A correlogram analysis of the activity in the rostral ventromedial medulla of awake rats and in rats anesthetized with ketamine or pentobarbital following the administration of morphine

The present study was designed to determine the relative contribution of the gastrocnemius muscle to isometric plantar flexor torque production at varying knee angles, while investigating the activation of the gastrocnemius muscle at standardised non-optimal lengths. Voluntary plantar flexor torque, supramaximally stimulated twitch torque and myoelectric activity (EMG) from the triceps surae were measured at different knee angles. Surface and intra-muscular EMG were recorded from the soleus muscle and the medial and lateral heads of the gastrocnemius muscle in 10 male subjects. With the ankle angle held constant, knee angle was changed in steps of 30° ranging from 180° (extended) to 60° (extreme flexion), while voluntary torque from a 5-s contraction was determined at 10 different levels of voluntary effort, ranging from 10% of maximal effort to maximal effort. To assess effort, supramaximal twitches were superimposed on all voluntary contractions, and additionally during rest. Maximal plantar flexor torque and resting twitch torque decreased significantly in a sigmoidal fashion with increasing knee flexion to 60% of the maximum torque at 180° knee angle. For similar levels of voluntary effort, the EMG root mean square (RMS) of gastrocnemius was less with increased knee flexion, whereas soleus RMS remained unchanged. From these data, it is concluded that the contribution of gastrocnemius to plantar flexor torque is at least 40% of the total torque in the straight leg position. The decrease of gastrocnemius EMG RMS with decreasing muscle length may be brought about by a decrease in the number of fibres within the EMG electrode recording volume and/or impaired neuromuscular transmission.     

Differences in the electromyographic activity of the hamstring muscles during maximal eccentric knee flexion

This study investigated the effects of the knee joint angle and angular velocity on hamstring muscles’ activation patterns during maximum eccentric knee flexion contractions. Ten healthy young males (23.4 ± 1.3 years) performed eccentric knee flexion at constant velocities of 10, 60, 180, and 300 deg/s in random order. The eccentric knee flexion torque and the surface electromyographic (EMG) activity of the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles were measured. The results of torque during 10 deg/s were lower than the faster velocities. No significant change was found in eccentric torque output and the EMG amplitude with change in the faster test velocities, although those values showed a decreasing tendency as the knee approached extension. Furthermore, the EMG amplitude of the BF decreased significantly as the knee approached extension, although the EMG activity of the ST and SM remained constant. These results suggest that the neural inhibitory mechanism might be involved in decreasing in maximal voluntary force and hamstring muscles activation toward the knee extension during high-velocity eccentric movement and therefore subjects have difficulties to maintain high eccentric force level throughout the motion. Moreover, the possible mechanism reducing the BF muscle activation as the knee approaches extension was architectural differences in the hamstring muscles, which might reflect each muscle’s function.     

Activation of agonist and antagonist muscles at different joint angles during maximal isometric efforts

The purpose of this study was to investigate the influence of different angles of the knee joint on the activation level of an agonist (quadriceps femoris muscle) and antagonist (biceps femoris muscle) from electromyographic activities and activation levels (twitch interpolation). Isometric torque measurements were performed on 23 healthy subjects at 10° intervals between 40° and 110° of knee joint flexion. Superimposed twitches at maximal voluntary contraction were applied and the voluntary activation estimated. To quantify the antagonist muscle activity, we normalized its integrated EMG (iEMG) value at each joint angle with respect to its iEMG value at the same angle when acting as an agonist at maximal effort. The activation levels at the knee-flexed position (80–110°) were higher than that at the knee-extended position (40–70°). The co-activation levels at 90, 100, and 110° were significantly higher than that the other knee angle. These results suggest that the activation level of an agonist (quadriceps femoris) muscle and the co-activation level of an antagonist (biceps femoris) muscle were higher in longer muscles than in shorter muscles. It was also concluded that the risk of knee injuries could be reduced by applying these mutual relationships between activation levels of agonist and antagonist muscles.     

