Identification of feigned ankle plantar and dorsiflexors weakness in normal subjects

Based on the limited ability of the human being to voluntarily control submaximal eccentric exertions, previous studies have indicated that isokinetic testing with a combined concentric–eccentric exercise protocol could effectively identify submaximal (feigned) effort in various muscle groups by showing an abnormally high eccentric to concentric ratio (ECR). The objective of this study was to determine the validity and accuracy of an ECR-based isokinetic test in identifying feigned ankle weakness. Thirty-eight normal subjects performed maximal and feigned efforts in an isokinetic concentric and eccentric ankle plantar- and dorsiflexion protocol with two different velocities, 30 and 120° s⁻¹. The isokinetic parameters ECR and the derivatives DEC (difference between ECR at high speed of motion and ECR at low speed of motion) and SEC (sum of ECR at high speed of motion plus the ratio between eccentric peak torque at high speed and concentric peak torque at low speed) were calculated. The ECR, DEC and SEC scores were significantly greater in feigned conditions for ankle plantarflexion, but not for dorsiflexion. Using optimal cutoff scores based on 99% tolerance intervals, it was disclosed that the most efficient parameter was the SEC, identifying 92% of the feigned efforts with 99% confidence, indicating that the ankle plantarflexors are less controllable in fast eccentric conditions than that in concentric conditions. The ECR-based parameters are valid for effectively identifying feigned plantarflexion effort with high accuracy, but do not allow the detection of feigned dorsiflexion weakness.     

Differentiating between types and levels of isokinetic knee musculature efforts

This investigation assessed whether a measure of moment curve shape similarity, and a measure quantifying curve magnitude differences, enables differentiation between types (sincere vs. feigned) and levels (maximal vs. submaximal) of effort exerted during isokinetic testing of the knee. Healthy participants (n = 37) performed four sets of six concentric knee extension–flexion repetitions on two occasions. The sets consisted of: (1) maximal effort; (2) self-perceived 75% of maximal effort; (3) self-perceived 50% of maximal effort; and (4) a set attempting to feign injury. Average cross-correlation and percent root mean square difference values were computed between moment curves in each direction. Logistic regression was used to derive decision rules for differentiating between maximal and submaximal effort levels; and between sincere and feigned effort types. Using a cutoff criteria corresponding to 100% specificity, maximal effort production could be ascertained with 96% sensitivity within the sample. Feigned efforts, however, could be ascertained with only 31% sensitivity due to overlap with sincere submaximal effort. Using the proposed models, clinicians may be able to ascertain whether maximal efforts were produced during isokinetic knee musculature testing. Additionally, evidence regarding participant’s intentions with regard to influencing test results may be gauged, although to a lesser extent.     

Effects of age and sex on shoulder biomechanics and relative effort during functional tasks

Age-related decline in muscle strength can compromise shoulder function, which could increase the effort needed to perform activities of daily living (ADLs). The purpose of this cross-sectional study was to determine for the first time the relative shoulder effort during ADLs in healthy young and older adults.Ten healthy young adults and ten healthy older adults were tested for maximal isokinetic torque and on a set of ADL tasks. Using inverse dynamics, the shoulder torques during ADLs were referenced to the maximal isokinetic torque and relative effort was determined.Older compared to younger adults had >40% lower isokinetic shoulder abduction strength. The ratio of peak joint torque during six ADLs over the maximal isokinetic torque, i.e., relative effort, was higher in old (∼52%) compared with young adults (∼22%, p < 0.05). Relative effort in older adults was over 40% in overhead activities and particularly high in abduction and reaching tasks, over 60%.Healthy older compared with younger adults perform most ADL tasks involving the shoulder joint with nearly twice the level of relative effort. The concomitant reductions in maximal shoulder isokinetic torque and increases in relative effort may be related to the high prevalence of musculoskeletal pain and shoulder dysfunction in old age reported in epidemiological studies.     

