Alternate muscle activity observed between knee extensor synergists during low-level sustained contractions.

To determine quantitatively the features of alternate muscle activity between knee extensor synergists during low-level prolonged contraction, a surface electromyogram (EMG) was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) in 11 subjects during isometric knee extension exercise at 2.5% of maximal voluntary contraction (MVC) for 60 min (experiment 1). Furthermore, to examine the relation between alternate muscle activity and contraction levels, six of the subjects also performed sustained knee extension at 5.0, 7.5, and 10.0% of MVC (experiment 2). Alternate muscle activity among the three muscles was assessed by quantitative analysis on the basis of the rate of integrated EMG sequences. In experiment 1, the number of alternations was significantly higher between RF and either VL or VM than between VL and VM. Moreover, the frequency of alternate muscle activity increased with time. In experiment 2, alternating muscle activity was found during contractions at 2.5 and 5.0% of MVC, although not at 7.5 and 10.0% of MVC, and the number of alternations was higher at 2.5 than at 5.0% of MVC. Thus the findings of the present study demonstrated that alternate muscle activity in the quadriceps muscle 1) appears only between biarticular RF muscle and monoarticular vasti muscles (VL and VM), and its frequency of alternations progressively increases with time, and 2) emerges under sustained contraction with force production levels < or =5.0% of MVC.     

Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions

The objective of this study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during fatiguing knee extensions. Thirty young adults were evaluated for their one-repetition maximum (1RM) during a seated, right-leg, inertial knee extension. All subjects then completed a single set of repeated knee extensions at 50% 1RM, to failure. Subjects performed a knee extension (concentric phase), held the weight with the knee extended for 2s (isometric phase), and lowered the weight in a controlled manner (eccentric phase). Raw EMG of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscles were full-wave rectified, integrated and normalized to the 1RM EMG, for each respective phase and repetition. The EMG median frequency (f(med)) was computed during the isometric phase. An increase in QF muscle EMG was observed during the concentric phase across the exercise duration. VL EMG was greater than the VM and RF muscles during the isometric phase, in which no significant changes occurred in any of the muscles across the exercise duration. A significant decrease in EMG across the exercise duration was observed during the eccentric phase, with the VL EMG greater than the VM and RF muscles. A greater decrease in VL and RF muscle f(med) during the isometric phase, than the VM muscle, was observed with no gender differences. The findings demonstrated differential recruitment of the superficial QF muscle, depending on the contraction mode during dynamic knee extension exercise, where VL muscle dominance appears to manifest across the concentric-isometric-eccentric transition.     

Non-uniform mechanical activity of quadriceps muscle during fatigue by repeated maximal voluntary contraction in humans

To determine the non-uniform surface mechanical activity of human quadriceps muscle during fatiguing activity, surface mechanomyogram (MMG), or muscle sound, and surface electromyogram (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles of seven subjects during unilateral isometric knee extension exercise. Time- and frequency-domain analyses of MMG and of EMG fatigued by 50 repeated maximal voluntary contractions (MVC) for 3 s, with 3-s relaxation in between, were compared among the muscles. The mean MVC force fell to 49.5 (SEM 2.0)% at the end of the repeated MVC. Integrated EMG decreased in a similar manner in each muscle head, but a marked non-uniformity was found for the decline in integrated MMG (iMMG). The fall in iMMG was most prominent for RF, followed by VM and VL. Moreover, the median frequency of MMG and the relative decrease in that of EMG in RF were significantly greater (P < 0.05) than those recorded for VL and VM. These results would suggest a divergence of mechanical activity within the quadriceps muscle during fatiguing activity by repeated MVC. Accepted: 19 January 1999     

Angle- and gender-specific quadriceps femoris muscle recruitment and knee extensor torque

