Assessment of Lower Limb Muscle Strength and Power Using Hand-Held and Fixed Dynamometry: A Reliability and Validity Study

Introduction

Hand-held dynamometry (HHD) has never previously been used to examine isometric muscle power. Rate of force development (RFD) is often used for muscle power assessment, however no consensus currently exists on the most appropriate method of calculation. The aim of this study was to examine the reliability of different algorithms for RFD calculation and to examine the intra-rater, inter-rater, and inter-device reliability of HHD as well as the concurrent validity of HHD for the assessment of isometric lower limb muscle strength and power.

Methods

30 healthy young adults (age: 23±5yrs, male: 15) were assessed on two sessions. Isometric muscle strength and power were measured using peak force and RFD respectively using two HHDs (Lafayette Model-01165 and Hoggan microFET2) and a criterion-reference KinCom dynamometer. Statistical analysis of reliability and validity comprised intraclass correlation coefficients (ICC), Pearson correlations, concordance correlations, standard error of measurement, and minimal detectable change.

Results

Comparison of RFD methods revealed that a peak 200ms moving window algorithm provided optimal reliability results. Intra-rater, inter-rater, and inter-device reliability analysis of peak force and RFD revealed mostly good to excellent reliability (coefficients ≥ 0.70) for all muscle groups. Concurrent validity analysis showed moderate to excellent relationships between HHD and fixed dynamometry for the hip and knee (ICCs ≥ 0.70) for both peak force and RFD, with mostly poor to good results shown for the ankle muscles (ICCs = 0.31–0.79).

Conclusions

Hand-held dynamometry has good to excellent reliability and validity for most measures of isometric lower limb strength and power in a healthy population, particularly for proximal muscle groups. To aid implementation we have created freely available software to extract these variables from data stored on the Lafayette device. Future research should examine the reliability and validity of these variables in clinical populations.     

Reliability and validity of three strength measures obtained from community-dwelling elderly persons

The purpose of this study was to describe the reliability and validity of 3 strength measures obtained from community-dwelling elderly individuals. The strength of 10 elders was tested initially and 6 and 12 weeks later using the MicroFET 2 hand-held dynamometer (knee extension strength), the Jamar dynamometer (grip strength), and the sit-to-stand (STS) test. Mobility was tested using the timed up-and-go (TUG) test and a timed walk test. Intraclass correlation coefficients, which were used to characterize the reliability of the strength tests, ranged from 0.807 to 0.981. Pearson correlations between the lower extremity strength measures and the TUG and gait speed ranged from 0.635 to -0.943. Our examination of the 3 measures for 12 weeks extends previous evidence of the stability of these strength measures and justifies the use of hand-held dynamometry and the STS test when investigating limitations in mobility.     

Relationship between measures of balance and strength in middle-aged adults

The purpose of this study was to investigate the relationship between variables of static and dynamic postural control as well as between isometric and dynamic muscle strength. A single-group design was used. Thirty-two middle-aged healthy adults (mean age: 56 ± 4 years) performed measurements of static (unperturbed)/dynamic (perturbed) balance and of isometric (i.e., maximal isometric torque [MIT]; rate of torque development [RTD] of the plantar flexor)/dynamic (i.e., countermovement jump [CMJ] height and power) lower extremity muscle strength. No significant associations were observed between variables of static and dynamic postural control (r = +0.128-0.341, p > 0.05) and between measures of balance and strength (r = -0.189 to +0.316, p > 0.05). Significant positive correlations were detected between variables of isometric and dynamic strength ranging from r = +0.361 to +0.501 (p < 0.05). Further, simple regression analyses revealed that a 10% increase in the mean CMJ height (3.1 cm) was associated with 44.4 N·m and 118.4 N·m·s better MIT and RTD, respectively. The nonsignificant correlations between static and dynamic balance measures and between balance and strength variables imply that static and dynamic postural control and balance and strength are independent of each other and may have to be tested and trained complementarily.     

Polymorphisms in the CNTF and CNTF receptor genes are associated with muscle strength in men and women.

