Effects of agonist and antagonist muscle fatigue on muscle coactivation around the knee in pubertal boys.

In many activities the knee joint flexes and extends actively with the involvement of both knee extensor and flexor muscle groups. Consequently the examination of the muscle activity during reciprocal movements may provide useful information on the function of these two muscle groups during fatigued conditions. Therefore, the purpose of this study was to examine the activity of antagonist muscles during a reciprocal isokinetic fatigue test of the knee extensors and flexors. Fifteen healthy pubertal males (age 13.8+/-0.8 years) performed 22 maximal isokinetic concentric efforts of the knee extensors at 60 degrees s(-1). The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) was recorded using surface electrodes. The motion ranged from 100 to 0 degrees of knee flexion. The average moment and average EMG (AEMG) at 10-30 degrees, 31-50 degrees, 51-70 degrees and 71-90 degrees angular position intervals were calculated for each repetition. Twenty efforts were further analyzed. Two-way repeated measures analysis of variance (ANOVA) tests indicated a significant decline of moment during the test (p<0.025). The VM and VL AEMG at longer muscle lengths increased significantly as the test progressed whereas the AEMG at short muscle lengths (10-30 degrees ) did not significantly change. The agonist AEMG of BF during the first repetition demonstrated a significant increase after the ninth repetition (p<0.025). The antagonist AEMG of all muscles did not change significantly during the test. These results indicate that there is consistent antagonist activity during both extension and flexion phases of an isokinetic reciprocal fatigue test. It can be concluded that during an isokinetic reciprocal fatigue test, both knee extensors and flexors are fatigued. However, this condition does not have a significant effect on the EMG patterns of these muscles when acting as antagonists during the test.     

Agonist-antagonist common drive during fatiguing knee extension efforts using surface electromyography.

AIM: This study examined the electromyographic (EMG) activity of knee extensor agonists and a knee extensor antagonist muscle during fatiguing isometric extensions across a range of force levels. METHODS: Five female subjects performed isometric knee extensions at 25%, 50%, 75% and 100% of their maximal voluntary contraction (MVC) with the knee flexed to 75 degrees. Surface EMG (SEMG) was recorded with bipolar electrodes from the vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF) and biceps femoris (BF) and the root-mean-squared (RMS) amplitude and the percentage frequency compression of these recordings were calculated. Commonality and cross talk between recordings were also examined. RESULTS: Cross talk between recordings was deemed negligible despite significant levels of commonality between the agonist and antagonist SEMG, which was attributed to common drive. SEMG RMS amplitude increased significantly for all muscles during the 25%, 50%, 75% MVC knee extensions until task failure, and decreased significantly for 100% MVC. The frequency spectrum of the SEMG compressed significantly for all muscles and % MVC levels. The VM, VL and BF SEMG recordings responded similarly to fatigue. The RF's frequency spectrum compressed to a significantly higher degree. CONCLUSIONS: The VM, VL, RF, and BF fatigue in parallel, with high similarity between VM, VL and BF, giving support to the concept of a shared agonist-antagonist motoneuron pool.     

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.     

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.     

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.     

Power output,mechanomyographic, and electromyographic responses to maximal,concentric, isokinetic muscle actions in men and women

The purpose of this study was to examine the responses of peak torque (PT), mean power output (MP), mechanomyographic (MMG) and electromyographic (EMG) amplitudes, and mean power frequencies (MPFs) of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) in men and women during dynamic muscle actions. Twelve women (mean +/- SD age = 22 +/- 3 years) and 11 men (22 +/- 3 years) performed maximal, concentric, isokinetic leg extensions at velocities of 60, 120, 180, 240, and 300 degrees x s(-1) on a Cybex 6000 dynamometer. Piezoelectric MMG-recording sensors and bipolar surface EMG electrodes were placed over the VL, RF, and VM muscles. No sex-related differences were found among the velocity-related patterns for PT, MP, MMG amplitude, MMG MPF, or EMG MPF. There were, however, sex-related differences in the patterns of EMG amplitude across velocity. The results indicated similar velocity-related patterns of increase of MP and MMG amplitude for all 3 muscles and of EMG amplitude for the VL and VM in the women. Velocity-related decreases (p 0.05) across velocity. MMG MPF increased (p < or = 0.05) only between 240 and 300 degrees x s(-1). Overall, these findings suggested that there were sex- and muscle-specific, velocity-related differences in the associations among motor unit activation strategies (EMG amplitude and MPF) and the mechanical aspects of muscular activity (MMG amplitude and MPF). With additional examination and validation, however, MMG may prove useful to practitioners for monitoring training-induced changes in muscle power output.     

