The effect of leg muscle activation state and localized muscle fatigue on tibial response during impact

The purpose of this research was to examine the effects of voluntarily manipulating muscle activation and localized muscle fatigue on tibial response parameters, including peak tibial acceleration, time to peak tibial acceleration, and the acceleration slope, measured at the knee during unshod heel impacts. A human pendulum delivered consistent impacts to 15 female and 15 male subjects. The tibialis anterior and lateral gastrocnemius were examined using electromyography, thus allowing voluntary contraction to various activation states (baseline, 15%, 30%, 45%, and 60% of the maximum activation state) and assessing localized muscle fatigue. A skin-mounted uniaxial accelerometer, preloaded medial to the tibial tuberosity, allowed tibial response parameter determination. There were significant decreases in peak acceleration during tibialis anterior fatigue, compared to baseline and all other activation states. In females, increased time to peak acceleration and decreased acceleration slope occurred during fatigue compared to 30% and 45%, and compared to 15% through 60% of the maximum activation state, respectively. Slight peak acceleration and acceleration slope increases, and decreased time to peak acceleration as activation state increased during tibialis anterior testing, were noted. When examining the lateral gastrocnemius, the time to peak acceleration was significantly higher across gender in the middle activation states than at the baseline and fatigue states. The acceleration slope decreased at all activation states above baseline in females, and decreased at 60% of the maximum activation state in males compared to the baseline and fatigue states. Findings agree with localized muscle fatigue literature, suggesting that with fatigue there is decreased impact transmission, which may protect the leg. The relative effects of leg stiffness and ankle angle on tibial response need to be verified.     

The frequency of alternate muscle activity is associated with the attenuation in muscle fatigue.

Alternate muscle activity between synergist muscles has been demonstrated during low-level sustained contractions [< or =5% of maximal voluntary contraction (MVC) force]. To determine the functional significance of the alternate muscle activity, the association between the frequency of alternate muscle activity during a low-level sustained knee extension and the reduction in knee extension MVC force was studied. Forty-one healthy subjects performed a sustained knee extension at 2.5% MVC force for 1 h. Before and after the sustained knee extension, MVC force was measured. The surface electromyogram was recorded from the rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM) muscles. The frequency of alternate muscle activity for RF-VL, RF-VM, and VL-VM pairs was determined during the sustained contraction. The frequency of alternate muscle activity ranged from 4 to 11 times/h for RF-VL (7.0 +/- 2.0 times/h) and RF-VM (7.0 +/- 1.9 times/h) pairs, but it was only 0 to 2 times/h for the VL-VM pair (0.5 +/- 0.7 times/h). MVC force after the sustained contraction decreased by 14% (P < 0.01) from 573.6 +/- 145.2 N to 483.3 +/- 130.5 N. The amount of reduction in MVC force was negatively correlated with the frequency of alternate muscle activity for the RF-VL and RF-VM pairs (P < 0.001 and r = 0.65 for both) but not for the VL-VM pair. The results demonstrate that subjects with more frequent alternate muscle activity experience less muscle fatigue. We conclude that the alternate muscle activity between synergist muscles attenuates muscle fatigue.     

Cross-spectral analysis of physiological tremor and muscle activity

We investigate the relationship between the extensor electromyogram (EMG) and tremor time series in physiological hand tremor by cross-spectral analysis. Special attention is directed to the phase spectrum and the effects of observational noise. We calculate the theoretical phase spectrum for a second-order linear stochastic process and compare the results to measured tremor data recorded from subjects who did not show a synchronized EMG activity in the corresponding extensor muscle. The results show that physiological tremor is well described by the proposed model and that the measured EMG represents a Newtonian force by which the muscle acts on the hand. Received: 7 October 1996 / Accepted in revised form: 29 January 1998     

Effects of postural muscle fatigue on the relation between segmental posture and movement.

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Delto?deus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.     

The effects of an attentional demand tasks on standing posture control

The present study sought to determine if the postural sway of a subject required to grasp a tray (motor task) holding a cup filled with water and prevent spilling (mental task), would be reduced by consciously redirecting attention to maintain the tray in a horizontal position. We hypothesized the mental task would increase the stabilization of standing postural balance. Postural sway was measured in 17 normal subjects under the following conditions: 1) holding a 100 g weight in each hand (total 200 g; no mental task), 2) holding with both hands a tray on which 200 g was placed (tray-holding task), and 3) holding with both hands a tray on which a cup filled with water weighing 200 g was placed in the center (mental task). Postural sway was significantly reduced during the mental task versus other tasks. Standing posture balance was stabilized when a mental task was added. Thus, we concluded that higher brain functions such as attention and consciousness exerted a significant influence over the control of standing posture.     

