The combined effects of weight training and plyometrics on dynamic leg strength and leg power

Abstract

The purpose of this study was to determine the effects of plyometric exercise (depth drops), combined with weight training, on dynamic leg strength and leg power. Plyometrics are exercises that force a rapid lengthening of muscle prior to contraction, to result in increased force output during contraction. Thirty-one volunteer university students were randomly assigned to three groups according to height of drop (1.1m=high, 0.4m=low and no height). Subjects in each group were classified in two conditions according to leg strength-body weight ratio (low=less than 2, and high=greater than 2). All groups were administered a dynamic leg strength test and Magaria anaerobic power test prior to and following an eight-week plyometric and weight training program. A two-way ANOVA (groups (3) x conditions (2)) revealed no significant differences between groups, conditions, and no significant interactions for leg strength and Magaria power scores. Independent T-tests for mean differences between pre- and post-test scores demonstrated significant gains in both strength and power for each group. 1¢t was concluded that participating in a combined 8-week

program of plyometrics and weight training will improve leg strength and power. Furthermore, coaches and athletes should be apprised that neither the level of strength nor height of drop variables altered the resultant training effects of the combination program used in this study. The possibility of reducing the time between forced stretch at impact and initiation of contraction to improve plyometric training effects was discussed.

Introduction

Training techniques for athletes often consist of a variety of exercises, all aimed at peak performance during com- petition. Plyometrics is one such exercise. It is becoming increasingly popular for the improvement of leg strength and power. Theoretically, plyometrics are exercises which use the rapid lengthening of a muscle, just prior to con- traction, resulting in an increased force output during contraction (3). Any exercise which forces a rapid lengthening of the muscle prior to its contraction may be considered plyometric. One of the most popular plyometric exercises is the depth jump. Depth jumps require a person to drop from a controlled height and, upon landing,

immediately . perform a maximum vertical

jump.

Verhoshanski (11), in a discussion of plyometric exercise, has divided depth jumps into three phases. The first phase is called amortization and occurs as a result of yielding work forcing a rapid stretch of the lower body extensor muscles. In the second phase, muscles perform a reactive switch from yielding work to overcoming work to initiatea positive vertical velocity. The third phase is the phase of active take-off. The extensor muscles contract to perform the jump. The first phase stretches the extensor muscle groups, the second isa reactive recovery, and the third uses the benefit of a reciprocal increase of force during contraction.

Plyometrics were first popularized by Russian athletes and coaches. The Soviets proclaim depth jumps will “stimulate maximum contraction capabibilities, those

above the athlete's conscious will” and “will improve the viscoelasticity of the contractile properties of soft tissues” (7). While there is no evidence to directly support or refute

the Russian propositions, a number of investigators offer vidence to support the plyometric theory. Cavagna (3) has

shown that stored elastic energy within a stretched muscle affects the production of contractile force following muscle stretch. Other factors which may contribute to this elastic energy storage and the increase in the contractile force of muscle are: time between muscle stretch and contraction (8), amplitude of