Agonist versus antagonist muscle fatigue effects on thigh muscle activity and vertical ground reaction during drop landing

BackgroundAgonist and antagonist co-activation plays an important role for stabilizing the knee joint, especially after fatigue. However, whether selective fatigue of agonists or antagonist muscles would cause different changes in muscle activation patterns is unknown.HypothesisKnee extension fatigue would have a higher influence on landing biomechanics compared with a knee flexion protocol.Study designRepeated-measures design.MethodsTwenty healthy subjects (10 males and 10 females) performed two sets of repeated maximal isokinetic concentric efforts of the knee extensors (KE) at 120° s^?1 until they could no longer consistently produce 30% of maximum torque. On a separate day, a similar knee flexion (KF) fatigue protocol was also performed. Single leg landings from 30 cm drop height were performed before, in the middle and after the end of the fatigue test. The mean normalized electromyographic (EMG) signal of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF) and gastrocnemius (GAS) at selected landing phases were determined before, during and after fatigue. Quadriceps:hamstrings (Q:H) EMG ratio as well as sagittal hip and knee angles and vertical ground reaction force (GRF) were also recorded.ResultsTwo-way analysis of variance designs showed that KE fatigue resulted in significantly lower GRF and higher knee flexion angles at initial contact while maximum hip and knee flexion also increased (p < 0.05). This was accompanied by a significant decline of BF EMG, unaltered EMG of vastii and GAS muscles and increased Q:H ratio. In contrast, KF fatigue had no effects on vGRFs but it was accompanied by increased activation of VM, BF and GAS while the Q:H increased during before landing and decreased after impact.ConclusionFatigue responses during landing are highly dependent on the muscle which is fatigued.     

Quadriceps cross-sectional area changes in young healthy men with different magnitude of Q angle

Knee pain and dysfunction have been often associated with an ineffective pull of the patella by the vastus medialis (VM) relative to the vastus lateralis (VL), particularly in individuals with knee joint malalignment. Such changes in muscular behavior may be attributed to muscle inhibition and/or atrophy that precedes the onset of symptoms. The aim of this study was to investigate possible effects of knee joint malalignment, indicated by a high quadriceps (Q) angle (HQ angle >15 degrees ), on the anatomic cross-sectional area (aCSA) of the entire quadriceps and its individual parts, in a group of 17 young asymptomatic men compared with a group of 19 asymptomatic individuals with low Q angle (LQ angle <15 degrees ). The aCSA of the entire quadriceps (TQ), VM, VL, vastus intermedius (VI), rectus femoris (RF), and patellar tendon (PT) were measured during static and dynamic magnetic resonance imaging (MRI) with the quadriceps relaxed and under contraction, respectively. A statistically significant lower aCSA was obtained in the HQ angle group, compared with the LQ angle group, for the TQ, VL, and VI in both static (TQ = 9.9%, VL = 12.9%, and VI = 9.1%; P < 0.05) and dynamic imaging (TQ = 10.7%, P < 0.001; VL = 13.4%, P < 0.01; and VI = 9.8%, P < 0.05) and the aCSA of the VM in dynamic MRI (11.9%; P < 0.01). The muscle atrophy obtained in the HQ angle group may be the result of a protective mechanism that inhibits and progressively adapts muscle behavior to reduce abnormal loading and wear of joint structures.     

Criterion validation of surface EMG variables as fatigue indicators using peak torque: a study of repetitive maximum isokinetic knee extensions.

A number of studies have been published that have used variables of the electromyogram (EMG) power spectrum during dynamic exercise. Despite these studies there is a shortage of studies of the validity of surface EMG registrations during repetitive dynamic contractions with respect to fatigue. The aim of this study was to investigate if the surface EMG variables mean frequency (MNF [Hz]) and the signal amplitude (RMS [microV]) are valid indicators of muscular fatigue (defined as "any exercise-induced reduction in the capacity to generate force or power output") during maximum repeated isokinetic knee extensions (i.e. criterion validity using peak torque). Twenty-one healthy volunteers performed 100 isokinetic knee extensions at 90 degrees s(-1). EMG signals were recorded from the vastus lateralis, the rectus femoris and the vastus medialis of the right thigh by surface electrodes. MNF and RMS of the EMG together with peak torque (PT [Nm]) were determined for each contraction. MNF showed consequently higher correlation coefficients with PT than RMS did. Positive correlations generally existed between MNF and PT. The majority of the subjects had positive correlations between RMS and PT (i.e. decreases both in PT and in RMS).In conclusion, at the individual level MNF generally - in contrast to RMS - showed good criterion validity with respect to biomechanical fatigue during dynamic maximum contractions.     

