*3.2.1. Acute effects*

Neuromuscular stimulation derived from WBV is the likely source of previously observed changes in athletic performance. The tonic vibration reflex is a response elicited from vibration directly applied to a muscle belly or tendon. This reflex is characterized by activation of muscle spindles primarily though Ia afferents and activation of extrafusal muscle fibers through alpha motor neurons (Cormie et al., 2006).

Vibration has been shown to stimulate transient increases in certain hormones, such as growth hormone and IGF-I (Cormie et al., 2006).

A study exists in which the flexibility of hamstring muscle of the lower limb was examined by varying the frequency of vibration stimulation. It showed that there was a 10% increase in flexibility at 20 Hz and no change at 40 Hz (Cardinale et al., 2003).

Vibration is detected not only by spindle, but also by the skin, the joint and secondary endings. All those structures contribute to the facilitatory input to the gamma-system which in turn affects sensitivity of the primary endings. Modulation of neuromuscular response to vibration is then to be referred not just to spindle activation, but to all the sensory systems in the body. Various parameters can affect the synergies in the sensory system and determine specific responses. Vibration is thought mainly to inhibit the contraction on antagonist muscles via Ia inhibitory neurons. However there is some evidence as well that vibrations can also produce co-activation (Cardinale et al., 2003).

A previous study investigated the acute effects of WBV on back and abdominal muscle activity. Muscle activity with vibration showed a low to moderate increase in trunk muscle activation (Wirth et al., 2011).

## *3.2.2. Long-term effects*

A 4-month WBV-loading induced a significant 8.5% mean increase in jump height of young healthy adults. This improvement was already seen after 2 months of the vibration. Lower limb extension strength was also enhanced by the 2-month vibration period. Intervention with a 4-month WBV enhanced jumping power in young adults, suggesting neuromuscular adaptation to the vibration stimulation. On the other hand, the vibration-intervention showed no effect on dynamic or static balance of the subjects (Torvinen et al., 2002).

## *3.2.3. The difference between athletes and normal healthy subjects*

There are many reports in which knee extensor strength and jump performance improved after stimulation with WBV in healthy subjects. In contrast, no improvement was observed in sprinttrained athletes and elite female field hockey players. This means either that the effect of daily training was greater than the stimulatory effect with WBV or that its cumulative activity and inhibitory effects were retained in the body system. It is suggested that the stimulating effect of WBV is probably greater in subjects that do not exercise every day.
