Power output and work in different muscle groups during ergometer cycling

The aim of this study was to calculate the magnitude of the instantaneous muscular power output at the hip, knee and ankle joints during ergometer cycling. Six healthy subjects pedalled a weight-braked bicycle ergometer at 120 watts (W) and 60 revolutions per minute (rpm). The subjects were filmed with a cine camera, and pedal reaction forces were recorded from a force transducer mounted in the pedal. The muscular work at the hip, knee and ankle joint was calculated using a model based upon dynamic mechanics described elsewhere. The mean peak concentric power output was, for the hip extensors, 74.4 W, hip flexors, 18.0 W, knee extensors, 110.1 W, knee flexors, 30.0 W and ankle plantar flexors, 59.4 W. At the ankle joint, energy absorption through eccentric plantar flexor action was observed, with a mean peak power of 11.4 W and negative work of 3.4 J for each limb and complete pedal revolution. The energy production relationships between the different major muscle groups were computed and the contributions to the total positive work were: hip extensors, 27%; hip flexors, 4%; knee extensors, 39%; knee flexors, 10%; and ankle plantar flexors 20%.     

Effects of movement speed and joint position on knee flexor torque in healthy and post-surgical subjects

Coactivation of knee flexors during knee extension assists in joint stability by exerting an opposing torque to the anterior tibial displacement induced by the quadriceps. This opposing torque is believed to be generated by eccentric muscle actions that stiffen the knee, thereby attenuating strain to joint ligaments, particularly the anterior cruciate ligament (ACL). However, as the lengths of knee muscles vary with changes in joint position, the magnitude of flexor/extensor muscle force coupling may likewise vary, possibly affecting the capacity for active knee stabilization. The purpose of this study was to assess the effect of changes in movement speed and joint position on eccentric/concentric muscle action relationships in the knees of uninjured (UNI) and post-ACL-surgery (INJ) subjects (n = 14). All subjects were tested for maximum eccentric and concentric torque of the contralateral knee flexors and extensor muscles at four isokinetic speeds (15 degrees-60 degrees x s(-1)) and four joint position intervals (20 degrees-60 degrees of knee flexion). Eccentric flexor torque was normalized to the percentage of concentric flexor torque generated at each joint position interval for each speed tested (flexor E-C ratio). In order to estimate the capacity of the knee flexors to resist active knee extension, the eccentric-flexor/concentric-extensor ratios were also computed for each joint position interval and speed (flexor/extensor E-C ratio). The results revealed that eccentric torque surpassed concentric torque by 3%-144% across movement speeds and joint position intervals. The magnitude of the flexor E-C ratio and flexor/extensor E-C increased significantly with speed in both groups of subjects (P < 0.05) and tended to rise with muscle length as the knee was extended; peak values were generated at the most extended joint position (20 degrees-30 degrees). Although torque development patterns were symmetrical between the contralateral limbs in both groups, between-group comparisons revealed significantly higher flexor/extensor E-C ratios for the INJ group compared to the UNI group (P < 0.05), particularly at the fastest speed tested (60 degrees x s(-1)). The results indicate that joint position and movement speed influence the eccentric/concentric relationships of knee flexors and extensors. The INJ subjects appeared to accommodate to surgery by developing the eccentric function of their ACL and normal knee flexors, particularly at higher speeds and at more extended knee joint positions. This may assist in the dynamic stabilization of the knee at positions where ACL grafts have been reported to be most vulnerable to strain.     

Joint moment and muscle power output characteristics of below knee amputees during running: the influence of energy storing prosthetic feet

Stance phase joint moments, muscle power outputs and mechanical energy characteristics were determined in five normal and five below knee amputee subjects running at 2.8 m s-1. The amputees were studied sequentially on three different prosthetic feet: the SACH foot (solid ankle cushion heel), and two energy storing feet, Seattle and Flex. While wearing the SACH foot, the amputees exhibited major alterations in the distribution and magnitude of muscle power output and muscle work: (1) the total work done by the lower extremity was reduced; (2) the hip extensors became the main source of energy absorption and generation, while in normal subjects the ankle plantarflexors were the major energy generators and the knee extensors the major energy absorbers; (3) the eccentric and concentric knee extensor power outputs were reduced and an abnormal concentric knee flexor power output was noted immediately after heel contact. In four of the amputees, energy storing feet resulted in improvements in the power output and mechanical work characteristics of the lower extremity: (1) the energy storing prosthetic feet generated 2-3 times greater energy than the SACH foot; (2) with the Flex foot the amputees exhibited a more normal pattern and magnitude of hip and knee extensor muscle work. One of the subjects, however, exhibited increased abnormalities with the energy storing prosthetic feet. The amount of energy restored relative to the amount of energy absorbed by each of the prosthetic feet was greater with the energy storing feet than the SACH foot (Flex 84%, Seattle 52%, SACH 31%).     