Effects of posture,movement and hand load on shoulder muscle activity

The influence of external factors such as arm posture, hand loading and dynamic exertion on shoulder muscle activity is needed to provide insight into the relationship between internal and external loading of the shoulder joint. Surface electromyography was collected from 8 upper extremity muscles on 16 participants who performed isometric and dynamic shoulder exertions in three shoulder planes (flexion, mid-abduction and abduction) covering four shoulder elevation angles (30°, 60°, 90° and 120°). Shoulder exertions were performed under three hand load conditions: no load, holding a 0.5 kg load and 30% grip. It was found that adding a 0.5 kg load to the hand increased shoulder muscle activity by 4% maximum voluntary excitation (MVE), across all postures and velocities. Performing a simultaneous shoulder exertion and hand grip led to posture specific redistribution of shoulder muscle activity that was consistent for both isometric and dynamic exertions. When gripping, anterior and middle deltoid activity decreased by 2% MVE, while posterior deltoid, infraspinatus and trapezius activity increased by 2% MVE and biceps brachii activity increased by 6% MVE. Increased biceps brachii activity with gripping may be an initiating factor for the changes in shoulder muscle activity. The finding that hand gripping altered muscle activation, and thus the internal loading, of the shoulder may play an important role in shoulder injury development and rehabilitation.     

Performance of soccer passing skills under moderate and high-intensity localized muscle fatigue

Despite the acknowledged importance of fatigue on performance, ecologically sound studies investigating fatigue and its effects on sport-specific skills are surprisingly limited. The aim of this study was to investigate the effects of moderate and high-intensity localized muscle fatigue on passing performance in soccer. Twenty physically active male college students (age, 22.9 +/- 5.3 years) participated in this study. Subjects performed the modified Loughborough Soccer Passing Test under the following three conditions: rest, moderate fatigue, and high-intensity fatigue. Fatigue intensity was established using a percentage of the maximal number of alternate split squats performed by the subject in 1 minute. Results revealed a significant difference between performance at rest and performance following high-intensity, localized muscle fatigue. The results suggest the need for trainers and coaches to incorporate high-intensity exercise drills into training plans to help players better cope with the demands of the game on the pitch.     

Characteristics of human knee muscle coordination during isometric contractions in a standing posture: The effect of limb task

Different functional roles for the hands have been demonstrated, however leg control is not as well understood. The purpose of the present study was to evaluate bilateral knee neuromuscular control to determine if the limb receiving greater attention would have more well-tuned control compared to an unattended limb. Surface electrodes were placed on seven muscles of each limb, before standing on two force platforms. Visual feedback was given of the forces and moments of the “focus limb,” but not the “unattended limb.” Static isometric forces were matched with their focus limb, requiring their unattended limb to push in the opposite direction, using a combination of forward–backward–medial–lateral shear forces while muscle activity was collected bilaterally. There was a significant main effect for limb task (p = 0.02), with the medial hamstrings being more specific (p = 0.001) while performing the unattended limb and the lateral hamstring being more well-tuned (p = 0.007) while performing the focus limb task. The focus limb’s medial and lateral gastrocnemius were principally active in the forwards direction, but only the unattended limb’s lateral gastrocnemius was active in the backwards direction. Findings suggest unique neuromuscular control strategies are used for the legs depending on limb task.     

Differential effects of mental concentration and acute psychosocial stress on cervical muscle activity and posture

Physical and psychosocial stressors in the workplace have been independently associated with the development of neck pain, yet interactions among these risk factors remain unclear. The purpose of this study was to compare the effects of mentally challenging computer work performed with and without exposure to a psychosocial stressor on cervical muscle activity and posture. Changes in cervical posture and electromyography of upper trapezius, cervical extensor, and sternocleidomastoid muscles were compared between a resting seated posture at baseline, a low stress condition with mental concentration, and a high stress condition with mental concentration and psychosocial stress in sixty healthy office workers. Forward head posture significantly increased with mental concentration compared to baseline, but did not change with further introduction of the stressor. Muscle activity significantly increased from the low stress to high stress condition for both the dominant and non-dominant upper trapezius, with no corresponding change in activity of the cervical extensors or flexors between stress conditions. These findings suggest that upper trapezius muscles are selectively activated by psychosocial stress independent of changes in concentration or posture, which may have implications for the prevention of stress-related trapezius myalgia in the workplace.     