The objectives were to examine knee angle-, and gender-specific knee extensor torque output and quadriceps femoris (QF) muscle recruitment during maximal effort, voluntary contractions. Fourteen young adult men and 15 young adult women performed three isometric maximal voluntary contractions (MVC), in a random order, with the knee at 0 degrees (terminal extension), 10 degrees, 30 degrees, 50 degrees, 70 degrees, and 90 degrees flexion. Knee extensor peak torque (PT), and average torque (AT) were expressed in absolute (N m), relative (N m kg(-1)) and allometric-modeled (N m kg(-n)) units. Vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscle EMG signals were full-wave rectified and integrated over the middle 3 s of each contraction, averaged over the three trials at each knee angle, and normalized to the activity recorded at 0 degrees. Muscle recruitment efficiency was calculated as the ratio of the normalized EMG of each muscle to the allometric-modeled average torque (normalized to the values at 0 degrees flexion), and expressed as a percent. Men generated significantly greater knee extensor PT and AT than women in absolute, relative and allometric-modeled units. Absolute and relative PT and AT were significantly highest at 70 degrees, while allometric-modeled values were observed to increase significantly across knee joint angles 10-90 degrees. VM EMG was significantly greater than the VL and RF muscles across all angles, and followed a similar pattern to absolute knee extensor torque. Recruitment efficiency improved across knee joint angles 10-90 degrees and was highest for the VL muscle. VM recruitment efficiency improved more than the VL and RF muscles across 70-90 degrees flexion. The findings demonstrate angle-, and gender-specific responses of knee extensor torque to maximal-effort contractions, while superficial QF muscle recruitment was most efficient at 90 degrees, and less dependent on gender.     

Decrease in maximal voluntary contraction by tonic vibration applied to a single synergist muscle in humans.

The purpose of the study was to examine the effect of prolonged tonic vibration applied to a single synergist muscle on maximal voluntary contraction (MVC) and maximal rate of force development (dF/dt(max)). The knee extension MVC force and surface electromyogram (EMG) from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) during MVC were recorded before and after vibration of RF muscle at 30 Hz for 30 min. MVC, dF/dt(max), and the integrated EMG (iEMG) of RF decreased significantly after prolonged tonic vibration in spite of no changes in iEMG of VL and VM. The present results indicate that MVC and dF/dt(max) may be influenced by the attenuated Ia afferent functions of a single synergist muscle.     

Contraction mode shift in quadriceps femoris muscle activation during dynamic knee extensor exercise with increasing loads

The objective of the present study was to examine the superficial quadriceps femoris (QF) muscle electromyogram (EMG) during dynamic sub-maximal knee extension exercise between young adult men and women. Thirty subjects completed, in a random order, 2 sub-maximal repetitions of single-leg knee extensions at 20-90% of their one-repetition maximum (1RM). Vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) muscle integrated EMG (IEMG) during each sub-maximal lift was normalized to the respective 1RM for concentric, isometric and eccentric modes. The EMG median frequency (f(med)) was determined over the isometric mode. Men attained a significantly (p<0.05) greater knee angular velocity than the women during the concentric mode (83.6+/-19.1 degrees /s and 67.4+/-19.8 degrees /s, respectively). RF IEMG was significantly lesser than the VM (p=0.014) and VL (p<0.001) muscles, when collapsed across all contraction modes, loads, and sex. Overall IEMG was significantly greater during the concentric (p<0.001) and isometric (p<0.001) modes, than the eccentric mode. Men generated significantly (p=0.03) greater VL muscle IEMG than the women, while the opposite pattern emerged for the RF muscle. VM f(med) (105.1+/-11.1Hz) was significantly lesser than the VL (180.3+/-19.5Hz) and RF (127.7+/-13.9Hz) muscles across all lifting intensities, while the men (137.7+/-10.7Hz) generated greater values than the women (129.0+/-11.4Hz). The findings demonstrate a reduction in QF muscle activation across the concentric to eccentric transition, which may be related to the mode-specific velocity pattern.     