Genotypic associations between polymorphisms in the ciliary neurotrophic factor (CNTF) and CNTF receptor (CNTFR) genes and muscular strength phenotypes in 154 middle-aged men (45-49 yr) and 138 women (38-44 yr) and 99 older men (60-78 yr) and 102 older women (60-80 yr) were tested to validate earlier association studies. Allelic interaction effects were hypothesized between alleles of CNTF and CNTFR. We performed analysis of covariance with age, height, and fat-free mass (FFM) as covariates. FFM was anthropometrically estimated by the equation of Durnin-Womersley. Isometric, concentric, and eccentric torques for the knee flexors (KF) and extensors (KE) were measured using Biodex dynamometry. In the older male group, T-allele carriers of the C-1703T polymorphism in CNTFR performed significantly better on all noncorrected KF torques, whereas only noncorrected KE isometric torque at 120 degrees and concentric torque at 240 degrees/s were higher than the C/C homozygotes (P < 0.05). When age, height, and FFM were used as covariates, T-allele carriers performed only better on KE and KF isometric torque at 120 degrees (P < 0.05). Concentric KF torque at 180 degrees/s was lower in middle-aged female A-allele carriers compared with the T/T subjects for the T1069A polymorphism in CNTFR. After correction for age, height, and FFM, middle-aged female A-allele carriers exhibited lower values on all concentric KF strength measures and isometric torque at 120 degrees . There was a lack of association with the CNTF G-6A polymorphism in men, with inconclusive results for a limited number of phenotypes in women. No significant CNTF/CNTFR allele interaction effects were found. Results indicate that CNTFR C-1703T and T1069A polymorphisms are significantly associated with muscle strength in humans.     

Is there an association between variables of postural control and strength in prepubertal children?

The risk of sustaining falls and sports-related injuries is particularly high in children. Deficits in balance and muscle strength represent 2 important intrinsic fall and injury-risk factors. Therefore, the purpose of this study was to investigate the relationship between variables of static and dynamic postural control and isometric and dynamic muscle strength and to find out whether there is an association between measures of postural control and muscle strength in prepubertal children. Thirty children participated in this study (age 6.7 ± 0.5 years; body mass index 16.0 ± 1.8 kg·m(-2)). Biomechanic tests included the measurements of maximal isometric torque and rate of force development (RFD) of the plantar flexors on an isokinetic device, jumping power and height (countermovement jump [CMJ]) on a force plate, and the assessment of static and dynamic posture during bipedal stance on a balance platform. The significance level was set at p < 0.05. No significant associations were observed between variables of static and dynamic postural control. Significant positive correlations were detected between the RFD of the plantar flexors and CMJ height (r = 0.425, p < 0.01). No statistically significant associations were found between measures of postural control and muscle strength. The nonsignificant correlations between static and dynamic postural control and muscle strength imply that primarily dynamic measures of postural control should be incorporated in fall and injury-risk assessment and that postural control and muscle strength appear to be independent of each other and may have to be trained in a complementary manner for fall and injury-preventive purposes.     

Relationship between neuromuscular fatigue and spasticity in chronic stroke patients: A pilot study

IntroductionThe aim of this study was to assess the effects of neuromuscular fatigue on stretch reflex-related torque and electromyographic activity of spastic knee extensor muscles in hemiplegic patients. The second aim was to characterize the time course of quadriceps muscle fatigue during repetitive concentric contractions.MethodsEighteen patients performed passive, isometric and concentric isokinetic evaluations before and after a fatigue protocol using an isokinetic dynamometer. Voluntary strength and spasticity were evaluated following the simultaneous recording of torque and electromyographic activity of rectus femoris (RF), vastus lateralis (VL) and biceps femoris (BF).ResultsIsometric knee extension torque and the root mean square (RMS) value of VL decreased in the fatigued state. During the fatigue protocol, the normalized peak torque decreased whereas the RMS of RF and BF increased between the first five and last five contractions. There was a linear decrease in the neuromuscular efficiency-repetitions relationships for RF and VL. The peak resistive torque and the normalized RMS of RF and VL during passive stretching movements were not modified by the fatigue protocol for any stretch velocity.DiscussionThis study showed that localized quadriceps muscle fatigue caused a decrease in voluntary strength which did not modify spasticity intensity. Changes in the distribution of muscle fiber type, with a greater number of slow fibers on the paretic side, may explain why the stretch reflex was not affected by fatigue.     