Initial cardiovascular response on change of posture from squatting to standing

The immediate cardiovascular responses on active change from the squatting (control) to the standing position differ from those obtained in the lying-to-standing manoeuvre. Without exception, the first beat after changing from squatting to standing showed a decrease in systolic, diastolic and mean pressure by 2.0 +/- 1.1 kPa (14.6 +/- 8.3 mm Hg), 1.4 +/- 1.7 kPa (10.6 +/- 12.6 mm Hg) and 1.9 +/- 1.0 kPa (13.9 +/- 7.3 mm Hg), respectively. During the 4th or 5th pulse after standing the pulse pressure was significantly higher than when lying (P less than 0.01). Mean pressure reached a minimum of 7.7 +/- 1.9 kPa (57.8 +/- 14.4 mm Hg) after 7.1 +/- 1.1 s. Thereafter the blood pressure increased to a new level within about 15 s. 11 of 16 subjects demonstrated a biphasic heart rate (HR) response. The maximum HR was reached after 11.0 +/- 2.4 s of standing. In all experiments, the peaks in HR were distinctly delayed after the blood pressure dips. We conclude that an arterial baroreflex could be implicated in the immediate HR increase after a squatting-to-standing manoeuvre. The subsequent time course of the initial HR response, however, might be induced by other mechanisms.     

Effects of standing vs. seated posture on repeated Wingate performance

Standing during cycling may increase overall muscular activity. However, effects of standing vs. seated posture on performance measures during repeated bouts have not been extensively explored. The purpose of this study was to examine the effects of standing vs. seated posture on repeated Wingate performance. Healthy volunteers (n = 35) performed 3 consecutive Wingate anaerobic power tests (W(1), W(2), W(3)) in a standing (STA) as well as seated (SIT) posture. Within-group comparisons were made for peak power, mean power, minimum power, and fatigue index. Results were considered significant at p < or = 0.05. No significant differences were found for peak power in W(1), W(2), or W(3). No significant difference was found for mean power in W(1) or W(2), but significant differences were found for mean power in W(3) (STA: 451.5 +/- 105.3, SIT: 425.7 +/- 110.0); minimum power in W(1) (STA: 433.6 +/- 100.8, SIT: 381.5 +/- 96.9), W(2) (STA: 348.1 +/- 112.9, SIT: 308.0 +/- 95.8), W(3) (STA: 292.0 +/- 103.6, SIT: 265.3 +/- 90.8); and fatigue index: W(1) (STA: 51.3 +/- 10.7, SIT: 56.9 +/- 9.3), W(2) (STA: 56.5 +/- 12.6, SIT: 61.8 +/- 12.2), W(3) (STA: 59.4 +/- 13.1, SIT: 63.6 +/- 12.4). Results suggest that a standing posture enhances performance during repeated Wingate cycling. The enhancement is most likely due to an attenuated loss in power, which in turn improves fatigue index.     

The deleterious effects of bed rest on human skeletal muscle fibers are exacerbated by hypercortisolemia and ameliorated by dietary supplementation

Prolonged inactivity associated with bed rest in a clinical setting or spaceflight is frequently associated with hypercortisolemia and inadequate caloric intake. Here, we determined the effect of 28 days of bed rest (BR); bed rest plus hypercortisolemia (BRHC); and bed rest plus essential amino acid (AA) and carbohydrate (CHO) supplement (BRAA) on the size and function of single slow- and fast-twitch muscle fibers. Supplementing meals, the BRAA group consumed 16.5 g essential amino acids and 30 g sucrose at 1100, 1600, and 2100 h, and the BRHC subjects received 5 daily doses of 10–15 mg of oral hydrocortisone sodium succinate throughout bed rest. Bed rest induced atrophy and loss of force (mN) and power (µN·FL·s^–1) in single fibers was exacerbated by hypercortisolemia where soleus peak force declined by 23% in the type I fiber from a prevalue of 0.78 ± 0.02 to 0.60 ± 0.02 mN post bed rest (compared to a 7% decline with bed rest alone) and 27% in the type II fiber (1.10 ± 0.08 vs. 0.81 ± 0.05 mN). In the BRHC group, peak power dropped by 19, 15, and 11% in the soleus type I, and vastus lateralis (VL) type I and II fibers, respectively. The AA/CHO supplement protected against the bed rest-induced loss of peak force in the type I soleus and peak power in the VL type II fibers. These results provide evidence that an AA/CHO supplement might serve as a successful countermeasure to help preserve muscle function during periods of relative inactivity. isotonic contractile properties; peak force and power; calcium sensitivity; essential amino acids     