Nature of the coupling between neural drive and force-generating capacity in the human quadriceps muscle

The force produced by a muscle depends on both the neural drive it receives and several biomechanical factors. When multiple muscles act on a single joint, the nature of the relationship between the neural drive and force-generating capacity of the synergistic muscles is largely unknown. This study aimed to determine the relationship between the ratio of neural drive and the ratio of muscle force-generating capacity between two synergist muscles (vastus lateralis (VL) and vastus medialis (VM)) in humans. Twenty-one participants performed isometric knee extensions at 20 and 50% of maximal voluntary contractions (MVC). Myoelectric activity (surface electromyography (EMG)) provided an index of neural drive. Physiological cross-sectional area (PCSA) was estimated from measurements of muscle volume (magnetic resonance imaging) and muscle fascicle length (three-dimensional ultrasound imaging) to represent the muscles'' force-generating capacities. Neither PCSA nor neural drive was balanced between VL and VM. There was a large (r = 0.68) and moderate (r = 0.43) correlation between the ratio of VL/VM EMG amplitude and the ratio of VL/VM PCSA at 20 and 50% of MVC, respectively. This study provides evidence that neural drive is biased by muscle force-generating capacity, the greater the force-generating capacity of VL compared with VM, the stronger bias of drive to the VL.     

Neuromuscular activation of vastus intermedius muscle during fatiguing exercise

The purpose of this study was to investigate neuromuscular activation of the vastus intermedius (VI) muscle during fatiguing contraction. Seven healthy men performed sustained isometric knee extension exercise at 50% of maximal voluntary contraction until exhaustion. During the fatiguing task, surface electromyograms (EMGs) were recorded from four muscle components of the quadriceps femoris muscle group: VI; vastus lateralis (VL); vastus medialis (VM); and rectus femoris (RF) muscles. For the VI muscle, our recently developed technique was used. Root mean square (RMS) and median frequency (MF) of the surface EMG signal were calculated and these variables were then normalized by the value at the beginning of the task. Normalized RMS of the VI muscle resembled those of the other three muscles at all given times. At 95% of exhaustion time, normalized MF of the VI muscle was significantly higher than that of the VL muscle (p < 0.05). These results suggested that neuromuscular activation is not consistent between the VI and VL muscles at the exhaustion for isometric submaximal contraction and this could reflect the dissimilar intramuscular metabolism between these muscles.     

Local blood circulation among knee extensor synergists in relation to alternate muscle activity during low-level sustained contraction.

The relation between local circulation and alternate muscle activity among knee extensor synergists was determined during low-level sustained knee extension at 2.5% of maximal voluntary contraction for 60 min in seven subjects. Blood volume of rectus femoris (RF) and vastus lateralis (VL) was assessed by using near-infrared spectroscopy. Surface electromyogram (EMG) was recorded from RF, VL, and vastus medialis (VM). Alternate muscle activity was observed between RF and either VL or VM. Cross-correlation analysis was used to investigate the relation between blood volume and integrated EMG (iEMG) sequences throughout the task. One negative peak in the cross-correlation function was seen between the iEMG and blood volume with time lag of 30-60 s, indicating that muscle activity increases (or decreases) with the decrease (or increase) in local circulation with the corresponding time lag. Two cases in the emergence of alternate muscle activities, i.e., an increase in the EMG of RF accompanied by a decline of EMG in VL (case I) and vice versa (case II) were further analyzed. The time lag between iEMG and blood volume was longer in case I than that in case II. These results were statistically significant in the RF but not in the VL. It is concluded that even during low-level sustained contraction, local circulation is modulated by the alternate muscle activity of knee extensor synergists, and a negative correlation between the muscle activity and blood volume sequences was found in only RF but not in VL.     

EMG frequency content changes with increasing force and during fatigue in the quadriceps femoris muscle of men and women.

