The Effect of Prestretch on Skeletal Muscle Behavior

(1992). Pulmonary and leg VO, during submaximal exercise: Implications for muscular efficiency. The article points out the controversial question related to the mechanical behavior of skeletal muscle during the stretch-shortening cycle (SSC) and its effective capacity in enhancing muscle efficiency. Though Ingen Schenau et al. analyzed a large spectrum of theories, suggestions , and reported findings, the present contribution is restricted solely to four observations. First, the authors, comparing vertical jumps performed without and with counter-movement (CMJ), state that "this enhancement [of work output due to the stretch-shortening cycle] was attributed to the fact that the prestretch allows the muscles to develop a high level of active state and force before shortening begins" (p. 408). This is not a novel explanation, since this phenomenon was used to demonstrate the positive relationship found between mechanical delay (MD) and percentage of slow twitch fibers (Bosco, 1987). MD indicates the time spent by the contractile elements (CE) of the muscles for stretching the series elastic elements (SEE). In my 1987 study, 1 measured MD by calculating the differences between the time to perform a simple squat jump (SJ), in which the tension developed by the CE is transmitted through the muscle-tendon-bone linkage with some delay, and the corresponding time to perform positive (Tpos) work in the CMJ. In the latter case, the tension developed by CE is instantaneously transmitted to the bones. The relationship found between MD and ST fibers was not attributed to the different stiffnesses found in slow and fast muscles but was explained only through the differences in firing frequency between phasic and tonic motor units (Steg, 1964). In fact, the Tps for CMJ was similar for both slow and fast subjects, whereas in the latter subjects the TPOF for SJ was shorter than for slow subjects (p < .05). In accordance, the results were explained through the fact that fast subjects required less time to reach the maximal neural firing frequency than slow subjects (Bosco, 1987). On the other hand, at the beginning of the positive work of CMJ, all of the a-motoneurons should have been fully activated in all subjects studied (Bosco & Viitasalo, 1982), since the time for eccentric work was relatively long (128 ms on the average). The biomechanical principles and benefits of accelerating a body in two different ways were presented by Hochmuth (1960). As shown in Figure 1, the acceleration will be small in …