Evaluation of a method to scale muscle strength for gait simulations of children with cerebral palsy

Cerebral palsy (CP) is a neurological disorder that results in life-long mobility impairments. Musculoskeletal models used to investigate mobility deficits for children with CP often lack subject-specific characteristics such as altered muscle strength, despite a high prevalence of muscle weakness in this population. We hypothesized that incorporating subject-specific strength scaling within musculoskeletal models of children with CP would improve accuracy of muscle excitation predictions in walking simulations. Ten children (13.5 ± 3.3 years; GMFCS level II) with spastic CP participated in a gait analysis session where lower-limb kinematics, ground reaction forces, and bilateral electromyography (EMG) of five lower-limb muscles were collected. Isometric strength was measured for each child using handheld dynamometry. Three musculoskeletal models were generated for each child including a ‘Default’ model with the generic musculoskeletal model’s muscle strength, a ‘Uniform’ model with muscle strength scaled allometrically, and a ‘Custom’ model with muscle strength scaled based on handheld dynamometry strength measures. Muscle-driven gait simulations were generated using each model for each child. Simulation accuracy was evaluated by comparing predicted muscle excitations and measured EMG signals, both in the duration of muscle activity and the root-mean-square difference (RMSD) between signals. Improved agreement with EMG were found in both the ‘Custom’ and ‘Uniform’ models compared to the ‘Default’ model indicated by improvement in RMSD summed across all muscles, as well as RMSD and duration of activity for individual muscles. Incorporating strength scaling into musculoskeletal models can improve the accuracy of walking simulations for children with CP.     

The reliability of electromechanical delay and torque during isometric and concentric isokinetic contractions

The purpose of this investigation was to examine the intra-subject reliability of electromechanical delay (EMD) and torque of the dominant and non-dominant elbow flexors during isometric and isokinetic muscle contractions repeated over five consecutive days. Eleven volunteers that were unfamiliar with isokinetic dynamometry participated in this study and were asked to attend the laboratory on five consecutive days. An isokinetic dynamometer was used to exercise the elbow flexors under isometric, slow (60° s⁻¹) and fast (210° s⁻¹) isokinetic conditions; surface electromyography was recorded from the belly of biceps brachii and the signal was synchronised with the dynamometer to determine EMD. Intra-subject reliability for all measures was good (CV range, 3.1–6.5%) with no discernable difference between the dominant and non-dominant arms during isometric and isokinetic conditions. In addition, there was little difference in EMD and torque variability between the dominant and non-dominant arms which may have applications for clinicians and future research design when monitoring and investigating human muscle function. These data provide researchers and clinicians with an indication of the magnitude of change that is required to elucidate the presence of a meaningful change to muscle function in the elbow flexors.     

Muscular strength and power are correlated with motor unit action potential amplitudes,but not myosin heavy chain isoforms in sedentary males and females

It remains unclear if the sizes of higher-threshold motor units (MU) are associated with muscular strength and power. Therefore, the purpose of this study was to examine sex-related differences in muscle cross-sectional area (mCSA), percent myosin heavy chain (%MHC) isoform expression, and the MU action potential amplitudes (MUAP_AMPS)-recruitment threshold (RT) relationships of the vastus lateralis and isometric peak torque, isokinetic peak torque and mean power at 1.05 rad·s⁻¹ of the leg extensors. Surface electromyographic decomposition techniques were used to quantify MUAP_AMPS recorded during isometric muscle actions at 70% of maximal voluntary contractions and regressed against RTs with the slopes calculated. Ultrasound images were used to measure mCSA. Males had greater slopes from the MUAP_AMP-RT relationship than the females (P < 0.05). The greater slopes likely reflected larger higher-threshold MUs for the males. The mCSAs and slopes from the relationships were strongly correlated with isometric and isokinetic peak torque and isokinetic mean power (r = 0.78–0.82), however, type I %MHC isoform was only moderately correlated with isometric peak torque (r = −0.54). The results indicated that sex-related differences in muscular strength and power were associated more so with the sizes of the higher-threshold MUs (slopes) and mCSA than MHC isoforms. The amount of cross-bridge activity within muscle fibers that comprise higher-threshold MUs may be the primary contributor to muscular strength and power rather than the contractile properties of the muscle.     