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.     

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.     

膝骨性关节炎患者登梯时下肢肌肉活动和膝关节负荷的分析

目的:分析膝骨性关节炎患者(KOA)登梯时下肢肌群肌电活动与关节角冲量与正常人的差异,为康复方案设计提供生物力学参考。方法:采用Qualisys三维运动分析系统以及Delsys无线表面肌电系统对招募10名符合纳排标准的膝骨性关节炎患者和10名正常人进行登梯活动的步态检测,采用下肢肌群均方根值、股内外侧肌协同收缩比值、股二头肌和股外侧肌共同活动比值和髋、膝关节在冠状面和矢状面上角冲量对比分析与两组登梯时下肢肌群收缩模式对关节负荷的影响。结果:与正常对照相比,上梯时膝骨性关节炎患者股直肌均方根值RMS(Root Mean Square)增大(P0.05),膝骨性关节炎患者股内外侧肌收缩均方根值比值(RMS(Vastus Medialis)VM/(Vastus Lateralis)VL)减小(P0.05),膝骨性关节炎患者腘绳肌与股外侧肌收缩比值(RMS(Biceps Femoris)BF/VL增大(P0.05)。下梯时,膝骨性关节炎患者股直肌均方根值(RMS)增大(P0.05),臀大肌均方根值(RMS)减小(P0.05),股内外侧肌收缩均方根比值(RMS VM/VL)减小(P0.05)。上梯时,膝骨性关节炎患者髋、膝关节冠状面上的关节角冲量大于正常人(P0.05),膝关节在矢状面上关节角冲量大于正常组(P0.05),下梯髋、膝关节冠状面、矢状面上的角冲量无统计学差异(P0.05)。KOA组VM/VL、BF/VL与膝关节在冠状面和矢状面上的角冲量的改变没有直接的相关性(P0.05)。结论:膝骨性关节炎患者在登梯活动时股直肌的收缩活动增加,股内外侧肌的协同收缩下降,主动肌与拮抗肌的共同收缩增加,膝骨性关节炎患者在面对登梯活动时下肢肌群选择性激活和高激活状态协调一致,促进关节稳定。虽然下肢神经肌肉的收缩模式和膝关节负荷之间没有直接的相关性,可能是对膝关节负荷产生影响的生物力学因素较多,神经肌肉的收缩模式只是部分影响因素,后续将增加其他生物力学因素进一步研究。     

Knee and Hip Joint Kinematics Predict Quadriceps and Hamstrings Neuromuscular Activation Patterns in Drop Jump Landings

PurposeThe purpose was to assess if variation in sagittal plane landing kinematics is associated with variation in neuromuscular activation patterns of the quadriceps-hamstrings muscle groups during drop vertical jumps (DVJ).MethodsFifty female athletes performed three DVJ. The relationship between peak knee and hip flexion angles and the amplitude of four EMG vectors was investigated with trajectory-level canonical correlation analyses over the entire time period of the landing phase. EMG vectors consisted of the {vastus medialis(VM),vastus lateralis(VL)}, {vastus medialis(VM),hamstring medialis(HM)}, {hamstring medialis(HM),hamstring lateralis(HL)} and the {vastus lateralis(VL),hamstring lateralis(HL)}. To estimate the contribution of each individual muscle, linear regressions were also conducted using one-dimensional statistical parametric mapping.ResultsThe peak knee flexion angle was significantly positively associated with the amplitudes of the {VM,HM} and {HM,HL} during the preparatory and initial contact phase and with the {VL,HL} vector during the peak loading phase (p<0.05). Small peak knee flexion angles were significantly associated with higher HM amplitudes during the preparatory and initial contact phase (p<0.001). The amplitudes of the {VM,VL} and {VL,HL} were significantly positively associated with the peak hip flexion angle during the peak loading phase (p<0.05). Small peak hip flexion angles were significantly associated with higher VL amplitudes during the peak loading phase (p = 0.001). Higher external knee abduction and flexion moments were found in participants landing with less flexed knee and hip joints (p<0.001).ConclusionThis study demonstrated clear associations between neuromuscular activation patterns and landing kinematics in the sagittal plane during specific parts of the landing. These findings have indicated that an erect landing pattern, characterized by less hip and knee flexion, was significantly associated with an increased medial and posterior neuromuscular activation (dominant hamstrings medialis activity) during the preparatory and initial contact phase and an increased lateral neuromuscular activation (dominant vastus lateralis activity) during the peak loading phase.     

