Angle- and velocity-specific alterations in torque and semg activity of the quadriceps and hamstrings during isokinetic extension-flexion movements.

The purpose of this study was to investigate the effect of movement velocity (100 degrees, 200 degrees , 300 degrees s(-1), and 400 degrees s(-1)) and joint position (0 degrees - 15 degrees [L0], 25 degrees - 40 degees [L25], 55 degrees - 70 degrees [L55], and 75 degrees - 90 degrees [L75]) on peak torque (PT) parameters and surface electromyography (SEMG) of the knee-joint muscles during reciprocal isokinetic extension and flexion movements. Thirteen subjects (age = 22.7 +/- 2.1 years, mean height = 161.1 +/- 6.6 cm, mean weight = 63.5 +/- 5.8 kg) participated in the study. Bipolar surface electrodes were placed over the vastus medialis, vastus lateralis, biceps femoris, and medial hamstrings for determination of the root mean square (SEMGrms) and median frequency (SEMGmf) of the SEMG. Peak torque, angle of peak torque (PTang), percentage of peak torque (PTper), SEMGrms, and SEMGmf were analyzed using separate repeated measures analysis of variance (ANOVA). The following main results, significant at p < or = 0.05 or better, were found: The PTang was influenced by movement velocity (in extension there was a decrease in PTang moving from 300 degrees x s(-1) to 400 degrees x s(-1) and inflexion there was an increase in PTang moving from 300 degrees x s(-1) to 400 degrees x s(-1)). Secondly, a greater percentage of peak torque (PTper) was maintained during knee flexion than knee extension. And thirdly, both the quadriceps and hamstrings exhibited changing amplitudes and spectral frequencies based on joint position and movement velocity. There was a trend of decreasing SEMGrms for the quadriceps as the knee moved into extension, and a lower SEMGmf during early (L75) and end stages of knee extension (L0). For the hamstrings, SEMGrms was lowest at the more shortened position (L75) and highest near the mid-position (L25); the lowest SEMGmf occurred at the more lengthened position (L0) and the highest occurred at the more shortened position (L75). Finally, velocity influenced hamstrings and quadriceps muscle amplitude such that SEMGrms was highest at the slower velocities and lowest at the higher velocities. Velocity had no impact on quadriceps spectral properties (p > 0.05), but had a cyclic effect on hamstrings spectral properties. Changes in amplitude and frequency spectrum in tested muscles could be explained, in part, by neural drive to these muscles. Data support the hypothesis of lower activation levels of the quadriceps muscle in the extended position espoused by several authors as a way to protect the knee-joint in the knee-extended position.

Coactivation patterns of the medial and lateral hamstrings based on joint position and movement velocity during isokinetic movements.

The purpose of this investigation was to determine the effect of movement velocity (100 degrees x s(-1), 200 degrees x s(-1), 300 degees x s(-1), and 400 degrees x s(-1)) and joint position (0 degrees - 20 degrees [L0] 30 degrees - 50 degrees [L30], and 70 degrees - 90 degrees [L70] knee flexion) on reciprocal coactivation patterns of the medial and lateral hamstrings as determined by the amplitude and frequency spectrum of surface electromyography (SEMG). Thirteen female subjects (age = 22.7 +/- 2.1 years, mean height = 161.1 +/- 6.6 cm, mean weight = 63.5 +/- 5.8 kg) participated in the study. Bipolar surface electrodes were placed over the biceps femoris (BF) and medial hamstrings (MH) for determination of the root mean square (SEMGrms) and median frequency (SEMGmf) of the SEMG. Normalized SEMGrms values for the MH and BF were determined as a percentage of agonist SEMGrms activity for the same muscle during its agonist phase. Data were analyzed using separate 2 x 3 x 4 (muscle x position x angular velocity) repeated measures analysis of variance (ANOVA). For SEMGrms, there were significant muscle (p < 0.01) and position (p < or = 0.0001) main effects. Post-hoc analyses indicated the BF displayed greater muscle amplitude than the MH and that there was greater muscle amplitude at the L0 position (as the knee approached terminal extension). No velocity effect was noted (p > 0.05). For SEMGmf there were muscle x position (p < or = 0.05) and muscle x position x velocity (p < or = 0.01) interaction effects. Post-hoc analyses indicated the BF displayed a higher frequency spectrum than the MH at the L0 position. Secondly, velocity affected the BF and MH frequency spectrum such that values for both the MH and BF were lowest at 200 degrees x s(-1) and highest at 300 degrees x s(-1) (BF) and 400 degrees x s(-1) (MH). Velocity had little impact on the frequency spectrum in the midrange of the ROM (L30 position). Higher SEMGrms and SEMGmf values for the BF could be explained by the locking or screw home mechanism of the knee, and a way in which the human motor control system provides the limb with a dynamic braking system to control both extension and lateral rotational forces during the final stage of knee extension. It would appear that the way in which the body performs this function is not only to increase the amplitude of BF muscle firing but also to shift toward the recruitment of more fast-twitch motor units.