Scapular kinematics during unloaded and maximal loaded isokinetic concentric and eccentric shoulder flexion and extension movements

Characterization of scapular kinematics under demanding load conditions might aid to distinguish between physiological and clinically relevant alterations. Previous investigations focused only on submaximal external load situations. How scapular movement changes with maximal load remains unclear. Therefore, the present study aimed to evaluate 3D scapular kinematics during unloaded and maximal loaded shoulder flexion and extension. Twelve asymptomatic individuals performed shoulder flexion and extension movements under unloaded and maximal concentric and eccentric loaded isokinetic conditions. 3D scapular kinematics assessed with a motion capture system was analyzed for 20° intervals of humeral positions from 20° to 120° flexion. Repeated measures ANOVAs were used to evaluate kinematic differences between load conditions for scapular position angles, scapulohumeral rhythm and scapular motion extent. Increased scapular upward rotation was seen during shoulder flexion and extension as well as decreased posterior tilt and external rotation during eccentric and concentric arm descents of maximal loaded compared to unloaded conditions. Load effects were further seen for the scapulohumeral rhythm with greater scapular involvement at lower humeral positions and increased scapular motion extent under maximal loaded shoulder movements. With maximal load applied to the arm physiological scapular movement pattern are induced that may imply both impingement sparing and causing mechanisms.     

The dynamic control ratio and its equilibrium point: A preliminary study of isokinetic fatiguing internal-external rotational effort of the shoulder joint in healthy subjects

The shoulder joint-related dynamic control ratio (DCR) is derived by dividing the peak eccentric moment of the external rotators (ER) by the peak concentric moment of the internal rotators (IR). However, given the inherent limitation associated with a single value DCR, an alternative approach is to calculate it at fixed angular intervals. This preliminary study aimed at exploring the variation in the DCR at a resolution of 1° and under fatiguing external and internal rotation exertions. Eighteen young men, 10 experienced and 8 without experience in overhead sporting activities completed two separate series of 45 ER eccentric and 45 IR concentric isokinetic repetitions at 120°/s. The analysis focused on repetitions 1–3 (TR1), 21–23 (TR2) and 41–43 (TR3). The relative fatigue values for both muscle groups and for both E and NE participants were 25–40% with significantly higher fatigue resistance in eccentric compared with concentric exertions. The DCR traces varied substantially linearly for most of the internal rotation range of motion while significant differences (p < 0.001) were found in their values within group between TR1, TR2 and TR3 and between experienced and non-experienced participants. An antagonistic moment equilibrium (DCR = 1) was reached in all instances and for both groups only during TR3 with a significant progressive reduction in this moment as fatigue increased. Thus, considering the DCR as an angle-based variable rather than a single value isokinetic parameter, may add new insight regarding the interplay between the rotatory muscles of the shoulder joint.     

Effect of muscle fatigue on the compliance of the gastrocnemius medialis tendon and aponeurosis

The aim of the present study was to examine whether or not the compliance of the gastrocnemius medialis (GM) tendon and aponeurosis is influenced by submaximal fatiguing efforts. Fourteen elderly male subjects performed isometric maximal voluntary plantarflexion contractions (MVC) on a dynamometer before and after two fatiguing protocols. The protocols consisted of: (1) submaximal concentric isokinetic contractions (70% isokinetic MVC) at 60 degrees /s and (2) a sustained isometric contraction (40% isometric MVC) until failure to hold the defined moment. Ultrasonography was used to determine the elongation and strain of the GM tendon and aponeurosis. To account for the axis misalignment between ankle and dynamometer, the kinematics of the leg were captured at 120 Hz. The maximum moment decreased from 85.9+/-17.9 Nm prior fatigue to 79.2+/-19 Nm after isokinetic fatigue and to 69.9+/-16.4 Nm after isometric fatigue. The maximal strain of the GM tendon and aponeurosis before fatigue, after isokinetic and after isometric fatigue were 4.9+/-1.1%, 4.4+/-1.1% and 4.3+/-1.1% respectively. Neither the strain nor the elongation showed significant differences before and after each fatiguing task at any 100 N step of the calculated tendon force. This implies that the compliance was not altered after either the isokinetic or the isometric fatiguing task. Therefore it was concluded that the strains during the performed submaximal fatiguing tasks, were too small to provoke any structural changes in tendon and aponeurosis.     