Activity of mono- and biarticular leg muscles during sprint running

A cinematographic recording of the movements of the lower limbs together with simultaneous emg tracings from nine lower limb muscles were obtained from two male track sprinters during three phases of a 100 m sprint run. The extensor muscles of the hip joint were found to be the primary movers by acceleration of the body's center of gravity (C.G.) during the ground phase of the running cycle. The extensors of the knee joint were also important in this, but to a minor extent, while the plantar flexors of the ankle joint showed the least contribution. The biarticular muscles functioned in a way different from the monoarticular muscles in the sense that they perform eccentric work during the flight and recovery phases and concentric work during the whole ground phase (support), whereas the monoarticular muscles are restricted first to eccentric work and then to concentric work during the ground phase. Furthermore, the biarticular muscles show variation (and rate of variation) in muscle length to a larger extent than the monoarticular muscles. Paradoxical muscle actions appear to take place around the knee joint, where the hamstring muscles, m. gastrocnemius, m. vastus laterialis and m. vastus medialis act as synergists by extending the knee joint during the last part of the ground phase.     

Effect of squat depth and barbell load on relative muscular effort in squatting

ABSTRACT: Bryanton, MA, Kennedy, MD, Carey, JP, and Chiu, LZF. Effect of squat depth and barbell load on relative muscular effort in squatting. J Strength Cond Res 26(10): 2820-2828, 2012-Resistance training is used to develop muscular strength and hypertrophy. Large muscle forces, in relation to the muscle's maximum force-generating ability, are required to elicit these adaptations. Previous biomechanical analyses of multi-joint resistance exercises provide estimates of muscle force but not relative muscular effort (RME). The purpose of this investigation was to determine the RME during the squat exercise. Specifically, the effects of barbell load and squat depth on hip extensor, knee extensor, and ankle plantar flexor RME were examined. Ten strength-trained women performed squats (50-90% 1 repetition maximum) in a motion analysis laboratory to determine hip extensor, knee extensor, and ankle plantar flexor net joint moment (NJM). Maximum isometric strength in relation to joint angle for these muscle groups was also determined. Relative muscular effect was determined as the ratio of NJM to maximum voluntary torque matched for joint angle. Barbell load and squat depth had significant interaction effects on hip extensor, knee extensor, and ankle plantar flexor RME (p < 0.05). Knee extensor RME increased with greater squat depth but not barbell load, whereas the opposite was found for the ankle plantar flexors. Both greater squat depth and barbell load increased hip extensor RME. These data suggest that training for the knee extensors can be performed with low relative intensities but require a deep squat depth. Heavier barbell loads are required to train the hip extensors and ankle plantar flexors. In designing resistance training programs with multi-joint exercises, how external factors influence RME of different muscle groups should be considered to meet training objectives.     

Recovery Kinetics of Knee Flexor and Extensor Strength after a Football Match

We examined the temporal changes of isokinetic strength performance of knee flexor (KF) and extensor (KE) strength after a football match. Players were randomly assigned to a control (N = 14, participated only in measurements and practices) or an experimental group (N = 20, participated also in a football match). Participants trained daily during the two days after the match. Match and training overload was monitored with GPS devices. Venous blood was sampled and muscle damage was assessed pre-match, post-match and at 12h, 36h and 60h post-match. Isometric strength as well as eccentric and concentric peak torque of knee flexors and extensors in both limbs (dominant and non-dominant) were measured on an isokinetic dynamometer at baseline and at 12h, 36h and 60h after the match. Functional (KF_ecc/KE_con) and conventional (KF_con/KE_con) ratios were then calculated. Only eccentric peak torque of knee flexors declined at 60h after the match in the control group. In the experimental group: a) isometric strength of knee extensors and knee flexors declined (P<0.05) at 12h (both limbs) and 36h (dominant limb only), b) eccentric and concentric peak torque of knee extensors and flexors declined (P<0.05) in both limbs for 36h at 60°/s and for 60h at 180°/s with eccentric peak torque of knee flexors demonstrating a greater (P<0.05) reduction than concentric peak torque, c) strength deterioration was greater (P<0.05) at 180°/s and in dominant limb, d) the functional ratio was more sensitive to match-induced fatigue demonstrating a more prolonged decline. Discriminant and regression analysis revealed that strength deterioration and recovery may be related to the amount of eccentric actions performed during the match and athletes'' football-specific conditioning. Our data suggest that recovery kinetics of knee flexor and extensor strength after a football match demonstrate strength, limb and velocity specificity and may depend on match physical overload and players'' physical conditioning level.     