The purpose of this study was to determine the effect of gender on changes in electromyographic (EMG) signal characteristics of the quadriceps muscles with increasing force and with fatigue. A total of fourteen healthy adults (seven men, seven women) participated in the study. Subjects had to perform isometric ramp contractions in knee extension with the force gradually increasing from 0 to 100% of the maximal voluntary contraction (MVC) in a 6-s period. Subjects then performed a fatigue task, consisting of a sustained maximum isometric knee extension contraction held until force decreased below 50% of the pre-fatigue MVC. Subjects also performed a single ramp contraction immediately after the fatigue task. The Root Mean Square (RMS) amplitude, mean power frequency (MPF) and median frequency (MF) of EMG signals obtained from the vastus lateralis, vastus medialis and rectus femoris were calculated at nine different force levels from the ramp contractions (10, 20, 30, 40, 50, 60, 70, 80 and 90% MVC), as well as every 5 s during the fatigue task. The main results were a more pronounced increase in EMG RMS amplitude for the three muscles and in MPF for the VL muscle with force in men compared with women. No significant effect of gender was found with regards to fatigue. These observations most likely reflect a moderately greater type II fiber content and/or area in the VL muscle of men compared to that of women.     

Force fluctuations are modulated by alternate muscle activity of knee extensor synergists during low-level sustained contraction.

The study examined the hypothesis that altered synergistic activation of the knee extensors leads to cyclic modulation of the force fluctuations. To test this hypothesis, the force fluctuations were investigated during sustained knee extension at 2.5% of maximal voluntary contraction force for 60 min in 11 men. Surface electromyograms (EMG) were recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The SD of force and average EMG (AEMG) of each muscle were calculated for 30-s periods during alternate muscle activity. Power spectrum of force was calculated for the low- (< or =3 Hz), middle- (4-6 Hz), and high-frequency (8-12 Hz) components. Alternate muscle activity was observed between RF and the set of VL and VM muscles. The SD of force was not constant but variable due to the alternate muscle activity. The SD was significantly greater during high RF activity compared with high VL and VM activity (P < 0.05), and the correlation coefficient between the SD and AEMG was significantly greater in RF [0.736 (SD 0.095), P < 0.05] compared with VL and VM. Large changes were found in the high-frequency component. During high RF activity, the correlation coefficient between the SD and high-frequency component [0.832 (SD 0.087)] was significantly (P < 0.05) greater compared with other frequency components. It is suggested that modulations in knee extension force fluctuations are caused by the unique muscle activity in RF during the alternate muscle activity, which augments the high-frequency component of the fluctuations.     

Agonist muscle activity and antagonist muscle co-activity levels during standardized isotonic and isokinetic knee extensions

This study aimed to analyze the effects of the contraction mode (isotonic vs. isokinetic concentric conditions), the joint angle and the investigated muscle on agonist muscle activity and antagonist muscle co-activity during standardized knee extensions. Twelve healthy adult subjects performed three sets of isotonic knee extensions at 40% of their maximal voluntary isometric torque followed by three sets of maximal isokinetic knee extensions on an isokinetic dynamometer. For each set, the mean angular velocity and the total external amount of work performed were standardized during the two contraction modes. Surface electromyographic activity of vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), semitendinosus (ST) and biceps femoris (BF) muscles was recorded. Root mean square values were then calculated for each 10° between 85° and 45° of knee extension (0° = horizontal position). Results show that agonist muscle activity and antagonist muscle co-activity levels are significantly greater in isotonic mode compared to isokinetic mode. Quadriceps activity and hamstrings co-activity are significantly lower at knee extended position in both contraction modes. Considering agonist muscles, VL reveals a specific pattern of activity compared to VM and RF; whereas considering hamstring muscles, BF shows a significantly higher co-activity than ST in both contraction modes. Results of this study confirmed our hypothesis that higher quadriceps activity is required during isotonic movements compared to isokinetic movements leading to a higher hamstrings co-activity.     

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.     

Reliability and performance-dependent variations of muscle function variables during isometric knee extension.