Relative torque contribution of vastus medialis muscle at different knee angles

Aim: We investigated the relative contribution of the vastus medialis (VM) muscle to total isometric knee extension torque at 10°, 30°, 60° and 90° knee flexion. In the past a more prominent role of the VM muscle at more extended knee angles has been put forward. However, different components of the quadriceps muscle converge via a common distal tendon. We therefore hypothesized that the relative contribution of the VM to total knee extension torque would be similar across angles. Methods: At each knee angle the EMG isometric torque relations [20%, 25%, 30%, 35% maximal voluntary contraction (MVC)] of the rectus femoris (RF), vastus lateralis (VL) and VM muscle were established in 10 healthy male subjects; rectified surface EMG was normalized to M‐wave area. Subsequently, the VM was functionally eliminated by selective electrical surface stimulation with occluded blood flow. Results: There was no evidence for preferential activation of VM at any of the knee angles. Following VM elimination, total knee extension torque during maximal femoral nerve stimulation (three pulses at 300Hz) at 10°, 30°, 60° and 90°, respectively, decreased (P < 0.05) to (mean ± SD): 75.7 ± 12.2, 75.1 ± 9.3, 78.2 ± 7.2 and 76.0 ± 5.8% (P > 0.05 among knee angles). In addition, during voluntary contractions at 20% MVC the increases in torque output of RF and VL compensating for the loss of VM function were calculated from the increases in EMG and found to be similar (P > 0.05) at 10°, 30°, 60° and 90° values (%MVC), respectively, were: 9.1 ± 6.8, 7.5 ± 2.9, 5.9 ± 3.7 and 6.9 ± 3.4. Conclusion: The present findings support our hypothesis that the VM contributes similarly to total knee extension torque at different knee angles.     

Fatigue induced changes in phasic muscle activation patterns for fast elbow flexion movements

The present study investigated how muscle fatigue influences single degree-of-freedom elbow flexion movements and their associated patterns of phasic muscle activation. Maximal unfatigued voluntary isometric elbow flexor and extensor joint torque was measured at the beginning of the experiment. Subjects then performed elbow flexion movements over two distances as fast as possible, and movements over the longer distance at an intentionally slower speed. The slower speed was close to what would become the maximal speed in the fatigued state. Subjects then performed a fatiguing protocol of 20 sustained isometric flexion contractions of 25 s duration with 5 s rest at 50% maximal unfatigued voluntary force. After a recovery period they repeated the movements. The fatigue protocol was successful in inducing muscle fatigue, the evidence being decreased isometric maximal joint torque of over 20%. Fatigued movements had lower peak muscle torque and speed. Our principal finding was of changes in the timing of the phasic patterns of fatigued muscle activation. There was an increase in the duration of the agonist burst and a delay in the timing of the antagonist muscle as measured by the centroid of the EMG signals. We conclude that these changes serve as partial but incomplete, centrally driven compensation for fatigue induced changes in muscle function. An additional, unexpected finding was how small an effect fatigue had on movement performance when using a recovery time of 10 min that is long enough to allow muscle membrane conduction velocity to return to normal. This raises questions concerning the behavioral significance of classical laboratory studies of human fatigue mechanisms.     

Motor impairment in the human hand following eccentric exercise

Motor impairment was induced by having subjects perform two sets of 50 maximal contractions, using the first dorsal interosseus (FDI) muscle to abduct the index finger, while the muscle was being stretched. Tests were conducted prior to the exercise (pre-exercise) and 24 h following the exercise (post-exercise). There were declines of 19% in maximal abduction torque and 15% in maximal flexion torque at the metacarpaphalangeal joint, during isometric contraction post-exercise compared to pre-exercise. The ability to stabilize the metacarpophalangeal joint about the abduction/adduction axis was reduced by 14% post-exercise, and the variability in tracking an isometric torque target increased by 30%. There was a decrement of 7%–10% in the median frequency of the power density spectrum of FDI electromyogram (EMG) throughout a 60 s maintained abduction at 50% maximal voluntary contraction. The mean rectified EMG, on the other hand, increased by 100%–175% for torque levels below 40% of maximal voluntary contraction, post-exercise. The results were consistent with preferential injury of type II muscle fibres in FDI. Although non-exercised synergist muscles appeared to be inhibited during maximal voluntary flexion, there was evidence that they compensated for injured FDI muscle fibres during maintained contraction at sub-maximal flexion torque. Accepted: 19 September 2000     

Quadriceps muscle function characteristics in severely obese and nonobese adolescents