Cerebral cortical representation of external anal sphincter contraction: effect of effort

The external anal sphincter (EAS) plays a critical role in maintaining fecal continence; however, cerebral cortical control of voluntary EAS contraction is not completely understood. Our aims were to determine the cortical areas associated with voluntary EAS contraction and to determine the effect of two levels of sphincter contraction effort on brain activity. Seventeen asymptomatic adults (ages 21-48, 9 male) were studied using functional magnetic resonance imaging (fMRI) to detect brain activity. Studies were done in two stages. In stage 1 (10 subjects, 5 male), anal sphincter pressure was monitored from a catheter-affixed bag. Subjects performed maximal and submaximal EAS contractions during two fMRI scanning sessions consisting of alternating 10-s intervals of sustained contraction and rest. In stage 2 studies, seven subjects (4 male) performed only maximum effort sphincter contractions without a catheter. EAS contraction was associated with multifocal fMRI activity in sensory/motor, anterior cingulate, prefrontal, parietal, occipital, and insular regions. Total cortical activity volume was significantly larger (P < 0.05) for maximal (5,175 +/- 720 microl) compared with submaximal effort contractions (2,558 +/- 306 microl). Similarly, percent fMRI signal change was significantly higher (P < 0.05) for maximal (4.8 +/- 0.1%) compared with submaximal effort contractions (2.2 +/- 0.1%). Cortical region-of-interest analysis showed the incidence of insular activation to be more common in women compared with men. Other cortical regions showed no such gender differences. fMRI activity detected in stage 2 showed similar regions of cortical activation to those of the stage 1 study. Willful contraction of the EAS is associated with multifocal cerebral cortical activity. The volume and intensity of cerebral cortical activation is commensurate with the level of contractile effort.     

Agonist and antagonist muscle activation during maximal and submaximal isokinetic fatigue tests of the knee extensors

The purpose of this study was to examine the differences in electromyographic activity of agonist and antagonist knee musculature between a maximal and a submaximal isokinetic fatigue protocol. Fourteen healthy males (age: 24.3 ± 2.5 years) performed 25 maximal (MIFP) and 60 submaximal (SIFP) isokinetic concentric efforts of the knee extensors at 60° s⁻¹, across a 90° range of motion. The two protocols were performed a week apart. The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) were recorded using surface electrodes. The peak torque (PT) and average EMG (aEMG) were expressed as percentages of pre-fatigue maximal value. One-way analysis of variance indicated a significant (p < 0.05) decline of PT during the maximal (45.7%) and submaximal (46.8%) protocols. During the maximal test, the VM and VL aEMG initially increased and then decreased. In contrast, VM and VL aEMG continuously increased during submaximal testing (p < 0.05). The antagonist (BF) aEMG remained constant during maximal test but it increased significantly and then declined during the submaximal testing. The above results indicate that agonist and antagonist activity depends on the intensity of the selected isokinetic fatigue test.     

The effects of EMG biofeedback on strength acquisition

This study investigated the effectiveness of electromyographic (EMG) biofeedback in maximizing strength gains and integrated electromyographic (IEMG) levels of the quadriceps muscle group resulting from an isokinetic exercise program. Twenty-one male volunteers recruited from physical education classes at a large southwestern university were randomly assigned to one of the following three treatment groups: (1) a biofeedback (BF) trained group, (2) a deception (DEC) trained group, and (3) a nonfeedback (NF) trained group. Subjects were trained and tested for strength by extension on a Cybex Isokinetic Exercise Machine at a speed of 30 degrees per second. Training sessions were performed three times per week for five weeks; pretest and posttest data were based on the best score of three trials of a 1-RM maximum effort. A pretraining to posttraining comparison indicated significant increases in strength (p less than .001) and IEMG levels (p less than .001) for all treatment groups when a paired t test was applied to the data. A multivariate analysis of covariance (MANCOVA) revealed that the BF trained group showed significantly greater peak torque values than DEC and NF trained groups (p less than .01) and produced significantly greater IEMG levels than the NF trained group (p less than .05). Overall, these results were taken as supporting the hypothesis that a training program of combined isokinetics and EMG biofeedback produces significant gains in maximal force and IEMG activity of leg-extensor muscles.     