Maximum voluntary joint torque as a function of joint angle and angular velocity: model development and application to the lower limb

Measurements of human strength can be important during analyses of physical activities. Such measurements have often taken the form of the maximum voluntary torque at a single joint angle and angular velocity. However, the available strength varies substantially with joint position and velocity. When examining dynamic activities, strength measurements should account for these variations. A model is presented of maximum voluntary joint torque as a function of joint angle and angular velocity. The model is based on well-known physiological relationships between muscle force and length and between muscle force and velocity and was tested by fitting it to maximum voluntary joint torque data from six different exertions in the lower limb. Isometric, concentric and eccentric maximum voluntary contractions were collected during hip extension, hip flexion, knee extension, knee flexion, ankle plantar flexion and dorsiflexion. Model parameters are reported for each of these exertion directions by gender and age group. This model provides an efficient method by which strength variations with joint angle and angular velocity may be incorporated into comparisons between joint torques calculated by inverse dynamics and the maximum available joint torques.     

Force-time curve characteristics of dynamic and isometric muscle actions of elite women olympic weightlifters

Six elite women weightlifters were tested to evaluate force-time curve characteristics and intercorrelations of isometric and dynamic muscle actions. Subjects performed isometric and dynamic mid-thigh clean pulls at 30% of maximal isometric peak force and 100 kg from a standardized position on a 61.0 x 121.9 cm AMTI forceplate. Isometric peak force showed strong correlations to the athletes' competitive snatch, clean and jerk, and combined total (r = 0.93, 0.64, and 0.80 respectively). Isometric rate of force development showed moderate to strong relationships to the athletes' competitive snatch, clean and jerk, and combined total (r = 0.79, 0.69, and 0.80 respectively). The results of this study suggest that the ability to perform maximal snatch and clean and jerks shows some structural and functional foundation with the ability to generate high forces rapidly in elite women weightlifters.     

Generality versus specificity: a comparison of dynamic and isometric measures of strength and speed-strength

Considerable debate exists as to whether the qualities of muscle function exist as general or specific physiological capacities. If there is a generality of muscle function then strong relationships would exist between various measures of function for the same muscle(s), independent of the test contraction, mode or velocity. The purpose of this study was to examine the relationship between isometric and dynamic measures of muscle function to determine the existence of generality or specificity. A group of 22 men, experienced in weight training, were tested for lower and upper body dynamic and isometric measures of strength and speed-strength. The changes in these measures consequent to a resistance training programme were also investigated. The results of this study indicated that whilst isometric and dynamic measures of strength did significantly correlate (r=0.57-0.61), the relationship was below that required to denote statistical generality. More important, the changes in isometric and dynamic strength consequent to a dynamic heavy resistance training programme were unrelated (r=0.12-0.15). Thus the mechanisms that contribute to enhanced dynamic strength appearred unrelated to the mechanisms that contribute to enhanced isometric strength. Measures of dynamic and isometric speed-strength were unrelated, as were the changes in these measures resulting from training. The results of this study demonstrated that a generality of muscle function did not exist and that modality specific results were observed. Consequently this study calls into question the validity of isometric tests to monitor dynamically induced training adaptations.     

Variations in the relationship between the frequency content of EMG signals and the rate of torque development in voluntary and elicited contractions.