The purpose of this cross-sectional study was to compare quadriceps muscle strength and fatigue between severely obese (body mass index 34 kg/m²) and nonobese adolescents. Maximal isokinetic torque and angle of peak torque as well as isometric torque at short (40° of knee flexion) and long (80° of knee flexion) muscle length were measured using an isokinetic dynamometer. Muscle fatigue was quantified as the percent torque loss during an isokinetic voluntary protocol and an electrical stimulation isometric protocol. Obese adolescents produced greater absolute isokinetic (+16%; P < 0.05) and isometric torque at short (+25%; P < 0.01) but not at long muscle length (P > 0.05) compared to their lean counterparts. The angle of peak torque was significantly lower in obese than in nonobese subjects (−11%; P < 0.05), i.e., obese produced their maximal strength at shorter muscle length. Isokinetic and isometric torque normalized to the fat-free mass were not significantly different between the two groups. No significant difference in voluntary and stimulated torque loss was observed between groups. Muscle strength per unit of fat-free mass and muscle fatigue were similar in the obese and nonobese adolescents tested in this study, therefore suggesting that obesity has little or no effect on quadriceps muscle function characteristics. On the other hand, it remains to be confirmed whether the observed quadriceps muscle length specificity contributes to the reduced functional capacity of obese adolescents during complex motor tasks involving deep knee flexion (squatting, kneeling).     

Muscle fatigue and electromyographic changes are not different in women and men matched for strength

Previous gender comparisons of muscle performance have overlooked differences in absolute strength and have studied men and women at similar percentages of their maximal voluntary contraction (MVC). The purpose of this study was to examine agonist and antagonist muscle activation during submaximal fatigue, in men and women matched for strength. We compared plantar flexion muscle performance during a 10-min submaximal (20% MVC) fatigue protocol in ten healthy men and ten healthy women matched for plantar flexor MVC torque output [156.0 (17.6) N m for men and 152.9 (21.7) N m for women, mean (SD), P>0.05]. The results showed that after the fatigue protocol the torque output and the averaged electromyogram (EMG) amplitude of all muscles examined decreased significantly in both genders. The submaximal torque was sustained at the requested level (20% MVC) during the fatiguing contraction by increasing motor unit activity as indicated by the EMG. However, the post hoc tests revealed no significant differences between the two genders in any of the aforementioned tests. The similarity of agonist and antagonist muscle activation during a sustained submaximal contraction in both men and women suggests that the differences in fatigability between the two genders are diminished when the absolute force production is similar.     

连续疲劳干预对膝关节主被动生物力学特性的影响

目的 探究连续疲劳干预对膝关节主被动生物力学特性的影响。方法 招募27位健康大学生在等速力量测试系统上进行疲劳实验。疲劳实验包括3个疲劳周期,每个疲劳周期至少30次股四头肌连续等长收缩。对比不同疲劳周期的最大被动力矩、平均最大自主收缩（maximum voluntary contraction,MVC）力矩、股外侧肌和股二头肌肌电积分值以及协同收缩指数（co-contraction index,CI）。结果 疲劳周期3后,最大被动力矩显著下降（P<0.05）。疲劳周期1、2、3过程内平均MVC力矩、股外侧肌和股二头肌肌电积分值都显著下降（P<0.05）,然而二者CI全程无显著变化（P>0.05）。结论 股四头肌等长收缩训练剂量为90次的连续疲劳干预会显著影响膝关节主被动生物力学特性。疲劳干预后,膝关节抵抗被动屈伸能力下降。并且随着疲劳干预进程,股四头肌主动收缩能力下降,股四头肌和腘绳肌肌肉激活也下降,但二者肌群协同收缩水平不变。这种温和的肌群协同收缩模式有益于保持良性膝关节力学加载环境。研究结果有助于理解疲劳后膝关节主被动生物力学特性。     

Quadriceps voluntary activation at different joint angles measured by two stimulation techniques