Knee muscular response strategies differ by developmental level but not gender during jump landing.

The purpose of this investigation was to determine differences between pre- and post-pubescent males and females in quadriceps (vastus medialis; VM) and hamstrings (medial hamstrings and biceps femoris; HAMS) muscular activation patterns via the root mean square of surface electromyography (SEMG) during self-initiated vertical jump landing. Fifty-eight subjects, divided into age and gender groupings, were compared on kinematic variables during pre-landing (100 msec preceeding initial ground contact), post-landing (100 msec following initial ground contact), and initial-contact-to-maximum-knee-flexion stages. Kinematic variables investigated were (1) SEMG values during each stage of the vertical-jump landing; (2) Co-contraction ratios (CCR), which represented the ratio of normalized hamstrings' activity to normalized quadriceps' activity; and, (3) knee angle at initial contact. Results indicated (1) no significant gender differences in variables measured; and, (2) significant developmental level differences. Post-pubescent subjects displayed greater HAMS acitivity and CCR values in the pre-landing stage relative to post-landing stages, indicating that post-pubescent subjects had a greater level of hamstrings co-contraction prior to landing than pre-pubescent subjects. Conversely, pre-pubescent subjects displayed greater post-landing and initial-contact-to-maximum-knee-flexion ratios, indicating a greater level of hamstrings' co-contraction during post-landing stages than post-pubescent subjects. There were no significant differences in knee angle at initial contact. The greater level of hamstrings' co-activation prior to landing by post-pubescent subjects indicated that they used a strategy of pre-tuning the hamstrings prior to landing (more CNS pre-activation) to control the ground reaction forces and anterior tibial displacement experienced by the knee during landing. On the other hand, pre-pubescent subjects controlled these forces by having a greater level of hamstrings' co-activation during landing, which represents more of a reflexive activation in response to ground impact.

The effect of movement velocity and movement pattern on the reciprocal co-activation of the hamstrings.

The effect of velocity and movement pattern (reciprocal vs. non-reciprocal) on the reciprocal co-activation of the hamstrings was investigated through analysis of the root mean square (RMS) and the median frequency (MDF) of surface electromyography (SEMG). Fourteen subjects performed six continuous repetitions of a reciprocal isokinetic movement pattern (maximal extension followed by maximal flexion), and six continuous repetitions of a non-reciprocal movement pattern (maximal extension only) at 100 degrees, 200 degrees, 300 degrees s-1, and 400 degrees s-1. Data were analyzed using separate 2 x 4 (movement pattern x angular velocity) repeated measures analysis of variance (ANOVA). No significant differences (p > 0.05) were noted between reciprocal and non-reciprocal movement patterns for RMS. However, results did reveal a velocity effect for RMS (F = 5.0, p < 0.01), with significant differences observed between 100 degrees s-1 and 400 degrees s-1 (F = 9.4, p < 0.01), 200 degrees s-1 and 400 degrees s-1 (F = 9.5, p < 0.01), and 300 degrees s-1 and 400 degrees s-1 (F = 11.0, p < 0.001), with RMS values at 400 degrees s-1 being the highest. There was also a velocity effect for MDF (F = 8.03, p < 0.001), with significant differences observed between 100 degrees s-1 and 300 degrees s-1 (F = 4.2, p < 0.05), 100 degrees s-1 and 400 degrees s-1 (F = 20.2, p < 0.0001), 200 degrees s-1 and 400 degrees s-1 (F = 15.221, p < 0.001), and 300 degrees s-1 and 400 degrees s-1 (F = 5.9, p < 0.01). In all cases the highest MDF values were exhibited at the lower velocities. Lastly, there was an interaction effect when comparing movement patterns at 400 degrees s-1, with MDF values being significantly higher during the non-reciprocal movement pattern than during the reciprocal movement pattern (F = 10.9, p < 0.01). Results indicated that during isokinetic movements, RMS and MDF activity of the hamstrings are altered as velocity changes. More specifically, as velocity increases overall hamstrings' co-activation increases and there is a shift in the power spectrum toward the recruitment of slower-twitch muscle fibers. Results also indicate that movement pattern (reciprocal vs. non reciprocal) does not effect appreciably SEMG activity of hamstrings' co-contraction.