Influence of fatigue on upper limb muscle activity and performance in tennis

The study examined the fatigue effect on tennis performance and upper limb muscle activity. Ten players were tested before and after a strenuous tennis exercise. Velocity and accuracy of serve and forehand drives, as well as corresponding surface electromyographic (EMG) activity of eight upper limb muscles were measured. EMG and force were also evaluated during isometric maximal voluntary contractions (IMVC). Significant decreases were observed after exercise in serve accuracy (−11.7%) and velocity (−4.5%), forehand accuracy (−25.6%) and consistency (−15.6%), as well as pectoralis major (PM) and flexor carpi radialis (FCR) IMVC strength (−13.0% and −8.2%, respectively). EMG amplitude decreased for PM and FCR in serve, forehand and IMVC, and for extensor carpi radialis in forehand. No modification was observed in EMG activation timing during strokes or in EMG frequency content during IMVC. Several hypotheses can be put forward to explain these results. First, muscle fatigue may induce a reduction in activation level of PM and forearm muscles, which could decrease performance. Second, conscious or subconscious strategies could lead to a redistribution of muscle activity to non-fatigued muscles in order to protect the organism and/or limit performance losses. Otherwise, the modifications of EMG activity could also illustrate the strategies adopted to manage the speed-accuracy trade-off in such a complex task.     

Postural muscle activity patterns during standing at rest and on an oscillating floor.

Postural muscle activity pattern was examined in the eyes-closed state after adequate adaptation to floor anteroposterior oscillation. Twenty-three subjects were grouped almost evenly according to dominance of anterior or posterior postural muscles in the trunk and thigh during quiet stance. In the posterior-dominant group, this dominance was maintained at every frequency in most subjects. In the anterior-dominant group, this dominance was maintained in most subjects at 0.1 and 0.5 Hz but changed to posterior dominance at 1.0 and 1.5 Hz in about half the subjects. Periodicity of muscle activity was evaluated by EMG amplitude spectrum at the floor oscillation frequency. Periodicity of posterior-dominant muscles in the trunk and thigh increased with increasing oscillatory frequency. In the trunk, the periodicity did not differ significantly between posterior-dominant and anterior-dominant groups. However, in the thigh, periodicity was significantly lower in the anterior-dominant muscles. This was considered to be caused by nonperiodic alternating action of the anterior and posterior muscles. In the lower leg, posterior dominance was observed in quiet stance and at all oscillation frequencies. Periodicity of soleus and gastrocnemius increased at higher frequencies and was higher in gastrocnemius than in soleus. The periodicity difference between both muscles decreased with increasing oscillation frequency.     

Genetic isolation of fragmented populations is exacerbated by drift and selection

Reduced genetic variation at marker loci in small populations has been well documented, whereas the relationship between quantitative genetic variation and population size has attracted little empirical investigation. Here we demonstrate that both neutral and quantitative genetic variation are reduced in small populations of a fragmented plant metapopulation, and that both drift and selective change are enhanced in small populations. Measures of neutral genetic differentiation (F(ST)) and quantitative genetic differentiation (Q(ST)) in two traits were higher among small demes, and Q(ST) between small populations exceeded that expected from drift alone. This suggests that fragmented populations experience both enhanced genetic drift and divergent selection on phenotypic traits, and that drift affects variation in both neutral markers and quantitative traits. These results highlight the need to integrate natural selection into conservation genetic theory, and suggests that small populations may represent reservoirs of genetic variation adaptive within a wide range of environments.     

Head and shoulder posture affect scapular mechanics and muscle activity in overhead tasks

Forward head and rounded shoulder posture (FHRSP) is theorized to contribute to alterations in scapular kinematics and muscle activity leading to the development of shoulder pain. However, reported differences in scapular kinematics and muscle activity in those with forward head and rounded shoulder posture are confounded by the presence of shoulder pain. Therefore, the purpose of this study was to compare scapular kinematics and muscle activity in individuals free from shoulder pain, with and without FHRSP. Eighty volunteers were classified as having FHRSP or ideal posture. Scapular kinematics were collected concurrently with muscle activity from the upper and lower trapezius as well as the serratus anterior muscles during a loaded flexion and overhead reaching task using an electromagnetic tracking system and surface electromyography. Separate mixed model analyses of variance were used to compare three-dimensional scapular kinematics and muscle activity during the ascending phases of both tasks. Individuals with FHRSP displayed significantly greater scapular internal rotation with less serratus anterior activity, during both tasks as well as greater scapular upward rotation, anterior tilting during the flexion task when compared with the ideal posture group. These results provide support for the clinical hypothesis that FHRSP impacts shoulder mechanics independent of shoulder pain.