BILATERAL NEUROMUSCULAR AND FORCE DIFFERENCES DURING A PLYOMETRIC TASK NICK B. BALL AND JOANNA C. SCURR Department of Sport and Exercise Science, University of Portsmouth, Portsmouth, United Kingdom ABSTRACT Ball, NB and Scurr, JC. Bilateral neuromuscular and force differences during a plyometric task. J Strength Cond Res 23(5): 1433–1441, 2009—The purpose of this article is to compare the bilateral neuromuscular and force contribution during a plyometric bounce drop jump task and to assess the affects of nonsimultaneous foot placement. Sixteen male participants performed bounce drop jumps from a height of 0.4 m. Mean peak electromyography activity of the soleus, medial, and lateral gastrocnemius of both legs was recorded from each phase of the drop jump and normalized to a reference dynamic muscle action. Resultant ground reaction force, ground contact time, and duration of the drop jumps were recorded from each leg. Multivariate analysis of variance was used to compare bilateral electromyographic activity, resultant peak ground reaction force, and contact duration. Pearson’s correlations (r) ascertained relationships between normalized electromyographic activity and contact time. Significant differences were shown between left and right triceps surae normalized electromyography during precontact and contact 40ms (p , 0.01). No significant differ- ences were present in the contact post40ms phase (p . 0.01). Significant differences were found between normalized soleus electromyography and both gastrocnemii for both legs during precontact (p , 0.01). No significant differences were found for within-leg normalized electromyography for the contact 40ms phases and contact post40ms phase (p . 0.01). Weak relationships were found between normalized electromyographic activity and nonsimultaneous foot contact (r , 0.2). This study showed differences between left and right triceps surae in neuromuscular strategies engaged in the early stages of a drop jump task. Differences in contact time initiation were present; however, they are not significant enough to cause neuromuscular differences in the plantar flexor muscles. KEY WORDS electromyography, drop jump, normalization, triceps surae INTRODUCTION P lyometrics is a method of developing explosive power with exercises such as bounding, hopping, and jumping used to parallel the movement patterns of sprinting and jumping (17). Plyometrics have been shown to improve performance as part of a periodized training program for athletic development (20). Plyometric exercises work through use of the stretch shortening cycle (SSC) and the elastic components found within muscles by training the neuromuscular apparatus to switch rapidly from force-resisting to force-producing actions (22). The SSC enables production of higher force and power outputs compared with concentric muscle actions alone (6). After 40 ms of ground contact during a lower body plyometric exercise, the isometric loading phase begins whereby energy is stored and the switch from eccentric to concentric occurs (29). This switch is called the amortization phase and its minimization is a primary aim of plyometric exercise to increase power production in a short time period (22). To assist in the use of stored energy, the neuromuscular system prepares for the impending load by activating muscles before contact (24). This preactivation serves to stiffen the joint to prevent collapse immediately after contact and assists in contributing to an efficient pushoff phase (8). The drop jump is a popular plyometric exercise and in- volves stepping off a box to prestretch the ankle plantar flexors, landing on the floor with both feet, and then imme- diately jumping causing rapid shortening of these muscles (27). The minimization of ground contact time is related to the muscles’ capability to use the stored elastic energy as a result of a reduced amortization phase. Drop jumps can be modified to focus on the amortization phase of particular muscle groups (14) with the use of the SSC during drop jumps being shown to be a factor of knee range of motion (25). This study uses the bounce method (6), whereby mini- mal knee flexion increases the use of the triceps surae and reduces the force absorbed on impact (14). Different phases of the drop jump have been proposed by many authors to establish muscular behavior during the task with precontact (100 ms before contact), initial contact (35–40 ms post- contact), isometric loading phase (commences 40 ms after contact), and propulsion the most common (11,14,15,29). Previous electromyographic (EMG) research on drop landings has shown precontact muscle activity to be greater Address correspondence to Dr. Nick Ball, nick.ball@canberra.edu.au. 23(5)/1433–1441 Journal of Strength and Conditioning Research Ó 2009 National Strength and Conditioning Association VOLUME 23 | NUMBER 5 | AUGUST 2009 | 1433