People are able to fully activate their quadriceps at mid-length during a brief isometric contraction but it is uncertain whether this is the case at other muscle lengths. With the twitch superimposition technique for determining levels of voluntary activation (VA), the muscle may be stimulated through the intramuscular branches of the nerve or via the nerve trunk itself. The former technique is easier to use, but different populations of motor units may be stimulated if the joint position is changed to alter muscle length. The purpose of this study was to investigate quadriceps VA at a range of knee joint angles using both magnetic stimulation of the motor nerve and percutaneous electrical stimulation over the muscle belly. Eight healthy subjects (six females, mean age 29 years) performed maximal voluntary contractions of the quadriceps at knee joint angles at 10–110° of flexion. Surface electromyography (EMG) of quadriceps and hamstrings was recorded as an indication of the amount of muscle activity. Nearly all subjects showed >95% VA at all joint angles. VA did not vary with joint angle nor were there significant differences between the two stimulation techniques. Similarly, there was no significant effect of knee joint angle on the EMG activity of either muscle group. These findings indicate that VA of the quadriceps during a brief isometric contraction is not affected by muscle length and can be measured by either stimulation technique. Electronic Publication     

The influence of torque and velocity on erector spinae muscle fatigue and its relationship to changes of electromyogram spectrum density

The influence of contraction force and velocity during isokinetic contractions on the development of fatigue in the erector spinae muscle was studied. Seven male subjects performed a series of 250 contractions at 25% and 50% of their isometric maximal voluntary contraction (MVC) at 40 and 80°·s^–1. Fatigue defined as a decrease of the contractile capacity of the muscles was studied by means of a 15-s maximal test-contraction following the exercise. Both the initial force and the force decrement during the test-contraction were studied. Surface electromyogram (EMG) signals of the main tracts of the erector spinae muscle were recorded. The frequency content was studied by calculating the zero-crossing rate for the signals obtained during dynamic contractions and by means of fast Fourier transformation for the test contraction. After the 50% MVC dynamic contractions the initial force during the postexercise test-contraction was significantly lower than after the 25% MVC contractions. No significant influence of contraction velocity on fatigue development was found. The force decrement during the test-contraction did not depend on the experimental conditions. The EMG amplitude indicated that the subjects were better able to relax their muscles during the counter movement (flexion) at high forces and high velocities compared to the other experimental conditions. The frequency content of the EMG signals during the dynamic contractions and the postexercise test-contraction showed only very weak relationships with fatigue. Therefore, spectrum EMG parameters as determined in the present study do not seem suitable as indicators of muscle fatigue as a consequence of dynamic contractions of trunk extensor muscles.     

Use of a Kin-Com dynamometer to study the stretch-shortening cycle during plantar flexion

Summary The aim of this study was to evaluate the Kin-Com II dynamometer in the study of the stretch-shortening cycle (a concentric muscle action preceded by an eccentric muscle action). Measurements were made of plantar flexion at different angular velocities (120° · s^–1 and 240° · s^–1) with the knee at two different angles (0° and 90°). Ten healthy women ranging in age from 22 to 41 years were studied. Torque values were recorded simultaneously with surface electromyograms (EMG); maximal voluntary concentric torque values were recorded and, after a short rest, the torque values of the concentric action which followed immediately after an eccentric action of the same velocity, both with maximal effort. Mean values were taken at different ankle positions and also averaged over different ranges. A concentric action preceded by an eccentric action generated a torque value on an average about 100% larger than a concentric action alone. The EMG activity was lower or unchanged. It was concluded that the present method could be useful in the study of the stretch-shortening cycle in plantar flexion and in the testing of the behaviour of the elastic components in people with disabilities in the lower limbs.     

Conventionally assessed voluntary activation does not represent relative voluntary torque production

The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT), which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects (n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship between superimposed and voluntary torque. This includes very high contraction intensities (90–100%VA), which are difficult to consistently obtain in regular healthy subjects (VA of ∼90%). Subjects were tested at 30, 60, and 90° knee angles on two experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation (triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles. Maximal VA was similar and very high across knee angles: 97 ± 2.3% (mean ± SD). At high contraction intensities, the increase in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque increased from 79.6 ± 6.1 to 100%MVC, superimposed torque decreased from 8.5 ± 2.6 to 2.8 ± 2.3% of resting triplet. Therefore, an increase in VA of 5.7% (from 91.5 ± 2.6 to 97 ± 2.3%) coincided with a much larger increase in voluntary torque (20.4 ± 6.1%MVC) and EMG (33.9 ± 6.6%max). Moreover, a conventionally assessed VA of 91.5 ± 2.6% represented a voluntary torque of only 79.6 ± 6.1%MVC. In conclusion, when maximal VA is calculated to be ∼90% (as in regular healthy subjects), this probably represents a considerable overestimation of the subjects’ ability to maximally drive their quadriceps muscles.     