Reliability and concurrent validity of the movement assessment battery for children.

Reliability and concurrent validity of the Movement Assessment Battery for Children were evaluated with a sample of 106 boys and girls, distributed into four age groups: 5-6 yr. (n = 20), 7-8 yr. (n = 20), 9-10 yr. (n = 46), and 11-12 yr. (n = 20). Test-retest reliability of the Movement Assessment Battery for Children, estimated using intraclass correlation coefficients, was high across all age groups, and concurrent validity yielded moderate Pearson correlation coefficients between the Move ment battery and long and short forms of the Bruininks-Oseretsky Test of Motor Proficiency. These results support the use of the Movement battery as a measure of motor ability in children, ages 5 to 12 years.

Effect of ankle position fixation on peak torque and electromyographic activity of the knee flexors and extensors.

The purpose of this study was to examine the effect of ankle position fixation on peak torque (PT) and electromyographic (EMG) activity of knee-joint muscles during isokinetic testing. Twelve female athletes performed isokinetic knee flexion and extension at 60 degrees and 180 degrees/s under two conditions: with the ankle fixed in a position of plantarflexion and with the ankle fixed in a position of dorsiflexion. Bipolar surface electrodes were placed on the vastus lateralis, vastus medialis, biceps femoris, medial hamstrings, and the lateral head of the gastrocnemius for determination of the root mean square of the EMG (rmsEMG) and the median frequency of the EMG (mfEMG). No significant differences in knee extensor PT were noted in either ankle position for each velocity tested. Significant differences were noted, however, in knee flexor PT (p < 0.05) at both 60 degrees and 180 degrees/s, with the greatest PT observed with the ankle fixed in dorsiflexion. Neither quadriceps, hamstrings, nor gastrocnemius rmsEMG activity was affected by ankle position; however, there was a significant difference in mfEMG for the gastrocnemius, with higher frequencies observed with the ankle fixed in plantarflexion (p < 0.01). These results suggest that ankle position effects knee flexor PT during open chain isokinetic movements. The reason for decreased knee flexor PT with the ankle fixed in plantarflexion is probably due to the gastrocnemius muscle being in a too shortened position, thereby preventing it from effectively producing force at the knee joint.

Reciprocal coactivation patterns of the medial and lateral quadriceps and hamstrings during slow, medium and high speed isokinetic movements.

The effect of movement velocity and fatigue on the reciprocal coactivation of the quadriceps and hamstrings was investigated through analysis of the root mean square (RMS) and the median frequency (MDF) of surface electromyography for the vastus medialis (VM), vastus lateralis (VL), medial hamstrings (MH) and biceps femoris (BF). Fourteen subjects performed six continuous isokinetic knee extension and flexion movements at 60 degrees, 180 degrees and 300 degrees s(-1), and 30 continuous movements at 300 degrees s(-1) to examine muscular fatigue patterns. Statistical analyses revealed that the RMS activity of the VM displayed greater coactivation than the VL (P<0.01) and the BF displayed greater coactivation than the MH (P<0.0001). There was no effect of velocity on the coactivation levels of the VM, the VL, or the MH; however, there was an effect of velocity on the coactivation levels of the BF (P<0.0001). Relative to MDF activity, the MH shifted upward as velocity increased (P<0. 01) while the BF decreased between 180 and 300 degrees s(-1) (P<0. 01). Results of the muscular fatigue test indicated that the RMS activity of the VM showed a higher degree of coactivation than the VL (P<0.01) and the BF showed approximately three times the coactivation level of the MH (P<0.001). The MDF of the VL and MH shifted downward as the repetitions progressed (P<0.01) with no changes for the VM or for the BF. Results of this study suggest that during isokinetic testing, both the VM and BF have significantly greater reciprocal coactivation levels when compared to the VL and MH, respectively. In addition, these results suggest that motor unit recruitment patterns of the VM and VL and the MH and BF differ with regard to the effects of velocity and fatigue.

