka [6] reported greater torque fluctuations for older adults compared to younger adults in knee extension at low torques levels of 10% MVC and below, but not at higher torque levels. In another study, Tracy [10] reported similar results to ours for ankle plantarflexion, with less steadiness in older adults than young adults at target torque levels of 5% MVC and lower; with no difference between age groups at target levels up to 80% MVC. Our findings of no significant differences between young and older adults for HF torque steadiness are consistent with findings reported for dorsiflexion steadiness by Tracy [10]. Thus, it appears that age differences in torque steadiness may or may not be present depending upon the specific muscle being tested. A consideration for the different results in the different lower extremity muscle groups is that both HE and plantarflexion involve larger and more powerful muscles with larger motor units than their antagonist muscle groups for HF and dorsiflexion, but further research is needed to distinguish mechanisms underlying these differences between muscles.
Our results also indicated women are less steady than men in HE at low load target levels, but no gender differences were observed for HF. Hip extension findings are in contrast with previous research reporting higher knee extension fluctuations (higher CVs) at low forces in men as compared with women [6].
Older adults had increased torque accuracy values (more error or further from target torque levels) compared to young adults at the 25 Nm absolute torque level. Our findings are consistent with Hortobagyi [19], who reported impaired knee extensor TA relative to a 25 Nm target torque level in older adults and found no significant age differences in torque steadiness SD. Differences in magnitude of exerted torque (percent of MVC) between young and older adults while attempting to match absolute target torque level could account for age-related differences in TA.
Our findings also show that age and gender differences in torque steadiness do not affiliate with muscle strength in HF, as compared to HE. To determine if age and gender differences in HE and HF MVC occur irrespectively of body composition, MVC was adjusted for FFM as an estimate of muscle mass. After adjustment for FFM no gender differences were observed. Thus, it appears that males had higher HE and HF MVC due to greater FFM. Older adults, however, still demonstrated lower strength as compared to young adults even after adjusting MVC for FFM, potentially indicating an age-related decrease in muscle quality and/or neuromuscular function. Our findings are consistent with previous research indicating age-associated reduction in muscle strength [20].
Based on our results and those of other investigations, the amount of torque fluctuation depends upon age, gender, muscle group and intensity of contraction (torque level). It thus appears multiple neuromuscular mechanisms control torque steadiness, although these remain to be determined [10].
To our knowledge, the current study was the first to measure and report on hip musculature torque steadiness and accuracy. An advantage of the current study was that a number of hip muscle function parameters were assessed including strength, torque steadiness and accuracy across low to high level relative (percent of MVC) target torques and at an absolute target torque level of 25 Nm. The study compared these measures across both young and older adults and across genders. Limitations of the current study include results applying only to healthy independent-living adults and not to the many functionally limited older adults who display difficulty in performing everyday functional tasks. Further, how torque steadiness and accuracy relate to specific functional task performance awaits further research. Although research relating torque steadiness and accuracy to function is very limited, Seynnes et al. [3] report knee extension isometric torque steadiness is significantly related to chair-rise time and stair-climbing power in older women with mild functional impairment, and torque steadiness accounts for 63% and 34% of the variance in these variables, respectively. Thus, since hip muscle function is crucial for many types of activities of daily living, it appears likely torque steadiness in the hip musculature may also be related to performance of functional tasks. Further research should be undertaken to assess these relationships and also whether improvements in hip steadiness may help reduce functional limitations or even risk of falls in older adults.
Another consideration for the current study is that both hip extension and flexion function were assessed in standing position. This places the hip extensors (gluteus maximus and hamstrings) in a position where their attachments are fairly close together and outside of the optimal muscle length for generating tension based on the length-tension relationship [13]. The antagonist hip flexor muscles are in a more lengthened position relative to the length of the extensor muscles. It is unknown if positioning the hip in more flexion for the hip extension assessments (thus lengthening the hip extensor muscles) would change results. We are not aware of any studies that have compared torque steadiness or accuracy at different muscle lengths, but it is possible that steadiness and/or accuracy may vary with different muscle lengths, and this factor should be considered for future investigations.
Conclusions
In summary, our findings suggest older as compared to young adults, and women as compared to men, are less steady in hip extension at 5% MVC target torque levels, but not at higher torque levels. For hip flexion, torque steadiness is similar across low to high target torque levels in both genders and across young and older adults. For both hip extension and flexion, torque accuracy is impaired in older compared to young adults at absolute target torque levels, but not at relative torque levels. Impaired hip torque steadiness and accuracy in older adults have implications for potentially explaining and addressing functional impairments and risk for falls in older adults.
Acknowledgment
Grant support for this project was provided by the University of Alabama at Birmingham Center of Aging.