A model of semitendinosus muscle sarcomere length, knee and hip joint interaction in the frog hindlimb

The interaction between the semitendinosus muscle and both hip and knee joint angles was examined in the frog (Rana pipiens) hindlimb. Sarcomere length was measured by laser diffraction in passive muscle during hip and knee rotation. A model was then developed to predict semitendinosus sarcomere length as a function of both hip and knee flexion angle. Based on published frog muscle fiber length-tension [Gordon, A. M. et al., J. Physiol. 184, 170-192 (1966)] and force-velocity [Edman, K. A. P., J. Physiol. 291, 143-159 (1979)] properties, and published joint angles during hopping [Calow, L. J. and Alexander, R. McN., J. Zool. (Lond.) 171, 293-321 (1973)], muscle sarcomere length, force and hip and knee torque during a hop were predicted. The semitendinosus muscle generally operated on the descending limb of the length-tension curve at normal joint angle combinations. The model predicted that, during a single coordinated movement, a period of sarcomere shortening (concentric) was followed by a period of sarcomere lengthening (eccentric). Based on calculated torque profiles at the hip and knee joints, this study suggested that the semitendinosus muscle probably functions more as a hip extensor than a knee flexor. In addition, based on the nature of the shortening-lengthening cycle, the semitendinosus may act to mechanically link the force of knee extension to hip extension.     

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.     

The relationships between local muscular endurance and kinematic changes during a run to exhaustion at vVO2max

The relationships between local muscular endurance and kinematic changes during a run to exhaustion at vVo2max. J Strength Cond. Res. 18(4):000-000. 2004.-A recent study suggested that runners who maintain a stable running style are able to run for longer at vVo2max velocity (vVo2max). This may be because of the capacity of various muscle groups to maintain their functions despite the onset of fatigue. The purpose of this study was to examine the relationship between local muscular endurance of both the hip and knee extensor and flexor muscle groups and the kinematic changes during a run to exhaustion at vVo2max. Six subelite runners (age 24.2 + 4.2) participated in this study;they were considered as a homogeneous group based upon theirvVo2max scores (coefficient of variation = 3.9%). They performed an incremental protocol to determine vVo2max, a series of isokinetic tests to determine the local muscular endurance of both knee and hip flexors and extensors, and a run to exhaustion at vVo2max. The change in kinematic variables between the beginning and the end of the run were correlated with the measures of muscular endurance. Several statistically significant negative correlations emerged between the change in stride length and concentric hip extension (HE(con)), r = -0.934; eccentric hip extension (HE(ecc)), r = -0.818; eccentric knee flexion(KF(ecc)), r = -0.957; and change in maximum hip extension (Delta max HE), r - -0.857; and Delta max HE with HE(con), r = -0.846.We concluded that the local muscular endurance of both HE(con) and KF(ecc) are important in maintaining a stable running style.     

