Stimulated and voluntary fatiguing contractions of quadriceps femoris differently disturb postural control

Local exercise based on voluntary contractions produces greater warm-up effects on balance control than electro-induced contractions

On the one hand, while general (involving the whole body) warm-up exercises have been extensively used to optimize motor and postural performance, the effect of a local (involving a particular muscular group) warm-up has not yet been addressed. On the other hand, voluntary (VOL) and electro-induced (EI) contractions produce different physiological effects likely to differently affect motor and postural performance. The aim was to analyze and compare the effects of two local warm-up modalities, voluntary and electro-induced (VOL or EI warm-up) on balance control. Balance control was evaluated with a force platform (recording the displacement of the centre of foot pressure - COP) in 27 healthy young subjects before (PRE), immediately after (POST), 5 minutes after (POST5), and 10 minutes after (POST10) either warm-up. Each warm-up included 6 sets of 5 contractions at 10% of maximal voluntary contraction of the quadriceps femoris. The results showed that the VOL warm-up improved balance control at POST, POST5 and POST10 while the EI warm-up improved it only at POST10. In addition, balance control was significantly better after the VOL warm-up than after the EI warm-up at POST5 and POST10. A short and local VOL warm-up improved balance control immediately after its completion and for, at least, ten minutes, while the EI warm-up required some minutes of recovery before producing its improving effects.

The fatigue-induced alteration in postural control is larger in hypobaric than in normobaric hypoxia

To test the hypothesis that postural control would be more affected by plantar flexors fatigue during acute exposure in hypobaric (HH) than in normobaric (NH) hypoxia or normobaric normoxia (NN). Twelve young male adults performed in a random order three experimental sessions (in HH and NH (F_iO₂ 0.139) at an altitude of 2950 m, and in NN at 500 m) composed of a bipedal postural control with eyes open on a posturographic platform before and after a plantar flexors fatiguing protocol. Center of pressure (CoP) trajectory and stabilogramm diffusion analyses (SDA) parameters were assessed. A two-way repeated measures analysis of variance was used to identify differences by examination of the group and time interaction. Surface of CoP trajectory analysis, increased at POST in HH (p < 0.001) and in NH (p < 0.01) compared to NN. SDA confirmed that PC was more altered in HH than in NH (p < 0.001) and NN (p < 0.05) at POST. The plantar flexor fatigue-induced alteration in postural control increased to a larger extent in HH than in NH or NN, suggesting an alleviating influence of the decreased barometric pressure per se and a mechanical influence of the higher breathing frequency in HH.

Muscle plasticity of aged subjects in response to electrical stimulation training and inversion and/or limitation of the sarcopenic process

This review addresses the possible structural and functional adaptations of the muscle function to neuromuscular electrical stimulation (NMES) training in frail and/or aged (without advanced chronic disease) subjects. Evidence suggests that the sarcopenic process and its structural and functional effects would be limited and/or reversed through NMES training using excito-motor currents (or direct currents). From a structural viewpoint, NMES helps reduce muscle atrophy. From a functional viewpoint, NMES enables the improvement of motor output (i.e., muscle strength), gait, balance and activities of daily living which enhances the quality of life of aged subjects. Muscle plasticity of aged subjects in response to NMES training turns out to be undeniable, although many mechanisms are not yet explained and deserve to be explore further. Mechanistic explanations as well as conceptual models are proposed to explain how muscle plasticity operates in aged subjects through NMES training. NMES could be seen as a clinically applicable training technique, safe and efficient among aged subjects and could be used more often as part of prevention of sarcopenia. Therapists and physical conditioners/trainers could exploit this new knowledge in their professional practice to improve life conditions (including the risk of fall) of frail and/or aged subjects.

Scapular kinematic and shoulder muscle activity alterations after serratus anterior muscle fatigue

BACKGROUND

Although the serratus anterior muscle has an important role in scapular movement, no study to date has investigated the effect of serratus anterior fatigue on scapular kinematics and shoulder muscle activity. The purpose of this study was to clarify the effect of serratus anterior fatigue on scapular movement and shoulder muscle activity.

METHODS

The study participants were 16 healthy men. Electrical muscle stimulation was used to fatigue the serratus anterior muscle. Shoulder muscle strength and endurance, scapular movement, and muscle activity were measured before and after the fatigue task. The muscle activity of the serratus anterior, upper and lower trapezius, anterior and middle deltoid, and infraspinatus muscles was recorded, and the median power frequency of these muscles was calculated to examine the degree of muscle fatigue.

RESULTS

The muscle endurance and median power frequency of the serratus anterior muscle decreased after the fatigue tasks, whereas the muscle activities of the serratus anterior, upper trapezius, and infraspinatus muscles increased. External rotation of the scapula at the shoulder elevated position increased after the fatigue task.

CONCLUSION

Selective serratus anterior fatigue due to electric muscle stimulation decreased the serratus anterior endurance at the flexed shoulder position. Furthermore, the muscle activities of the serratus anterior, upper trapezius, and infraspinatus increased and the scapular external rotation was greater after serratus anterior fatigue. These results suggest that the rotator cuff and scapular muscle compensated to avoid the increase in internal rotation of the scapula caused by the dysfunction of the serratus anterior muscle.