Despite the common use of standardised methods analysing neuromuscular function during knee extension, there is a lack of test-retest reliability studies. Furthermore, for most of the investigated variables it is unknown which changes of values indicate an enhancement of performance. The aim of the present study was to investigate performance-dependent variations of muscle functions during isometric contraction of knee extensors and to examine test-retest reliability of their measurement methods. For test-retest reliability sports students completed three test sessions. Highly skilled athletes, sports students and untrained subjects were investigated to determine the performance-dependent variations. The following variables were analysed: maximal voluntary contraction (MVC), voluntary activation (VA), absolute muscle reaction time (AR), muscle endurance (ME), and EMG frequency analysis (MF) of m. vastus lateralis (VL), m. vastus medialis (VM) and m. rectus femoris (RF). RESULTS TEST-RETEST-RELIABILITY: A high reliability between session 1 vs. 2 and session 2 vs. 3 was shown for MVC (ICC=0.92 and .97), VA (0.92/0.95) and ME (0.87/0.95). ICC in AR (0.23) was low between the first and second session and moderate between the second and third session (0.74). MF of VL, VM and RF showed low ICC between sessions. PERFORMANCE DEPENDENT VARIATIONS: Significant differences in nearly all variables (except VA) were found between trained (athletes and sports students) and untrained subjects.     

Influence of contraction intensity, muscle, and gender on median frequency of the quadriceps femoris.

The purpose of this study was to assess the effects of contraction intensity, gender, and muscle on median frequency of the three superficial portions of the quadriceps femoris muscle. Thirty healthy volunteers were assessed for isometric electromyogram activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees flexion. Subjects performed 5-s isometric contractions at 10, 20, 30, 40, 50, 60, 70, 80, and 90% of the average of three maximal voluntary contractions. Median frequency (f(med)) of the three muscles was assessed through a power spectral analysis performed over 11 consecutive 512-ms epochs overlapping each other by one-half their length. The f(med) for each of the 11 epochs was then determined, followed by calculation of the mean and SD. The major findings of this study demonstrated that overall f(med) was significantly highest for the VL and lowest for the VM, whereas RF f(med) was between that of the other two muscles. Similar findings were observed for f(med) variability as the VL was significantly higher than the VM and RF, with no gender differences or differences between the latter two muscles. The results demonstrate that the largest change in f(med) as a function of contraction intensity occurred for the VL in men (18.6%) and women (7.6%). These findings suggest that muscle fiber-type homogeneity may exist in the VM and RF, which displayed negligible changes in f(med), whereas the VL may possess greater morphological variability.     

Gender and muscle differences in EMG amplitude and median frequency, and variability during maximal voluntary contractions of the quadriceps femoris.

The purpose of this study was to evaluate gender and muscle differences in electromyographic (EMG) amplitude and median frequency mean and standard deviation during maximal voluntary contractions of the quadriceps femoris. Thirty recreationally active volunteers were assessed for isometric EMG activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during three 5-s maximal isometric voluntary contractions (MVCs). Median frequency of the three muscles was assessed through a power spectral analysis (fast Fourier transformation, Hanning window processing, 512 points). The power spectral analysis was performed during the middle 3 s of each contraction over 11 consecutive, 512 ms epochs overlapping each other by half their length (256 ms). The median frequency (F(med)) for each of the 11 windows was determined for each muscle. The mean and standard deviation of the F(med) across the 11 overlapping windows were then calculated for each contraction and muscle. EMG amplitude was determined by calculating the root mean square (RMS-50 ms time constant) over the same contraction period for each muscle. The mean amplitude and standard deviation about the mean value were then determined. A three-factor ANOVA with repeated measures was performed on the calculated F(med) mean and standard deviation values, and RMS standard deviations, to assess any gender, muscle, or trial differences, or interactions. A two-factor (gender by muscle) ANOVA with repeated measures was performed on the RMS mean amplitude for each muscle. Intraclass correlation coefficients (ICCs-2,1), standard errors of measurement (SEMs), and associated 95% confidence intervals were then calculated for maximal quadriceps torque and F(med) for each muscle. The results from this study demonstrated that the VL muscle displayed significantly higher F(med) values than the RF and VM muscles. The RF muscle showed significantly higher F(med) values (mean of 11 overlapping windows) than the VM muscle. Intrasession reliability was found to be high for the calculated mean values (ICC=0.85-0.96), but was shown to be low for variability (ICC=0.13-0.45). The major findings of this study support the notion that the EMG signal is "quasi-random" in nature, as demonstrated by the reproducible F(med) means and unreliable variability.     