The role of intersegmental dynamics during rapid limb oscillations

The interactive dynamic effects of muscular, inertial and gravitational moments on rapid, multi-segmented limb oscillations were studied. Using three-segment, rigid-body equations of motion, hip, knee and ankle intersegmental dynamics were calculated for the steady-state cycles of the paw-shake response in adult spinal cats. Hindlimb trajectories were filmed to obtain segmental kinematics, and myopotentials of flexors and extensors at each of the three joints were recorded synchronously with the ciné film. The segmental oscillations that emerged during the paw-shake response were a consequence of an interplay between active and passive musculotendinous forces, inertial forces, and gravity. During steady-state oscillations, the amplitudes of joint excursions, peak angular velocities, and peak angular accelerations increased monotonically and significantly in magnitude from the proximal joint (hip) to the most distal joint (ankle). In contrast to these kinematic relationships, the maximal values of net moments at the hip and knee were equal in magnitude, but of significantly lower magnitude than the large net moment at the ankle joint. At both the ankle and the knee, the flexor and extensor muscle moments were equal, but at the hip the magnitude of the peak flexor muscle moment was significantly greater than the extensor muscle moment. Muscle moments at the hip not only acted to counterbalance accelerations of the more distal segments, but also acted to maintain the postural orientation of the hindlimb. Large muscle moments at the knee functioned to counterbalance the large inertial moments generated by the large angular accelerations of the paw. At the ankle, the muscle moments dominated the generation of the paw accelerations. At the ankle and the knee, muscle moments controlled limb dynamics by slowing and reversing joint motions, and the active muscle forces contributing to ankle and knee moments were derived from lengthening of active musculotendinous units. In contrast to the more distal joints, the active muscles crossing the hip predominantly shortened as a result of the interplay among inertial forces and gravitational moments. The muscle function and kinetic data explain key features of the complex interactions that occur between central control mechanisms and multi-segmented, oscillating limb segments during the paw-shake response.     

Investigation of muscle tone in patients with Parkinson’s disease in unloading conditions

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, the main symptoms of which are hypertonicity and difficulties emerging during performance of stepping movements due to increased muscle stiffness. Biomechanical (stiffness) and electrophysiological (shortening reaction, SR) characteristics of hip and shank muscles were examined in 25 patients with mild and moderate stages of PD (1 to 3 of Hoehn and Yahr Rating Scale, 61 ± 9 years) and 22 age-matched healthy controls in unloading leg conditions during passive flexion/extension of hip, knee, and ankle joints, as well as the changes in the tonic state of muscles under the influence of levodopa. The data obtained were compared with similar findings in healthy subjects. Essentially greater stiffness in all leg muscle groups (except foot extensors) was observed in patients with PD as compared to the healthy subjects. In patients with PD, SR values in hip and shank extensors as well as in foot flexors and extensors were essentially greater then in the healthy subjects. The medicine essentially reduced the stiffness of hip flexors and knee flexors and extensors. The SR persisted, although the frequency of its occurrence decreased in half of studied muscles, and a significant decrease in the SR value was observed in foot extensors. The medicine had no marked effect on the SR in the proximal muscles. Thus, the increased muscle stiffness in patients with PD manifests itself as distorted reactions to external disturbances and increased reflectory reactions of muscles.     

Effects of 20 days of bed rest on physiological cross-sectional area of human thigh and leg muscles evaluated by magnetic resonance imaging

The present study was to investigate the effects of 20 days of bed rest on morphological characteristics of lower limb skeletal muscles. Ten sedentary volunteers (5 males and 5 females) were participating in this study. Magnetic resonance imaging techniques were used to measure the physiological cross-sectional areas (PCSAs) of the major muscles and muscle groups of the lower limb. Consecutive images were taken from the right thigh and leg of subjects, and muscle volumes (MV), muscle length, and fiber length were calculated. PCSA of each muscle was determined as MV times the cosine of the angle of fiber pennation divided by fiber length. PCSA of knee extensor and flexor muscles were significant reduced during and after bed rest. MV and PCSA of individual muscles in the knee extensors decreased by -5.1 % to -8.0%. In knee flexors, MV and PCSA in biceps femoris (long head), semitendinosus, semimembranosus, and sartorius decreased during and after bed rest. MV and PCSA in medial and lateralis [correction of andateralis] gastrocnemius, and soleus were remarkably reduced by -9.4 to -10.3% after bed rest. The results suggest that there is a great variability of muscle atrophy in the lower limb muscle groups or individual muscle after bed rest and that the plantar flexors primarily affected.     