Normalized EMG to normalized torque relationship of vastus intermedius muscle during isometric knee extension

The purpose of the present study was to investigate the electromyography (EMG) to torque relationship of the vastus intermedius (VI) muscle. Thirteen healthy men performed maximal voluntary contraction (MVC) and submaximal contraction during isometric knee extension at 10% of the MVC to 90% of the MVC at intervals of 10% of the MVC level. Surface EMG was detected from four muscle components of the QF muscle group, i.e., VI, vastus lateralis (VL), vastus medialis, and rectus femoris (RF) muscles. Normalized muscle activation in the VI muscle was significantly lower than in the VL muscle at a lower torque level (20 and 40% of MVC) and significantly lower compared to the RF muscle at a higher torque level (from 60 to 90% of MVC). These results suggest that neuromuscular activation in the VI muscle is not consistent with the other components of QF muscle group during submaximal knee extension contractions.     

Muscle activation during maximal voluntary eccentric and concentric knee extension

Summary The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360° · s^–1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30°–70°) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%–146%,P < 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%–31%,P < 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P > 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P < 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.     

Surface EMG signal alterations in Carpal Tunnel syndrome: a pilot study

Fatigability and muscle oxygen consumption (mVO₂) during sustained voluntary isometric knee extensions are less at extended (30° knee angle; 0°, full extension) versus flexed knee angles (90°). This lower energy consumption may partially result from lower neural activation at extended knee angles. We hypothesized a smaller difference in mVO₂ between extended and flexed knee angles during electrical stimulation, which guaranteed maximal activation, than during maximal voluntary contractions (MVC). In eight healthy young males, MVC extension torque was obtained at 30°, 60° and 90° knee angles. mVO₂ of the rectus femoris (RF), vastus lateralis (VL) and medialis muscle was measured using near-infrared spectroscopy during tetanic (10 s) and maximal voluntary (15 s) contractions (MVC₁₅). For electrically induced contractions, steady state mVO₂ was reached at similar (P > 0.05) times after torque onset (4.6 ± 0.7 s) at all knee angles. In contrast, during MVC₁₅ at 30° mVO₂ was reached at 7.1 ± 1.1 s, significantly later compared to 60° and 90° knee angles. The knee angle dependent differences in mVO₂ were not lower in electrically induced contractions (as hypothesised) but were similar as in voluntary contractions. Normalized mVO₂ at 30° (percentage 90° knee angle) was 79.0 ± 9.4% (across muscles) for electrically induced and 79.5 ± 7.6% (across muscles) for voluntary contractions (P < 0.05). We conclude that the slower onset of mVO₂ during voluntary effort at 30° may have been due to a lower maximal activation. However, because steady state mVO₂ both during electrically induced and voluntary contractions was ~20% less at extended versus flexed knee angles, the causes for the lower mVO₂ must reside within the muscle itself.     

Changes in muscle function in response to 10 days of lower limb unloading in humans

Force-generating capacity and electromyographic (EMG) activity of the knee extensor muscles were studied before and after short-term (10 d) unilateral lower limb unloading and during 4 days of recovery. Ten healthy males used crutches to prevent one of their lower limbs from weight-bearing while maintaining joint mobility as well as daily ambulatory activities. Knee extensor torque and quadriceps rectified EMG during maximal voluntary isometric contraction (MVC) was measured repeatedly before and after the intervention. Also, EMG at a fixed submaximal level (100 Nm; 30–45% MVC) and maximal angular velocity (AV_max), during unresisted knee extension, were assessed. Maximum torque decreased (P<0.05) by 13±8% in response to unloading while maximum EMG activity did not change after unloading or during recovery (P=0.35). Submaximum EMG increased (P<0.05) by 25±16% after unloading. Maximum and submaximum torque/EMG ratios decreased (P<0.05) after unloading. AV_max decreased (P<0.05) by 9±8% after unloading. The post value, however, was not different from that of the weight-bearing limb. Torque, EMG and AV_max were recovered (P>0.05) after 4 days of resumed weight-bearing. The pronounced decrease and the rapid recovery in maximum torque appears not to be attributed to a change in muscle mass alone. Because the findings of unchanged maximum EMG and increased EMG at a submaximal force level suggest no change in neural drive, we propose that unspecific tissue factors in part impair muscle function in response to short-term loss of weight-bearing activity. Results also indicate that recovery in muscle function after short-term unloading seems to be completed in a time span shorter than the period of preceding inactivity.     