Leg rotation and vastus medialis oblique/vastus lateralis electromyogram activity ratio during closed chain kinetic exercises prescribed for patellofemoral pain.

OBJECTIVE: To examine the effect of leg rotation on the vastus medialis oblique/vastus lateralis (VMO/VL) electromyogram (EMG) activity ratio during two closed chain knee extension exercises prescribed for patellofemoral pain syndrome (PFPS). DESIGN AND SETTING: Subjects performed step-up/step-down (SUSD) and modified wall-slide (MWS), exercises with the leg neutral (SUSD/N, MWS/N), internally rotated (SUSD/I, MWS/I), and externally rotated (SUSD/E, MWS/E), respectively, for determination of root mean square EMG. SUBJECTS: Six athletically active females symptomatic for PFPS and nine athletically active asymptomatic female control subjects. MEASUREMENTS: The EMG activity of the VMO and the VL was collected while the subjects performed the exercises. Root mean square EMG was determined and then normalized to the subjects' maximal voluntary isometric contractions for calculation and analysis of the VMO/VL activity ratio. RESULTS: Leg rotation had no effect on the VMO/VL activity ratio for the symptomatic group. The SUSD/E produced a lower VMO/VL activity ratio compared with the SUSD/N (p < .01) and the SUSD/I (p < .01) in the control group. Similarly, the VMO/VL activity ratio for the MWS/E was lower than for the MWS/I (p < 0.01). CONCLUSIONS: These results suggest that external rotation of the leg may affect VMO/VL activity ratio during closed chain knee extension exercises in individuals without PFPS but not in individuals symptomatic for PFPS.

Peak torque, average power, and hamstrings/quadriceps ratios in nondisabled adults and adults with mental retardation.

OBJECTIVE: To compare isokinetic hamstring and quadriceps peak torque (Nm), average power (watts), and corresponding hamstring/quadriceps (HQ) ratios (as percentages) of adult men with Down syndrome(DS), with mental retardation without Down syndrome (NDS), and nondisabled sedentary controls (SC). DESIGN: Repeated measures analysis of variance. SETTING: Subjects were tested at a university exercise science laboratory. SUBJECTS: Volunteer sample of 35 subjects: SC(n=13), DS(n=9), and NDS (n=13). INTERVENTION: Subjects performed isokinetic strength tests at 60 degrees /sec and 90 degrees/sec using gravity effected torque procedures. Subjects with DS and NDS performed the test on two separate days with best results selected for statistical comparisons. Sedentary controls performed the test once. MAIN OUTCOME MEASURES: Isokinetic hamstring and quadriceps peak torque and average power, and corresponding HQ ratios on a Cybex 340 isokinetic dynamometer. RESULTS: In all isokinetic parameters measured, sedentary controls demonstrated significantly higher scores than subjects with DS and NDS. There was no significant difference between subjects with DS and NDS, although mean peak torque and average power scores were greater in subjects with NDS. Finally, there were no significant differences in peak torque and average power HQ ratios across groups (p> .01), although group mean peak torque HQ ratios were greatest for sedentary controls (range=61% to 63%) and approximated accepted HQ ratio norms, and lowest for subjects with DS(range=40% to 46%). CONCLUSIONS: Individuals with mental retardation are in need of progressive resistance exercise programs to improve hamstring and quadriceps strength and normalize HQ strength and power ratios.

Effects of frequency and duration of exercise sessions on physical activity levels and adherence.

The purpose of this study was to assess the relations among physical activity, adherence, and frequency and duration of exercise sessions for 33 male and female sedentary university students. Analysis indicated that frequency and duration of the exercise program did not significantly affect adherence. However, frequency and duration of the exercise sessions did significantly affect leisure activity once the structured exercise program terminated.

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.