Postural control and physiological responses to a simulated match in U-20 judo competitors

The aim was to evaluate the effects of judo combat on the athletes' postural control (PC) and physiological loading before, during and after a simulated match. Seventeen under-20 regional and national level athletes completed one modified 7-min match. At baseline, during the combat (3rd and 7th minutes) and 2-min post-match centre of pressure (CoP) parameters were assessed. Heart rate (HR), blood lactate (BLa) and rating of perceived exertion (RPE) and local RPE (LRPE) were collected. Significant increments were observed in CoP mean positioning and velocity at 3rd and 7th minutes, but the CoP deviation in both axes was unaffected. HR and BLa were elevated at 3rd and 7th minutes, and they remained elevated 2-min post-match. However, CoP returned to baseline 2-min post-match. RPE was elevated at 3rd and 7th minutes and the greatest effort was displayed in the Deltoid and Quadriceps. Thus, one simulated judo match stimulates a significant metabolic response and balance is degraded, with the greatest effects on the anterior-posterior axis and it recovers to baseline level after 2 min of passive rest. The physiological load cannot be regarded as a potential predictor variable of CoP. Overall, a judo match predominantly affects the upper body than the other body parts.

Time to task failure influences the postural alteration more than the extent of muscles fatigued

The aim of this study was to compare the effects of unilateral and bilateral muscle fatigue on monopedal postural control. Nineteen subjects completed bilateral fatiguing contractions and unilateral fatiguing contractions of the quadriceps femoris until the torque output for both exercises dropped below 30% of the measured peak torque (the time to task failure was measured) for three consecutive contractions (independently measured for each leg). Postural control was evaluated by using a force platform which recorded center of foot pressure (COP) and was measured before and after the completion of both fatiguing tasks. Spatio-temporal COP parameters were used to evaluate postural control. The unilateral contractions affected monopedal postural control more than the bilateral fatiguing contractions (p<0.05). Moreover, the time to task failure was significantly longer for the unilateral contractions than for the bilateral contractions (p<0.05). The greater alteration of postural control for the unilateral fatiguing contractions compared to the bilateral fatiguing contractions could be related to a longer time to task failure which could provoke greater disturbances of the postural system in terms of sensory input and motor output.

Effects of unilateral knee extensor muscle fatigue induced by stimulated and voluntary contractions on postural control during bipedal stance

OBJECTIVE

This study aim was to compare the effects of unilateral knee extensor fatigue induced by electrically stimulated (ES) and voluntary (VOL) contractions on postural control during bipedal stance.

METHODS

Seventeen healthy active males (21.5±2 years) completed these two fatiguing exercises of the quadriceps femoris muscle. Both exercises were of equal duration (130 repetitions of 5 seconds, each separated by a rest period of 2 seconds) and intensity (20% isometric maximal voluntary contraction [MVC]). Both MVC and postural control were recorded using an ergometer and a force platform that registered the centre of foot pressure (COP). We analyzed the spatiotemporal COP parameters (in eyes closed condition) and the spectral power density given by the wavelet transform. Recordings were performed before (PRE condition) and after the completion of each fatiguing task (immediately: POST condition; and after a 5-minute recovery: POST 5 condition).

RESULTS

In the POST and POST 5 conditions, the ES exercise affected MVC more than the VOL exercise but the bipedal postural control was similarly deteriorated for both exercises.

CONCLUSIONS

The disturbance of the bipedal postural control after unilateral knee muscle fatigue is not only related to a reduction in muscle strength but also (especially) to an impairment of the effectiveness of sensory inputs. Unilateral knee muscle fatigue induced by ES similarly degrades the bipedal postural control as that induced by VOL, and the duration of the recovery of postural control did not differ between both fatiguing exercises.

Stimulated contractions delay and prolong central fatigue compared with voluntary contractions in men

Voluntary and stimulated contractions are commonly used in sports training and rehabilitation, and it is well known that both these kinds of contractions generate central fatigue. However, to date, there is a lack of research on the comparison of the mechanisms by which these 2 exercises induce central disturbances. Central fatigue can be characterized by central activation failure during maximal voluntary contraction (MVC). Superimposition of an electrical stimulation onto MVC has been used to detect central activation failure. Completeness of activation has been quantified by the central activation ratio (CAR) = MVC/(MVC + stimulated force). The aim was not only to evaluate the CAR immediately after fatiguing voluntary (VOL) and stimulated (STIM) contractions but also to compare recovery duration over different time periods (prefatigue: PRE condition; immediate postfatigue: POST condition; after a 5-minute recovery: POST 5 condition; after a 30-minute recovery: POST 30 condition) (n = 18). Results showed that in the POST condition, the CAR is more affected for the VOL contractions than for the STIM contractions (p < 0.001). The CAR was affected for the STIM contractions only in the POST 5 condition (p < 0.05). In the POST 30 condition, the CAR was incomplete for the STIM contractions, whereas it was complete for the VOL contractions (p < 0.01). In conclusion, the VOL contractions alter the CAR more than the STIM contractions immediately after their completion. However, the effects of the STIM contractions on the CAR are delayed and prolonged.