Effects of Three Modified Plyometric Depth Jumps and Periodized Weight Training on Lower Extremity Power

Introduction The term plyometrics refers to specific exercises which encompass a rapid stretching of muscle that is undergoing eccentric stress followed by a concentric, rapid contraction of that muscle for the purpose of developing a forceful movement over a short period of time (Chu, 1983). One particular plyometric activity, the depth jump, has been shown to improve power in the vertical jump (Batholemew, 1985; Miller, 1982; Parcells, 1977; Verkhoshanski & Tatyan, 1983). Depth jumps are a type of dynamic exercise where an individual steps off a box 20 to 80 centimeters in height, lands, and performs an explosive vertical jump (Wilson, Murphy, & Giorgi, 1996). The depth jump is thought to enhance vertical jump performance through the quickening of the amortization phase, which is the electromechanical delay from the initiation of eccentric to the initiation of concentric muscle actions of the movement (Steben & Steben, 1981). Plyometric depth jumps have been modified to generate greater stresses at the joints of the hip, knee, and ankle (Holcomb, Lander, Rutland, & Wilson, 1996a). These variations were identified as the hip depth jump (HDJ), knee depth jump (KDJ), and ankle depth jump (ADJ). Each variation included modifications to the range of motion of the joint being emphasized during the eccentric portion of the depth jump. The HDJ, KDJ, and ADJ are thought to increase the workload, and thus power, at the particular joint for which they are named. The need for such a modification stemmed from biomechanical analysis of both the vertical and depth jumps. In biomechanical analysis of the vertical jump, the hip was found to contribute 23-39% of the total work done during the vertical jump (Bobbert, Huijing, & Van Ingen Schenaue, 1987; Bobbert, MacKay, Schinkelshoek, Huijing, & Van Ingen Schenaue, 1986; Hubley & Wells, 1983; Van Soest, Roebroeck, Bobbert, Huijing, & Van Ingen Schenaue, 1985). However, two analyses of the depth jump revealed the hip contribution to be only 19% and 13% respectively (Bobbert et al., 1986, 1987). Consequently, the traditional plyometric depth jump does not stress the hip joint to the extent that it is used during the vertical jump, the functional task it was originally designed to enhance. Biomechanical analysis of the modified plyometric depth jumps was also performed to analyze joint contribution through total work done at each joint (Holcomb et al., 1996a). Total work at the hip, knee, and ankle joints was 80%, 5%, and 15%, respectively, during the HDJ. Analysis of the KDJ revealed contributions of 37% at the hip joint, 49% at the knee joint, and 14% at the ankle joint. The joint contributions during the ADJ were reported to be 24%, 20%, and 56% at the hip, knee, and ankle joints, respectively. Therefore, each depth jump primarily stressed the particular joint for which it was named. The effectiveness of training programs is routinely measured via functional test performance. Functional tests usually contain a series of movements that have high correlations with athletic activity and are used for research, evaluation, and rehabilitation purposes. Biomechanical analyses of functional tests can reveal percent joint contributions to the activity. Table 1 contains the percent joint contributions of modified plyometric depth jumps and selected functional tests for this study. Although specific joint contributions have not been calculated for the 30-meter sprint or seated single leg press, some research has examined the power output of these functional tests. Researchers have identified the hip to be a dominant force producer in sprints of short duration (Mero & Komi, 1990; Mero ,Komi, & Gregor, 1992; Mero & Peltola, 1989). Wilk et al. (1996) examined the electromyographic activity of the quadriceps and hamstring muscles during a two-legged seated leg press and found a high degree of quadriceps activity, suggesting significant power contributions from the knee joint. …