Our purpose was to characterize the relationship between EMG mean power frequency (MPF) or median frequency (MF) and rate of torque development in voluntary ballistic and electrically elicited isometric contractions. Twenty-three healthy adults participated in two sets of experiments performed on elbow flexor muscles. For Experiment 1, subjects were asked to generate voluntary ballistic contractions by reaching four different target torque levels (20, 40, 60 and 100% of the maximal voluntary contraction (MVC)) as fast as they could. For Experiment 2, electrical (M-waves) and mechanical (twitches) responses to electrical stimulation of the nerves supplying the biceps brachii and brachioradialis muscles were recorded with the subjects at rest and with a background isometric contraction of 15% MVC. MPF, MF and rate of torque development (% MVC/s) were calculated for both voluntary and elicited contractions. Significant positive correlations were observed between MPF and rate of torque development for the voluntary contractions, whereas significant negative correlations were observed between the two variables for elicited contractions. This suggests that factors other than muscle fiber composition influence the frequency content of EMG signals and/or the rate of torque development, and that the effect of these factors will vary between voluntary and elicited contractions.     

The mechanical response of active human muscle during and after stretch

Five subjects contracted forearm supinator muscles which were stretched after development of maximal isometric torque. The ratio of torque at the end of stretch over isometric torque at that position was calculated as excess torque. Excess torque increased with stretch velocity and decreased with stretch amplitude, and it was not dependent upon final muscle length. The rate of decay of torque following stretch could not be shown to depend upon stretch variables. The absence of significant changes in myoelectric activity suggested that with high initial forces, reflex activity did not account for the observed changes. Time-constants of decay (0.15 s to 1.8 s) were much greater than time-constants of rise (approx. 0.07 s) of isometric torque at the same muscle length. This indicates that interaction of series elastic and contractile elements is not the sole cause of prolonged torque following stretch. It is concluded that stretch temporarily enhances the intrinsic contractile properties of a group of human muscles in a manner similar to, but quantitatively different from that seen in isolated muscle preparations.     

Isometric and isokinetic muscle strength, anthropometry and physical activity in 8 and 13 year old Swedish children

Isometric muscle strength of the hand-grip and of trunk flexion and extension, and isokinetic torque of elbow and knee flexion and knee extension were assessed in a random sample of 8 and 13 year old Swedish children. The results were compared with respect to sex and age in absolute terms and relative to weight, height2 and estimates of lean body mass and cross-sectional muscle area. Daily physical activity was also estimated. The muscle strength variables were in general found to be very similar in the 8 year old boys and girls. In the 13 year old group the boys were generally stronger than the girls, in both absolute and relative terms, except for similar torque values during knee extension. The absolute and relative muscle strength and torque values were higher in the older than in the younger children, with the exception of trunk strength per unit of body weight and of lean body mass, which were similar in boys of both ages and significantly lower in the older than in the younger girls. No significant correlation was found between the estimates of physical activity and isometric and isokinetic muscle strength and torque.     

Validity of trunk extensor and flexor torque measurements using isokinetic dynamometry

This study aimed to evaluate the validity and test–retest reliability of trunk muscle strength testing performed with a latest-generation isokinetic dynamometer. Eccentric, isometric, and concentric peak torque of the trunk flexor and extensor muscles was measured in 15 healthy subjects. Muscle cross sectional area (CSA) and surface electromyographic (EMG) activity were respectively correlated to peak torque and submaximal isometric torque for erector spinae and rectus abdominis muscles. Reliability of peak torque measurements was determined during test and retest sessions. Significant correlations were consistently observed between muscle CSA and peak torque for all contraction types (r = 0.74−0.85; P < 0.001) and between EMG activity and submaximal isometric torque (r ⩾ 0.99; P < 0.05), for both extensor and flexor muscles. Intraclass correlation coefficients were comprised between 0.87 and 0.95, and standard errors of measurement were lower than 9% for all contraction modes. The mean difference in peak torque between test and retest ranged from −3.7% to 3.7% with no significant mean directional bias. Overall, our findings establish the validity of torque measurements using the tested trunk module. Also considering the excellent test–retest reliability of peak torque measurements, we conclude that this latest-generation isokinetic dynamometer could be used with confidence to evaluate trunk muscle function for clinical or athletic purposes.     

