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Whole-Body vibration (WBV) may lead to muscle contractions via reflex activation of the primary muscle spindle (Ia) fibres. WBV has been reported to increase muscle power in the short term by improved muscle activation. The present study set out to investigate the acute effects of a standard WBV training session on voluntary activation during maximal isometric force production (MVC) and maximal rate of force rise (MRFR) of the knee extensors. Twelve students underwent a single standard WBV training session: 5x1 min vibration (frequency 30 Hz, amplitude 8 mm) with 2 min rest in between. During vibration, subjects stood barefoot on the vibration platform with their knees at an angle of 110 degrees. At 90 s following vibration, maximal voluntary knee extensor force was reduced to 93 (5)% [mean (SD), P<0.05] of baseline value and recovered within the next 3 h. Voluntary activation remained significantly depressed (2-4%). Neither the electrically induced MRFR nor voluntary MRFR were significantly affected by WBV. In addition, six WBV training sessions in 2 weeks ( n=10) did not enhance either voluntary muscle activation during MVC [99 (2)% of the baseline value] or voluntary MRFR [98 (9)% of the baseline value]. It is concluded that in the short term, WBV training does not improve muscle activation during maximal isometric knee extensor force production and maximal rate of force rise in healthy untrained students.

The practice period had a favorable effect on some technique variables and mechanical efficiency, which may indicate a positive effect of improved technique on mechanical efficiency. Although muscle activation and kinematic segment characteristics were not measured in the present study, they may also impact mechanical efficiency. No changes occurred over time in most force application parameters, left-right symmetry, and intercycle variability during the 3-wk practice period; however, these variables may change on another time scale.

The purpose of the present study was to investigate whether 11 weeks of whole body vibration (WBV) training applied in a way that is commonly seen in practice, i.e. without additional loads, would improve muscle activation and/or contractile properties of the knee extensor muscles and counter movement jump height in healthy subjects. Ten subjects belonging to the experimental group trained three times a week and stood bare-foot with a 110 degrees knee angle on a vibration platform (30 Hz, 8 mm amplitude). They underwent five to eight sets of 1-min vibration with 1 min rest in between. Ten control subjects followed the same training programme but stood (110 degrees knee angle) beside the platform. Before, during and following the training period the subjects were tested. Values [mean (SEM)] obtained in the last test were expressed as percentages of the baseline value and presented for control and experimental groups. Quadriceps femoris isometric muscle force [105.4 (6.2)%, 99.9 (2.0)%; P=0.69], voluntary activation [107.1 (6.0)%, 101.1 (2.3)%; P=0.55] and maximal rate of voluntary force rise [95.4 (6.0)%, 103.3 (7.7)%; P=0.57] did not improve. The maximal rate of force rise during electrical stimulation was increased [102.3 (4.5)%, 123.6 (7.5)%; P=0.02]. Counter movement jump height was not affected by WBV [103.7 (1.8)%, 103.0 (2.8)%; P=0.71]. In conclusion, 11 weeks of standard two-legged WBV training without additional training loads did not improve functional knee extensor muscle strength in healthy young subjects.

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The shortening of the R-R interval in response to voluntary and electrically induced isometric muscle contractions of short duration was investigated in 15 volunteers. In some of those experiments the effect of vagal blockade was also studied. The results show: 1) a lag time between the start of the contraction and the following decrease in R-R interval duration of 550 milliseconds; 2) a similar R-R interval response due to voluntary and electrically induced contractions of the same force; 3) no shortening of the R-R interval when the skin is stimulated without ensuing muscular contraction; 4) a complete disappearance of the response to isometric contractions during vagal blockade. A difference in lag time between the onset of arm contraction and cardiac acceleration could not be demonstrated. Most of the results give strong evidence to the existence of a muscle-heart reflex in man, involved in the instantaneous cardiac acceleration at the onset of exercise, that has its origin in the muscles and the vagal nerves as its efferent pathway.

The wheelchair training at both intensities can have favorable effects on maximal physical work capacity in able-bodied subjects, and possibly also on mechanical efficiency at submaximal power output. Effects are seen in parameters for both aerobic and anaerobic work capacities. Although training at 70% intensity was more effective than the 50% intensity, training at 50% HRR may initially be more appropriate in untrained individuals, such as novice wheelchair users at the start of their rehabilitation, to prevent fatigue and enhance motivation.

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