Gear ratios at the limb joints of jumping dogs

An increase in gear ratio of the limb extensor muscles during joint extension has been suggested to be a mechanism that facilitates optimal power production by skeletal muscles. The objectives of this study were to: (1) measure gear ratios at the wrist, elbow, shoulder, ankle, knee, and hip joints of jumping dogs, (2) compute the work performed by each of these joints, and (3) measure muscle shortening velocity for a joint exhibiting an increasing gear ratio during joint extension. The gear ratio out-lever was computed by dividing the ground reaction force (GRF) moment by the GRF, whereas the in-lever was directly measured as the perpendicular distance from the joint center to the line of action of the extensor muscle. In addition, changes in fascicle length were measured from the vastus lateralis muscle using sonomicrometry. Of the joints examined, only the gear ratios at the shoulder and knee joints increased during jumping in a manner that could facilitate peak power production of actively shortening muscles. The vastus lateralis was found to shorten at an average velocity of 3.20 muscle lengths per second. This is similar to estimates of shortening velocity that produce peak muscular power in mammals the size of dogs. Additionally, the knee extensors were found to produce a large proportion (26.6%) of the positive external work of the limbs. These observations suggest that dynamic gearing in jumping dogs may allow the extensor muscles of the knee joint to shorten in a way that maximizes their power production.     

Potentiation of knee extensor contraction by antagonist conditioning contraction at several intensities

The purpose of this study was to examine the effect of graded conditioning contractions of the antagonist knee flexor muscles on the output characteristics of knee extensor muscles in healthy humans. Eight male university students performed maximum isometric contractions of knee extensors, preceded by isometric conditioning contractions of the antagonist knee flexors. The developed force and electromyographic (EMG) amplitudes of the knee extensors after the conditioning contraction were measured and compared with those of simple knee extension without conditioning. The forces of the conditioning flexor contraction were set at three levels: low (20% of maximum voluntary contraction: MVC), moderate (60% of MVC), and high (100% of MVC). The EMG amplitudes of the vastus medialis, vastus lateralis, and rectus femoris muscle were recorded and the root mean square amplitudes were calculated. The strongest enhancement of the extension force was obtained by moderate intensity conditioning contraction (108.95+/-1.87% of simple knee extension), although high intensity conditioning also induced a significant increase (105.41+/-2.69%). Low intensity conditioning did not cause a significant enhancement of the contraction force (103.17+/-2.99%). Similarly, the EMG amplitudes were significantly increased by moderate and/or high conditioning. These results suggest that antagonist conditioning contraction of moderate intensities is sufficient and may be optimal to potentiate knee extensor contraction.     

Effect of exercise-induced fatigue on postural control of the knee

Muscle fatigue is associated with reduced power output and work capacity of the skeletal muscle. Fatigue-induced impairments in muscle function are believed to be a potential cause of increased injury rates during the latter stages of athletic competition and often occur during unexpected perturbations. However the effect of fatigue on functionally relevant, full body destabilizing perturbations has not been investigated. This study examines the effect of muscle fatigue on the activation of the quadriceps and hamstrings to fast, full body perturbations evoked by a moveable platform. Surface electromyographic (EMG) signals were recorded from the knee extensor (vastus medialis, rectus femoris, and vastus lateralis) and flexor muscles (biceps femoris and semitendinosus) of the right leg in nine healthy men during full body perturbations performed at baseline and immediately following high intensity exercise performed on a bicycle ergometer. In each condition, participants stood on a moveable platform during which 16 randomized postural perturbations (eight repetitions of two perturbation types: 8 cm forward slides, 8 cm backward slides) with varying inter-perturbation time intervals were performed over a period of 2-3 min. Maximal voluntary knee extension force was measured before and after the high intensity exercise protocol to confirm the presence of fatigue. Immediately after exercise, the maximal force decreased by 63% and 66% for knee extensors and flexors, respectively (P<0.0001). During the post-exercise postural perturbations, the EMG average rectified value (ARV) was significantly lower than the baseline condition for both the knee extensors (average across all muscles; baseline: 19.7±25.4μV, post exercise: 16.2±19.4 μV) and flexors (baseline: 24.3±20.9 μV, post exercise: 13.8±11.0 μV) (both P<0.05). Moreover the EMG onset was significantly delayed for both the knee extensors (baseline: 132.7±32.9 ms, post exercise: 170.8±22.9 ms) and flexors (baseline: 139.1±38.8 ms, post exercise: 179.3±50.9 ms) (both P<0.05). A significant correlation (R(2)=0.53; P<0.05) was identified between the percent reduction of knee extension MVC and the percent change in onset time of the knee extensors post exercise. This study shows that muscle fatigue induces a reduction and delay in the activation of both the quadriceps and hamstring muscles in response to rapid destabilizing perturbations potentially reducing the stability around the knee.     