Effect of fatigue on hamstring coactivation during isokinetic knee extensions

We examined the effect of fatigue of the quadriceps muscles on coactivation of the hamstring muscles and determined if the response is different between two isokinetic speeds in ten males and ten females with no history of knee pathology. Electromyographic data were recorded from the vastus lateralis and biceps femoris muscles during 50 maximal knee extensions at isokinetic speeds of 1.75 rad · s⁻¹ (100° · s⁻¹) and 4.36 rad · s⁻¹ (250° · s⁻¹). A greater degree of coactivation was apparent at the higher speed, but the increase in coactivation of the hamstring muscles was similar at both speeds. The results revealed that: (1) coactivation is greater at a higher isokinetic speed, and (2) coactivation increases during fatigue, but the rate of increase is independent of contraction velocity. Accepted: 15 June 1998     

Torque-related changes in mechanomyographic intensity patterns for the superficial quadriceps femoris muscles

The purpose of this study was to investigate changes in mechanomyographic (MMG) intensity patterns for the vastus lateralis (VL), rectus femoris (RF) and vastus medialis (VM) during submaximal to maximal concentric isokinetic, eccentric isokinetic and isometric muscle actions of the leg extensors. Eleven men (mean ± SD age = 20.1 ± 1.1 years) performed concentric, eccentric and isometric muscle actions of the dominant leg extensors on 3 separate days. Surface MMG signals were detected from the VL, RF and VM, processed with a wavelet analysis and examined with a trend plot. The results indicated that the trend plot was capable of tracking systematic changes in MMG amplitude and frequency with an increase in torque. However, these changes were statistically significant in only 26% of the cases. There were also no consistent differences between muscles or contraction types for the significance of the trend plots.     

Isokinetic elbow flexion and coactivation following eccentric training.

The influence of an eccentric training on torque/angular velocity relationships and coactivation level during maximal voluntary isokinetic elbow flexion was examined. Seventeen subjects divided into two groups (Eccentric Group EG, n = 9 Control Group CG, n = 8) performed on an isokinetic dynamometer, before and after training, maximal isokinetic elbow flexions at eight angular velocities (from - 120 degrees s(-1) under eccentric conditions to 240 degrees s(-1) under concentric conditions), and held maximal and submaximal isometric actions. Under all conditions, the myoelectric activities (EMG) of the biceps and the triceps brachii muscles were recorded and quantified as the RMS value. Eccentric training of the EG consisted of 5x6 eccentric muscle actions at 100 and 120% of one maximal repetition (IRM) for 21 sessions and lasted 7 weeks. In the EG after training, torque was significantly increased at all angular velocities tested (ranging from 11.4% at 30 degrees (s-1) to 45.5% at - 120 degrees s(-1)) (p < 0.05). These changes were accompanied by an increase in the RMS activities of the BB muscle under eccentric conditions (from - 120 to - 30 degrees (s-1)) and at the highest concentric angular velocities (180 and 24 degrees s(-1)) (p < 0.05). The RMS activity of the TB muscle was not affected by the angular velocity in either group for all action modes. The influence of eccentric training on the torque gains under eccentric conditions and for the highest velocities was attributed essentially to neural adaptations.     

Mechanomyographic and electromyographic responses to repeated concentric muscle actions of the quadriceps femoris.

In comparison to isometric muscle action models, little is known about the electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) responses to fatiguing dynamic muscle actions. Simultaneous examination of the EMG and MMG amplitude and MPF may provide additional insight with regard to the motor control strategies utilized by the superficial muscles of the quadriceps femoris during a concentric fatiguing task. Thus, the purpose of this study was to examine the EMG and MMG amplitude and MPF responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) during repeated, concentric muscle actions of the dominant leg. Seventeen adults (21.8+/-1.7 yr) performed 50 consecutive, maximal concentric muscle actions of the dominant leg extensors on a Biodex System 3 Dynamometer at velocities of 60 degrees s(-1) and 300 degrees s(-1). Bipolar surface electrode arrangements were placed over the mid portion of the VL, RF, and VM muscles with a MMG contact sensor placed adjacent to the superior EMG electrode on each muscle. Torque, MMG and EMG amplitude and MPF values were calculated for each of the 50 repetitions. All values were normalized to the value recorded during the first repetition and then averaged across all subjects. The cubic decreases in torque at 60 degrees s(-1) (R2 = 0.972) and 300 degrees s(-1) (R2 = 0.931) was associated with a decline in torque of 59+/-24% and 53+/-11%, respectively. The muscle and velocity specific responses for the MMG amplitude and MPF demonstrated that each of the superficial muscles of the quadriceps femoris uniquely contributed to the control of force output across the 50 repetitions. These results suggested that the MMG responses for the VL, RF, VM during a fatiguing task may be influenced by a number of factors such as fiber type differences, alterations in activation strategy including motor unit recruitment and firing rate and possibly muscle wisdom.     

