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Introduction
Vibration applied to muscle or tendon induces a non-voluntar y m uscular contraction term ed the `tonic vibration reX ex' (Eklund and H agbarth, 1966). The voluntary im petus increases such a m uscular contraction, and thus the maximum voluntar y contraction can be facilitated (M atyas et al., 1986). M oreover, vibratory stimulation com bined with a substantial voluntary effort was show n to elicit m ovem ent in neuromuscular patients w ho were unable to contract their paretic muscles (Hagbarth and Eklund, 1966). T he technique
is widely used in neurophysiology and physiotherapy (G ranit, 1970; Bishop, 1974). Attempts to use vibratory stimulation in the training of athletes have been undertaken only recently (Nazarov and Spivak, 1987). A substantial increase in muscle strength was observed after 3 weeks of vibratory stim ulation strength training Author to w hom all correspondence should be addressed. when com pared w ith regular strength training (Issurin et al., 1994). Explosive strength, or the ability to develop force within a ver y short time, is of prim ary im portance in any sports. Typical exercises for explosive strength training are characterized by fast m uscular contractions with an external load of about 50± 70% of m axim al strength (Vrijens, 1990). The im m ediate eV ect of
such exercises can be assesse d by the power which an
athlete can generate in a m ovem ent. Several additional
training techniques have been used to accentuate power
training: the quick release technique, pre-stretching of active m uscles before contraction, electrical stimulation and biofeedback. The objectives of these techniques are to im prove upon previous achievements, to facilitate motor learning effects and to enhance muscular capacity (Torrey,1985). Based on the results of a previous study (Issurin et al., 1994), it is likely that similar outcomes may also be achieved using vibratory stimulation.
Participants
Altogether, 28 male athletes aged 18-42 years volunteered to participate in the study. They were divided into two groups (Table 1). The First group consisted of athletes from the Israeli national judo, wrestling, weightlifting, gymnastics and track and field teams. These athletes regularly engaged in highly intensive power training. The second group consisted of amateur athletes participating in club or college sports, such as basketball, volleyball, judo, weightlifting, bodbuilding, boxing and track and Weld. The amateur athletes were also engaged in power exercises but not as extensively as their elite counterparts (2± 4 times a week). Because all of the athletes were familiar with power exercises, they were able to perform several repetitions w ith m axim al effort and high reproducibility (see Table 2). This was one reason w hy elite and highly qualiW ed athletes were enrolled as participants. The study was approved by the local ethics committee and inform ed consent was obtained from the participants before the study began. Instrumentation and tests. The athletes perform ed bilateral biceps curl exercises in a sitting position on a `Schnell' dynam ic bilateral biceps machine (Schnell, G erm any, D.B. Pat. 2213440). They were secured to the m achine by pads placed at the elbow, chest and back (Fig. 1). The pulling action began from a position of maximal forearm extension and finished with the elbow at an angle of 90 degrees.
The athletes were instructed to perform each repetition as quickly as possible. The superim posed vibration during the exercise was transmitted to the muscles by a specially designed vibratory stim ulation device (Issurin et al., 1994). It
consists of an electrom otor w ith a speed reduction and eccentric wheel. The load is held by a cable which is passed through the eccentric wheel via the pulleys. The eccentric rotation elicited peak-to-peak oscillations of 3 m m with a frequency of 44 H z. After vibration dam ping owing to cable transmission, the acceleration on the handle was about 30 Vibration from the two-arm s handle was transmitted through the contracting m uscles involved in the pulling action.
The power of the active phase of exercise was measured using a `Power Teach' instrument (G E SportS.A.S., Rome, Italy). Two probes were installed on thecounterweight frame. The locations of the probes were established during the warm -up; the lower probe was placed 2 cm above the counterweight start position and the upper probe was placed opposite the W nal counterweight position. Therefore, the probes covered the complete range of m ovem ent. The distance between the probes and the counterweight was transferred to a Vibratory stimulation.
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