Optimal whole-body vibration settings for muscle strength and power enhancement in human knee extensors

This study compared the effects of 6-week whole-body vibration (WBV) training programs with different frequency and peak-to-peak displacement settings on knee extensor muscle strength and power. The underlying mechanisms of the expected gains were also investigated. Thirty-two physically active male subjects were randomly assigned to a high-frequency/high peak-to-peak displacement group (HH; n = 12), a low-frequency/low peak-to-peak displacement group (LL; n = 10) or a sham training group (SHAM; n = 10). Maximal voluntary isometric, concentric and eccentric torque of the knee extensors, maximal voluntary isometric torque of the knee flexors, jump performance, voluntary muscle activation, and contractile properties of the knee extensors were assessed before and after the training period. Significant improvement in knee extensor eccentric voluntary torque (P < 0.01), knee flexor isometric voluntary torque (P < 0.05), and jump performance (P < 0.05) was observed only for HH group. Regardless of the group, knee extensor muscle contractile properties (P < 0.05) were enhanced. No modification was observed for voluntary muscle activation or electrical activity of agonist and antagonist muscles. We concluded that high-frequency/high peak-to-peak displacement was the most effective vibration setting to enhance knee extensor muscle strength and jump performance during a 6-week WBV training program and that these improvements were not mediated by central neural adaptations.     

Muscular contributions to hip and knee extension during the single limb stance phase of normal gait: a framework for investigating the causes of crouch gait

Crouch gait, a troublesome movement abnormality among persons with cerebral palsy, is characterized by excessive flexion of the hips and knees during stance. Treatment of crouch gait is challenging, at present, because the factors that contribute to hip and knee extension during normal gait are not well understood, and because the potential of individual muscles to produce flexion or extension of the joints during stance is unknown. This study analyzed a three-dimensional, muscle-actuated dynamic simulation of walking to quantify the angular accelerations of the hip and knee induced by muscles during normal gait, and to rank the potential of the muscles to alter motions of these joints. Examination of the muscle actions during single limb stance showed that the gluteus maximus, vasti, and soleus make substantial contributions to hip and knee extension during normal gait. Per unit force, the gluteus maximus had greater potential than the vasti to accelerate the knee toward extension. These data suggest that weak hip extensors, knee extensors, or ankle plantar flexors may contribute to crouch gait, and strengthening these muscles--particularly gluteus maximus--may improve hip and knee extension. Abnormal forces generated by the iliopsoas or adductors may also contribute to crouch gait, as our analysis showed that these muscles have the potential to accelerate the hip and knee toward flexion. This work emphasizes the need to consider how muscular forces contribute to multijoint movements when attempting to identify the causes of abnormal gait.     

Co-activation of vastus lateralis and biceps femoris muscles in pubertal children and adults

Determining the mechanisms of co-activation around the knee joint with respect to age and sex is important in terms of our greater understanding of strength development. The purpose of this study was to examine the effects of age, sex and muscle action on moment of force and electromyographic (EMG) activity of the agonist and antagonist muscle groups during isokinetic eccentric and concentric knee extension and flexion. The study comprised nine pubertal boys [mean age 12.6 (SD 0.5) years], nine girls [12.7 (SD 0.5) years] nine adult men [23.1 (SD 2.1) years] and nine adult women [23.7 (SD 3.1) years] who performed maximal isometric eccentric and concentric efforts of knee extensors and flexors on a dynamometer at 30 degrees x s(-1). The moment of force and surface EMG activity of vastus lateralis and biceps femoris muscles were recorded. The moment of force:agonist averaged EMG (aEMG) ratios were calculated. The antagonist aEMG values were expressed as a percentage of the aEMG activity of the same muscle, at the same angle, angular velocity and muscle action when the muscle was acting as agonist. Three-way analysis of variance (ANOVA) designs indicated no significant effects of age or sex on moment:aEMG ratios. Eccentric ratios were significantly higher than the corresponding concentric ones (P < 0.05). The results also indicated no significant effect of age and sex on the aEMG of the vastus lateralis and biceps femoris muscles when acting as antagonists. The antagonist aEMG was significantly greater during concentric agonist efforts compared with the corresponding eccentric ones (P < 0.05). These findings would suggest that the moment exerted per unit of agonist EMG and the antagonist activity are similar in children compared with adults and are not sex dependent. Future comparisons between eccentric and concentric moments of force and agonist ENG should take into consideration the antagonist effects, irrespective of age or sex.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.