Standing strength training of the ankle plantar and dorsiflexors in older women, using concentric and eccentric contractions

Many studies have reported strength gains in older adults following high-intensity resistance training. However, the muscle contraction types examined have been primarily isometric (static) or concentric (CONC; shortening). Less is known about how eccentric (ECC) strength in older adults responds to training or about the efficacy of ECC contractions as training stimuli in these subjects, even though muscle contractions of this type are performed in most training regimens and daily physical activities. In this study, 15 physically active, healthy older women [68 (5) years; mean (SD)] completed an 8-week resistance training program of two sessions per week. Training consisted of three sets of eight repetitions of CONC ankle plantar flexion (PF) and ECC dorsiflexion (DF), at greater than 80% of the initial peak torque, in a standing position only. Subjects were tested in standing and supine positions for: (1) strength over a range of 10° DF to 20° PF for both CONC and ECC; DF and PF (2) passive resistive torque of the plantar flexors at 6°/s; and (3) DF and PF rate of torque development. All strength testing and training was done at 30°/s. Significant increases (P < 0.01) were found for both CONC DF (↑30%) and ECC DF (↑17%) peak torque in the standing position. No significant changes occurred for DF strength as measured with the subjects in the supine position, PF strength in either position, passive resistive torque, or rate of torque development. In summary, strength gains occurred only in the dorsiflexors, which were trained using ECC contractions. Improvements in DF strength were specific to the position of training, which has implications for the transferability of strength gains to functional tasks such as maintaining gait. Accepted: 17 January 1997     

Peak force and rate of force development during isometric and dynamic mid-thigh clean pulls performed at various intensities

Eight male collegiate weightlifters (age: 21.2 +/- 0.9 years; height: 177.6 +/- 2.3 cm; and body mass: 85.1 +/- 3.3 kg) participated in this study to compare isometric to dynamic force-time dependent variables. Subjects performed the isometric and dynamic mid-thigh clean pulls at 30-120% of their one repetition maximum (1RM) power clean (118.4 +/- 5.5 kg) on a 61 x 121.9-cm AMTI forceplate. Variables such as peak force (PF) and peak rate of force development (PRFD) were calculated and were compared between isometric and dynamic conditions. The relationships between force-time dependent variables and vertical jump performances also were examined. The data indicate that the isometric PF had no significant correlations with the dynamic PF against light loads. On the one hand, there was a general trend toward stronger relationships between the isometric and dynamic PF as the external load increased for dynamic muscle actions. On the other hand, the isometric and dynamic PRFD had no significant correlations regardless of the external load used for dynamic testing. In addition, the isometric PF and dynamic PRFD were shown to be strongly correlated with vertical jump performances, whereas the isometric PRFD and dynamic PF had no significant correlations with vertical jump performances. In conclusion, it appears that the isometric and dynamic measures of force-time curve characteristics represent relatively specific qualities, especially when dynamic testing involves small external loads. Additionally, the results suggest that athletes who possess greater isometric maximum strength and dynamic explosive strength tend to be able to jump higher.     

Correlation between plantarflexor moment arm and preferred gait velocity in slower elderly men

In this study, the relationship between musculoskeletal architecture of the lateral gastrocnemius muscle and gait velocity in elderly individuals was investigated using ultrasonography and standardized tests of physical performance in 20 older adult males. Musculoskeletal architecture parameters included moment arm, fascicle length, pennation angle, and muscle thickness. The Six Minute Walk Test (6MIN) and Four Metre Walk Velocity Test (4METRE) were used to determine preferred and maximum gait velocity, respectively. Only weak correlations were found for all 20 subjects taken together. After subjects were separated into faster and slower subgroups by preferred velocity using cluster analysis; however, a strong correlation was found between plantarflexion moment arm and 6MIN velocity in the slower group (R(2)=0.669, p=0.004). Examination of subgroup differences revealed that the slow subgroup was significantly older than the fast subgroup (p=0.034), and had average body mass (p=0.021) and body mass index (p=0.011) that were significantly greater. The strength of the correlation between plantarflexion moment arm and 6MIN velocity found for slower subjects is much greater than those previously reported for correlations between ankle strength or power and walking velocity. Further investigation is necessary to determine if a link exists between plantarflexor moment arm and gait velocity in older and heavier adults.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.