Effects of a shoulder injury prevention strength training program on eccentric external rotator muscle strength and glenohumeral joint imbalance in female overhead activity athletes

Imbalance of the eccentrically-activated external rotator cuff muscles versus the concentrically-activated internal rotator cuff muscles is a primary risk factor for glenohumeral joint injuries in overhead activity athletes. Nonisokinetic dynamometer based strength training studies, however, have focused exclusively on resulting concentric instead of applicable eccentric strength gains of the external rotator cuff muscles. Furthermore, previous strength training studies did not result in a reduction in glenoumeral joint muscle imbalance, thereby suggesting that currently used shoulder strength training programs do not effectively reduce the risk of shoulder injury to the overhead activity athlete. Two collegiate women tennis teams, consisting of 12 women, participated in this study throughout their preseason training. One team (n = 6) participated in a 5-week, 4 times a week, external shoulder rotator muscle strength training program next to their preseason tennis training. The other team (n = 6) participated in a comparable preseason tennis training program, but did not conduct any upper body strength training. Effects of this strength training program were evaluated by comparing pre- and posttraining data of 5 maximal eccentric external immediately followed by concentric internal contractions on a Kin-Com isokinetic dynamometer (Chattecx Corp., Hixson, Tennessee). Overall, the shoulder strength training program significantly increased eccentric external total work without significant effects on concentric internal total work, concentric internal mean peak force, or eccentric external mean peak force. In conclusion, by increasing the eccentric external total exercise capacity without a subsequent increase in the concentric internal total exercise capacity, this strength training program potentially decreases shoulder rotator muscle imbalances and the risk for shoulder injuries to overhead activity athletes.     

Torque steadiness and muscle activation are bilaterally impaired during shoulder abduction and flexion in chronic post-stroke subjects

ObjectiveTo characterize sensorimotor control and muscle activation in the shoulder of chronic hemiparetic during abduction and flexion in maximal and submaximal isometric contractions. Furthermore, to correlate submaximal sensorimotor control with motor impairment and degree of shoulder subluxation.MethodsThirteen chronic hemiparetic post-stroke age-gender matched with healthy were included. Isometric torques were assessed using a dynamometer. Electromyographic activity of the anterior and middle deltoid, upper trapezius, pectoralis major and serratus anterior muscles were collected. Variables were calculated for torque: peak, time to target, standard deviation (SD), coefficient of variation (CV), and standard error (RMSE); for muscle activity: maximum and minimum values, range and coefficient of activation. Motor impairment was determined by Fugl-Meyer and shoulder subluxation was measured with a caliper.ResultsParetic and non-paretic limbs reduced peak and muscle activation during maximal isometric contraction. Paretic limb generated lower force when compared with non-paretic and control. Paretic and non-paretic presented higher values of SD, CV, RMSE, and CV for prime mover muscles and minimum values for all muscles during steadiness. No correlation was found between sensorimotor control, motor impairment and shoulder subluxation.ConclusionChronic hemiparetic presented bilateral deficits in sensorimotor and muscle control during maximal and submaximal shoulder abduction and flexion.     

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.     

A differential suction electrode for recording electromyographic activity from the pelvic floor muscles: Crosstalk evaluation

The purpose of this study was to determine whether a differential suction electrode (DSE) probe is less prone to recording crosstalk from the hip adductors or external rotators than the Femiscan? probe when recording electromyography (EMG) data from the pelvic floor muscles (PFMs). Twenty nulliparous, continent women performed hip adduction and hip external rotation contractions at 25%, 50% and 100% of maximal voluntary effort both while keeping their PFMs relaxed and while contracting their PFMs as strongly as possible. All tasks were performed while DSEs were adhered to the vaginal wall at the level of the pubococcygeus group bilaterally, and also with the Femiscan? probe in situ. The order of the probes was randomized. For each task, the peak smoothed PFM EMG amplitude was compared between hip forces and probes using a two-way repeated measures analysis of variance (ANOVA) including the interaction between contraction level and probe (α = 0.05). There was a significant contraction level by probe interaction for each task. In most cases the Femiscan? probe recorded significantly higher PFM EMG activity during hip adduction and external rotation tasks at 25% and 50% MVC compared to what it recorded when the hip musculature remained relaxed, whereas the DSE probe did not. As such, the DSE probe appears to be less susceptible to crosstalk from the hip adductors and external rotators than the Femiscan? probe at these hip muscle contraction levels. Both probes recorded significantly higher EMG activity when maximal contractions of the hip adductors and external rotators were performed therefore, no conclusion can be made regarding whether the activity recorded from the PFMs during maximal hip adduction and external rotation is the result of crosstalk or co-activation.     

