The Effect Of Height And Post-Landing Movement Task On Landing Performance

Abstract

The purpose of this research was to evaluate selected biomechanical aspects of drop jumps and stable landings performed from increasing heights. The related literature indicates that landings performed in an experimental setting have been to a large degree isolated landing tasks, with subjects landing and remaining in a stable position. It was the intent of this research to address the interactive demands of the complete jumping/landing task. Seven subjects performed three stable landings (L) and three drop jumps (DJ) from each of four initial drop heights (15, 30, 45 and 60cm). All landings were performed with both feet contacting a force platform (500 Hz). Right knee sagittal plane angular displacement was recorded at 500 Hz via an electrogoniometer. Vertical GRF variables used to evaluate landing included maximum impact force (Fmax) normalized to subject body mass, and the time at which the amount of vertical impulse (Timp-land) necessary to account for the downward momentum of the body was achieved. To derive Timp-land, the total body vertical momentum at contact was estimated using contact velocity (calculated from initial drop height) and subject mass. Integration of the GRF curve was perforrned to establish the time relative to contact at which impulse sufficient to reduce landing momentum to zero was achieved. Timpland was therefore indicative of the time period over which the landing phase could be considered complete. In addition, maximum flexion angle at the knee (Kmax) was used in the analysis. Group mean values summarized across heights were as follows for the Land D movement tasks, respectively: Fmax: 52.0 and 41.8N /kg; Timp-land 0.118 and 0.126 sec; Kmax: 71.3 and 80.1 deg. Pearson product correrations were performed for each subject, relating initial drop height (Ht) to each of the three independent variables describing landing. Six of seven subjects (86%) exhibited strong positive correlations (r > .7071; explained variance> 50%) between Ht and Kmax and Ht and Timp-land . indicating an impact force increase and concomitant increase in time over which greater landing momentums were accounted for, observable for landing with and without the peformance of a subsequent jump. The Ht, Kmax relationship was strong and positive for the L conditions (71 % with r > .7071), but only one of seven subjects exhibited a significant Ht, Kmax relationship for the DJ performances. These results suggest that increased knee flexion is a common component of a strategy to absorb landing momentum over longer periods as height increases for stable landings. This does not, however, appear to allow for complete accommodation of Fmax across the full range of heights. The absence of a strong relationship between Ht and Kmax with DJ performance is suggestive of a change in kinematic strategy with the addition of the post-landing movement task.