Test-retest reliability of EMG and peak torque during repetitive maximum concentric knee extensions.

The aim of this study was to investigate the reliability of peak torque and surface electromyography (EMG) variable's root mean square (RMS) and mean frequency (MNF) during an endurance test consisting of repetitive maximum concentric knee extensions. Muscle fatigue has been quantified in several ways, and in isokinetic testing it is based on a set of repetitive contractions. To assess test-retest reliability, two sets of 100 dynamic maximum concentric knee extensions were performed using an isokinetic dynamometer. The two series were separated by 7-8 days. The subjects relaxed during the passive flexion phase. Twenty (10 men and 10 women) clinically healthy subjects volunteered.Peak torque and EMG from rectus femoris, vastus medialis, vastus lateralis and biceps femoris were recorded. RMS and MNF were calculated from the EMG signal. The reliability was calculated with intraclass correlation coefficient ICC (1.1) and standard error of measurements (SEM). The reliability of peak torque was good (ICC=0.93) and SEM showed low values. ICC was good for absolute RMS of rectus femoris (ICC>/=0.80), vastus medialis (ICC>/=0.88) and vastus lateralis (ICC>/=0.82) and MNF of rectus femoris (ICC>/=0.82) and vastus medialis (ICC>/=0.83). Peak torque, and MNF and RMS of rectus femoris and vastus medialis are reliable variables obtained from an isokinetic endurance test of the knee extensors.     

Influence of angular velocity and movement frequency on development of fatigue in repeated isokinetic knee extensions

Six sedentary students, six orienteers, and six soccer players were each subjected to 15 tests, comprising 120 s of repeated, maximal isokinetic knee extensions. The tests differed with respect to movement velocity (30 degrees.s-1, 120 degrees.s-1, and 300 degrees.s-1), and movement frequency (5 at each velocity). At a certain velocity, a rectilinear relationship was found between muscular performance intensity (expressed either as average power output or as exercise time ratio) and development of fatigue (expressed either as an absolute or as a fractional decline in work output). Significant inter-velocity differences existed between the slopes of these lines at some combinations of performance and fatigue expressions. Only tendencies towards a difference in x-intercept values were found. This x-intercept value can be taken as a measure of the greatest attainable intensity level of performance without the development of fatigue. This suggestion is valuable both in basic physiological research, and as a possible criterion for optimization of muscular performance. At a given exercise time ratio, increasing movement velocity produced increasing fatigue. However, at a given muscular power output--above 15 W approximately--fatigue developed to a greater extent at the low velocity than at the two higher ones, which did not differ significantly. Substantial individual variation was seen in the positions of the low-, medium-, and high-velocity lines. These variations did not depend on the training background. This implies that the validity of using single-velocity, single-frequency tests in determining isokinetic endurance is doubtful.     

Muscle activation profiles about the knee during Tai-Chi stepping movement compared to the normal gait step.

The purpose of this study was to investigate knee muscle activity patterns in experienced Tai-Chi (TC) practitioners during normal walking and TC stepping. The electromyographic (EMG) activity of vastus lateralis (VL), vastus medialis (VM), bicep femoris (BF), and gastrocnemius (GS) muscles of 11 subjects (five females and six males) during the stance phase of normal walking was compared to stance phase of a TC step. Knee joint motion was also monitored by using an Optotrak motion analysis system. Raw EMG was processed by root-mean-square (RMS) technique using a time constant of 50 ms, and normalized to maximum of voluntary contraction for each muscle, referred to as normalized RMS (nRMS). Peak nRMS and co-contraction (quantified by co-contraction index) during stance phase of a gait cycle and a TC step were calculated. Paired t-tests were used to compare the difference for each muscle group peak and co-contraction pair between the tasks. The results showed that only peak values of nRMS in quadriceps and co-contraction were significantly greater in TC stepping compared to normal walking (Peak values of nRMS for VL were 26.93% for normal walking and 52.14% for TC step, p=0.001; VM are 29.12% for normal walking and 51.93% for TC stepping, p=0.028). Mean co-contraction index for VL-BF muscle pairs was 13.24+/-11.02% during TC stepping and 9.47+/-7.77% in stance phase of normal walking (p=0.023). There was no significant difference in peak values of nRMS in the other two muscles during TC stepping compared to normal walking. Preliminary EMG profiles in this study demonstrated that experienced TC practitioners used relatively higher levels of knee muscle activation patterns with greater co-contraction during TC exercise compared to normal walking.