Electromyographic amplitude ratio of serratus anterior and upper trapezius muscles during modified push-ups and bench press exercises

Imbalance and weakness of the serratus anterior and upper trapezius force couple have been described in patients with shoulder dysfunction. There is interest in identifying exercises that selectively activate these muscles and including it in rehabilitation protocols. This study aims to verify the UT/SA electromyographic (EMG) amplitude ratio, performed in different upper limb exercises and on two bases of support. Twelve healthy men were tested (average age = 22.8 +/- 3.1 years), and surface EMG was recorded from the upper trapezius and serratus anterior using single differential surface electrodes. Volunteers performed isometric contractions over a stable base of support and on a Swiss ball during the wall push-up (WP), bench press (BP), and push-up (PU) exercises. All SEMG data are reported as a percentage of root mean square or integral of linear envelope from the maximal value obtained in one of three maximal voluntary contractions for each muscle studied. A linear mixed-effect model was performed to compare UT/SA ratio values. The WP, BP, and PU exercises showed UT/SA ratio mean +/- SD values of 0.69 +/- 0.72, 0.14 +/- 0.12, and 0.39 +/- 0.37 for stable surfaces, respectively, whereas for unstable surfaces, the values were 0.73 +/- 0.67, 0.43 +/- 0.39, and 0.32 +/- 0.30. The results demonstrate that UT/SA ratio was influenced by the exercises and by the upper limb base of support. The practical application is to show that BP on a stable surface is the exercise preferred over WP and PU on either surfaces for serratus anterior muscle training in patients with imbalance between the UT/SA force couple or serratus anterior weakness.     

The effects of EMG biofeedback on strength acquisition

This study investigated the effectiveness of electromyographic (EMG) biofeedback in maximizing strength gains and integrated electromyographic (IEMG) levels of the quadriceps muscle group resulting from an isokinetic exercise program. Twenty-one male volunteers recruited from physical education classes at a large southwestern university were randomly assigned to one of the following three treatment groups: (1) a biofeedback (BF) trained group, (2) a deception (DEC) trained group, and (3) a nonfeedback (NF) trained group. Subjects were trained and tested for strength by extension on a Cybex Isokinetic Exercise Machine at a speed of 30 degrees per second. Training sessions were performed three times per week for five weeks; pretest and posttest data were based on the best score of three trials of a 1-RM maximum effort. A pretraining to posttraining comparison indicated significant increases in strength (p<.001)and IEMG levels (p<.001) for all treatment groups when a pairedt test was applied to the data. A multivariate analysis of covariance (MANCOVA) revealed that the BF trained group showed significantly greater peak torque values than DEC and NF trained groups (p<.01) and produced significantly greater IEMG levels than the NF trained group (p<.05). Overall, these results were taken as supporting the hypothesis that a training program of combined isokinetics and EMG biofeedback produces significant gains in maximal force and IEMG activity of leg-extensor muscles.This research was conducted while the author was at the University of New Mexico, Albuquerque, New Mexico.     

Normal muscle strength and fatigability in patients with effort syndromes.

To examine fatigue mechanisms in an unselected series of patients with excess fatigue ("effort syndromes") their muscle function was compared with that of normal subjects. Voluntary performance was assessed with a cycle ergometer to exhaustion and by maximal isometric contractions of the quadriceps femoris. The mean maximal heart rate in patients during ergometry was 89% of the predicted rate, and quadriceps strength was either normal or was inappropriate for the available muscle, which suggested submaximal effort. Contractile performance was examined in the absence of volition with stimulated contractions of the adductor pollicis. During stimulated fatiguing activity patients were neither weaker nor more fatigable than controls; thus the excess fatigue experienced by the patients was not due to a defect of the contractile apparatus. The increased perception of effort must therefore be due to impairment of central rather than peripheral mechanisms. The optimal approach to treatment of effort syndromes combines physical and psychological techniques.     

Effect of submaximal exercise at low temperatures on pulmonary function in healthy young men

In order to understand more fully the effect on pulmonary function of whole body exposure to cold during submaximal exercise, we measured pulmonary function indices in ten healthy male students and ten healthy male forestry workers of similar age following submaximal treadmill walking at different temperatures in a climatic chamber. After measuring the maximal aerobic capacity with a cycle ergometer test, the subjects had to walk on four separate occasions in the climatic chamber at an intensity of 70%-75% of their individual maximal heart rate; the first at normal room temperature and then randomly, either at 0 degrees C or at -20 degrees C, and vice versa. The duration of each walk was 8 min. Finally, each subject had to walk in the chamber at -20 degrees C for 17 min. Flow volume spirometry was performed at room temperature 1, 5, 10, and 20 min after exercise and the values were compared to baseline values taken prior to the last walking test. There were only minor changes in pulmonary function indices following exercise at different temperatures. Only one student showed a reduction of over 15% in peak expiratory flow rate after an 8-min walk at -20 degrees C. It seems that submaximal exercise of short duration, even at a temperature as low as -20 degrees C, does not impair pulmonary function in healthy young men.     