References
- 1.Buchner DM, de Lateur BJ. The importance of skeletal muscle strength to physical function in older adults. Ann Behav Med. 1999;13:91–98. [Google Scholar]
- 2.Faulkner JA, Larkin LM, Claflin DR, Brooks SV. Age-related changes in the structure and function of skeletal muscles. Clin Exp Pharmaco Physio. 2007;34:1091–1096. doi: 10.1111/j.1440-1681.2007.04752.x. [DOI] [PubMed] [Google Scholar]
- 3.Seynnes O, Hue OA, Garrandes F, Colson SS, Bernard PL, Legros P, Fiatarone Singh MA. Force steadiness in the lower extremities as an independent predictor of functional performance in older women. J Aging Phys Act. 2005;13:395–408. doi: 10.1123/japa.13.4.395. [DOI] [PubMed] [Google Scholar]
- 4.Burnett RA, Laidlaw DH, Enoka RM. Coactivation of the antagonist muscle does not covary with steadiness in old adults. J Appl Physiol. 2000;89:61–71. doi: 10.1152/jappl.2000.89.1.61. [DOI] [PubMed] [Google Scholar]
- 5.Laidlaw DH, Bilodeau M, Enoka RM. Steadiness is reduced and motor unit discharge is more variable in old adults. Muscle Nerve. 2000;23:600–612. doi: 10.1002/(sici)1097-4598(200004)23:4<600::aid-mus20>3.0.co;2-d. [DOI] [PubMed] [Google Scholar]
- 6.Tracy BL, Enoka RM. Older adults are less steady during submaximal isometric contractions with the knee extensor muscles. J Appl Physiol. 2002;92:1004–1012. doi: 10.1152/japplphysiol.00954.2001. [DOI] [PubMed] [Google Scholar]
- 7.Lavender AP, Nosaka K. Changes in fluctuation of isometric force following eccentric and concentric exercise of the elbow flexors. Eur J Appl Physiol. 2006;96:235–240. doi: 10.1007/s00421-005-0069-5. [DOI] [PubMed] [Google Scholar]
- 8.Carville SF, Perry MC, Rutherford OM, Smith IC, Newham DJ. Steadiness of quadriceps contractions in young and older adults with and without a history of falling. Eur J Appl Physiol. 2007;100:527–533. doi: 10.1007/s00421-006-0245-2. [DOI] [PubMed] [Google Scholar]
- 9.Manini TM, Clark BC, Tracy BL, Burke J, Ploutz-Snyder L. Resistance and functional training reduces knee extensor position fluctuations in functionally limited older adults. Eur J Appl Physiol. 2005;95:436–446. doi: 10.1007/s00421-005-0048-x. [DOI] [PubMed] [Google Scholar]
- 10.Tracy BL. Force control is impaired in the ankle plantarflexors of elderly adults. Eur J Appl Physiol. 2007;101:629–636. doi: 10.1007/s00421-007-0538-0. [DOI] [PubMed] [Google Scholar]
- 11.Patten C, Kamen G. Adaptations in motor unit discharge activity with force control training in young and older human adults. Eur J Appl Physiol. 2000;83:128–143. doi: 10.1007/s004210000271. [DOI] [PubMed] [Google Scholar]
- 12.Enoka RM, Christou EA, Hunter SK, Kornatz KW, Semmler JG, Taylor AM, Tracy BL. Mechanisms that contribute to differences in motor performance between young and old adults. J Electromyogr Kinesiology. 2003;13:1–12. doi: 10.1016/s1050-6411(02)00084-6. [DOI] [PubMed] [Google Scholar]
- 13.Oatis CA. Kinesiology: the Mechanics and Pathomechanics of Human Movement. Philadelphia, Lippincott Williams & Wilkins. 2004 [Google Scholar]
- 14.Jackson AS, Pollock M, Ward A. Generalized equations for predicting body density for men. J Nutr. 1978;40:497–504. doi: 10.1079/bjn19780152. [DOI] [PubMed] [Google Scholar]
- 15.Jackson AS, Pollock M, Ward A. Generalized equations for predicting body density for women. Med Sci Sports Exerc. 1980;12:175–182. [PubMed] [Google Scholar]
- 16.Sale DG, Testing strength and power . Physiological Testing of the High-Performance Athlete. In: McDougall JD, Wenger HA, Green HJ, editors. Champaign, Human Kinetics. 1991. pp. pp 21–106. [Google Scholar]
- 17.Galganski ME, Fuglevand AJ, Enoka RM. Reduced control of motor output in a human hand muscle of elderly subjects during submaximal contractions. J Neurophysiol. 1993;69:2108–2115. doi: 10.1152/jn.1993.69.6.2108. [DOI] [PubMed] [Google Scholar]
- 18.Hamilton AF, Jones KE, Wolpert DM. The scaling of motor noise with muscle strength and motor unit number in humans. Exp Brain Res. 2004;157:416–429. doi: 10.1007/s00221-004-1856-7. [DOI] [PubMed] [Google Scholar]
- 19.Hortobagyi T, Tunnel D, Moody J, Beam S, DeVita P. Low- or high-intensity strength training partially restores impaired quadriceps force accuracy and steadiness in aged adults. J Gerontol Biol Sci. 2001;56A:B38–B47. doi: 10.1093/gerona/56.1.b38. [DOI] [PubMed] [Google Scholar]
- 20.Hunter GR, McCarthy JP, Bamman MM. Effects of resistance training on older adults. Sports Med. 2004;34:329–348. doi: 10.2165/00007256-200434050-00005. [DOI] [PubMed] [Google Scholar]