Electromyographic activity and rate of muscle fatigue of the quadriceps femoris during cycling exercise in the severe domain

This study compared the activation pattern and the fatigue rate among the superficial muscles of the quadriceps femoris (QF) during severe cycling exercise. Peak oxygen consumption (VO(2)peak) and maximal accumulated oxygen Deficit (MAOD) were established by 10 well-trained male cyclists (27.5 ± 4.1 years, 71.0 ± 10.3 kg, 173.4 ± 6.6 cm, mean VO(2)peak 56.7 ± 4.4 ml·kg·min(-1), mean MAOD 5.7 ± 1.1 L). Muscle activity (electromyographic [EMG] signals) was obtained during the supramaximal constant workload test (MAOD) and expressed by root mean square (RMS) and median frequency (MF slope). The RMS of the QF, vastus lateralis (VL) and vastus medialis (VM) muscles were significantly higher than at the beginning after 75% of exercise duration, whereas for the rectus femoris (RF), this was observed after 50% of exercise duration (p ≤ 0.05). The slope of the MF was significantly higher in the RF, followed by the VL and VM (-3.13 ± 0.52 vs. -2.61 ± 0.62 vs. -1.81 ±0.56, respectively; p < 0.05). We conclude that RF may play an important role in limiting performance during severe cycling exercise.     

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.     

Isometric squat force output and muscle activity in stable and unstable conditions

The purpose of this study was to assess the effect of stable vs. unstable conditions on force output and muscle activity during an isometric squat. Nine men involved in recreational resistance training participated in the investigation by completing a single testing session. Within this session subjects performed isometric squats either while standing directly on the force plate (stable condition, S) or while standing on inflatable balls placed on top of the force plate (unstable condition, U). Electromyography (EMG) was recorded during both conditions from the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and medial gastrocnemius (G) muscles. Results indicated peak force (PF) and rate of force development (RFD) were significantly lower, 45.6% and 40.5% respectively, in the U vs. S condition (p < or = 0.05). Average integrated EMG values for the VL and VM were significantly higher in the S vs. U condition. VL and VM muscle activity was 37.3% and 34.4% less in U in comparison to S. No significant differences were observed in muscle activity of the BF or G between U and S. The primary finding in this investigation is that isometric squatting in an unstable condition significantly reduces peak force, rate of force development, and agonist muscle activity with no change in antagonist or synergist muscle activity. In terms of providing a stimulus for strength gain no discernable benefit of performing a resistance exercise in an unstable condition was observed in the current study.     

Reliability of surface EMG during sustained contractions of the quadriceps.

The purpose of this study was to determine test-retest reliability for median frequency (MDF) and amplitude of surface EMG during sustained fatiguing contractions of the quadriceps. Twenty-two healthy subjects (11 males and 11 females) were tested on two days held one week apart. Surface EMG was recorded from rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) during sustained isometric contractions at 80% and 20% of maximal voluntary contraction (MVC) held to exhaustion. Quadriceps fatigue was described using four measures for both MDF and amplitude of EMG: initial, final, normalized final and slope. For both MDF and amplitude, the initial, final and normalized EMG showed moderate to high reliability for all three muscle groups at both contraction levels (ICC=0.59-0.88 for MDF; ICC=0.58-0.99 for amplitude). Slope of MDF and amplitude was associated with a large degree of variability and low ICCs for the 80% but not the 20% MVC. MDF and amplitude of EMG during sustained contractions of the quadriceps are reproducible; normalized final values of MDF and amplitude show better reliability than slope.