Technical report: Reliability and validity of the iSAM 9000 isokinetic dynamometer

This study assessed the mechanical reliability and validity of the INRTEK iSAM 9000 isokinetic dynamometer, and compared the obtained torque values of the prototype device with those from a traditional device. Sixty volunteers (40 men and 20 women) were tested at 60 degrees per second for shoulder, knee, and trunk flexion, and extension on both the Cybex 6000 and a new isokinetic dynamometer (iSAM 9000). Intraclass correlation coefficients (ICC) and standard errors of measurement (SEM) revealed a high level of reproducibility and precision in the device's torque measurements (ICC range = 0.94-0.98; SEM range = 5.2-29.7). Pearson r values revealed very high relationships between the two instruments (set 1: r = 0.84-0.93; set 2: r = 0.87-0.93; P < 0.05). Significantly higher peak torque for both sets of left and right knee flexion and extension, right shoulder extension and trunk extension was found for the iSAM 9000 compared to the Cybex 6000 (P < 0.05). The strong ICCs and small SEMs support the device's mechanical reliability and validity. The high correlation coefficients between the prototype dynamometer and the Cybex 6000 support the new device's validity in the measurement of isokinetic torque. The findings of this study will be used to refine the next generation of the INRTEK isokinetic device with respect to test protocols and the reliability of measuring human muscle performance.     

The effects of interelectrode distance on electromyographic amplitude and mean power frequency during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to examine the effects of interelectrode distance (IED) on the absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isokinetic and isometric torque relationships for the biceps brachii muscle. Ten adults [mean+/-SD age=22.0+/-3.4 years] performed submaximal to maximal, isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects performed randomly ordered, submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Surface EMG signals were recorded simultaneously from bipolar electrode arrangements placed over the biceps brachii muscle with IEDs of 20, 40, and 60mm. Absolute and normalized EMG amplitude (muVrms and %max) increased linearly with torque during the isokinetic and isometric muscle actions (r(2) range=0.988-0.998), but there were no significant changes for absolute or normalized EMG MPF (Hz or %max) from 10% to 100% PT and MVC. In some cases, there were significant (p<0.05) differences among the three IED arrangements for absolute EMG amplitude and MPF values, but not for the normalized values. These findings suggested that for the biceps brachii muscle, IEDs between 20 and 60mm resulted in similar patterns for the EMG amplitude or MPF versus dynamic and isometric torque relationships. Furthermore, unlike the absolute EMG amplitude and MPF values, the normalized EMG data were not influenced by changes in IED between 20 and 60mm. Thus, normalized EMG data can be compared among previous studies that have utilized different IED arrangements.     

Do relevant shear forces appear in isokinetic shoulder testing to be implemented in biomechanical models?

Isokinetic dynamometers measure joint torques about a single fixed rotational axis. Previous studies yet suggested that muscles produce both tangential and radial forces during a movement, so that the contact forces exerted to perform this movement are multidirectional. Then, isokinetic dynamometers might neglect the torque components about the two other Euclidean space axes. Our objective was to experimentally quantify the shear forces impact on the overall shoulder torque, by comparing the dynamometer torque to the torque computed from the contact forces at the hand and elbow. Ten healthy women performed isokinetic maximal internal/external concentric/eccentric shoulder rotation movements. The hand and elbow contact forces were measured using two six-axis force sensors. The main finding is that the contact forces at the hand were not purely tangential to the direction of the movement (effectiveness indexes from 0.26 ± 0.25 to 0.54 ± 0.20), such that the resulting shoulder torque computed from the two force sensors was three-dimensional. Therefore, the flexion and abduction components of the shoulder torque measured by the isokinetic dynamometer were significantly underestimated (up to 94.9%). These findings suggest that musculoskeletal models parameters should not be estimated without accounting for the torques about the three space axes.