Introduction: Rebound jumps (RJs), successive vertical jumps performed as quickly as possible with a brief foot-ground contact duration, are often used as part of plyometric training. Since RJs incur high mechanical outputs of the ankle joint, it was hypothesized that RJ height would be affected by the degree of plantar flexion angle prior to each landing. In this study, RJ heights were compared between two distinct plantar flexion conditions. Ankle joint kinematics, ground reaction force (GRF) and ground-contact duration were measured to identify factors that determined the difference in RJ height, if observed, resulting from the alteration of the plantar flexion angle.
Methods: Fifteen track-and-field sprinters and jumpers performed six repetitions each of RJs, putting great emphasis on the ankle joint movement, under two conditions: small planter flexion (PFS) and large plantar flexion (PFL). Arm swings were permitted during the testing to allow for the best RJ performance. RJ height, ankle joint kinematics and GRF (including foot-ground contact duration) were measured using video images (480fps), an electro-goniometer (1000Hz) and a single-axis force plate (500Hz), respectively.
Results: From landing to take-off, plantar flexion angles were verified to be less for PFS by approximately 10º compared to PFL (P<0.001). Angular velocities of the ankle at the instant of landing and take-off were not different between conditions. RJ height and the concentric impulse were greater for PFs than PFL by 0.06±0.04m (P<0.001) and 12.1k±19.5kN･s (P=0.031), respectively. GRF at the transition from the eccentric phase to the concentric phase (amortization) did not differ between conditions. The eccentric duration was slightly but significantly shorter for PFS than PFL by 0.007±0.007s (P=0.003). However, the concentric duration and the foot-ground contact duration were similar between conditions.
Discussion: During RJ training, smaller plantar flexion may enhance jump height as a result of increased concentric impulse. An alteration in plantar flexion angle was thought to influence the length of muscle-tendon unit, across which the stretch-shortening cycle (SSC) occurred. This may have impacted on the level SSC effects. Factors pertaining to the increased concentric impulse could be 1) increased force output at the amortization and 2) extended concentric duration, neither of which was however not evidenced by the pooled results. A third mechanism explaining the increased concentric impulse was proposed to be increased force output throughout the concentric phase following the amortization, which was not quantified by the current methodology. To be more precise, the mechanisms of the increased concentric impulse varied among subjects, attributed to one of the aforementioned factors or a combination of those.
Impact and application to the field:
・When performing rebound jumps during plyometric training, jump heights and concentric impulses are augmented with a small plantar flexion angle. This strategy may allow for more specific adaptive stimuli for the improvements in jump performance and other related power tasks.
My co-authors and I acknowledge that we have no conflict of interest of relevance to the submission of this abstract.