Contraction specific changes in passive torque in human skeletal muscle

The present investigation examined passive torque and electromyographic response in human skeletal muscle during passive static stretch within 60 s after maximal repetive eccentric and concentric contractions. Passive torque (Nm) offered by the hamstring muscle group was measured during passive knee extension in a modified dynamometer in 10 subjects. The distal thigh was elevated to 0.52 rad from horizontal and the backrest was positioned at 1.57 rad. The lever arm moved the leg passivelly at 0.09 rad s^-1 from a starting positioned of 1.48 rad below horizontal to the final position where it remained stationary for 90s. Gross electrical activity of the human hamstring muscle group was recorded simultaneously. The effect of concentric or eccentric contraction was tested on separate days. Two stretch manoeuvres with a 45 min hiatus were administered on a control and experimental side. The experimental side performed 40 maximal effort repetitive concentric or eccentric hamstring muscle contractions at 1.05 rad s^-1 prior to the second stretch. Passive torque during the 90s stretch declined 30–35% on the experimental and control side in all stretches. P<0.001 without a significant effect of prior contraction mode. Passive peak and final torques were lower on the experimental side, P<0.01 after concentric contractions. Passive peak and final torques remained unchanged after eccentric contractions on the experimental side. The low level EMG response of the hamstring muscle during the stretch was unchanged after maximal repetitive concentric or eccentric contractions. These data demonstrate acute contraction specific alteration in passive torque in human skeletal muscle, which cannot be accounted for by EMG activity. Furthermore, the lack of difference on the control side implies that one 90s stretch has no effect on passive torque of the muscle 45 min later.     

Effect of prior isometric muscle action on concentric torque output during plantar flexion

The influence of different levels of prior isometric muscle action on the concentric torque output during plantar flexion was examined at two angular velocities (60°·s^–1 and 120°·s^–1) in ten healthy female subjects. The levels of the prior muscle actions were 25%, 50%, 75% and 100% of the maximal voluntary isometric contraction (MVIC). A KINetic-COMmunicator II dynamometer was used to measure torque output during plantar flexion within a range of motion of 78°-120° of the ankle joint. Simultaneous recordings of electromyograms (low-pass filtered and rectified) were obtained from the gastrocnemius medialis muscle and the soleus muscle. Torque-angle curves were made for the plantar flexions using different prior muscle actions. Up to 75% of MVIC, the torque output in the first part of the range of motion increased with the level of the prior isometric muscle action; at higher levels of MVIC the torque did not appear to increase any further. Later in the range of motion, after 24° in the plantar flexion at a velocity of 60°·s^–1 and 31° at 120°·s^–1, the prior muscle actions had no further influence. No increase was found in the electromyograms, with one exception, during the concentric movements when preceded by higher levels of MVIC. It would seem therefore that the increase in torque output early in the range of motion cannot be explained on the basis of differences in electrical muscle activation in this study.     

Electromyographic analysis of the knee during jump landing in male and female athletes

Many noncontact anterior cruciate ligament (ACL) injuries in female athletes occur at foot strike during jump landing when the knee is extended. This study was undertaken to determine the activation level of the quadriceps and hamstring muscles electromyographically. Fifteen healthy volunteers (eight women and seven men), all of whom were collegiate basketball players, participated in the study. The maximum voluntary contraction (MVC) of the vastus medialis (VM) at a knee flexion angle of 15-45 degrees was significantly higher in women than in men. There was no significant difference in overall mean hamstring activity in men and women over the same knee flexion range. However, when the knee flexion angle was 15 degrees , 20 degrees , and 25 degrees , hamstring activity was significantly lower in female athletes. These results suggest that female athletes have a higher risk of ACL injury during jump landing due to increased anterior tibial translation force with quadriceps muscle activity. Female athletes require greater hamstring activation, and it is suggested that exercising this muscle will increase its activity when the knee is extended, thus preventing ACL injury during actual sport motions.