title: Force-displacement differences in the lower extremities of young healthy adults between drop jumps and drop landings.
abstract: We measured ground reaction force and lower extremity shortening in ten healthy, young adults in order to compare five trials of drop jumps to drop landings. Our dependent variable was the percentage of displacement (shortening) between the markers on the ASIS and second metatarsal heads on each LE, relative to the maximum shortening (100% displacement) for that trial at the point of greatest ground reaction force. We defined this as "percent displacement at maximum force" (%dFmax). The sample mean %dFmax was 0.73%±0.14% for the drop jumps, and 0.47%±0.09% for the drop landings. The mean within-subject difference score was 0.26%±0.20%. Two-tailed paired t test comparing %dFmax between the drop jump and drop landing yielded P=0.002. For all participants in this study, the %dFmax was greater in drop jumps than in drop landings. This indicates that in drop jumps, the point of maximum force and of maximum shortening was nearly simultaneous, compared to drop landings, where the point of maximum shortening followed that of maximum force by a greater proportion. This difference in force to displacement behavior is explained by linear spring behavior in drop jumps, and linear damping behavior in drop landings.
authors.affiliation: Department of Physical Therapy, Missouri State University, 901 South National Avenue, Springfield, MO 65897, USA. Electronic address: jameshackney@missouristate.edu.
authors.forename: James M
authors.initials: JM
authors.lastname: Hackney
authors.affiliation: Department of Physical Therapy, Missouri State University, 901 South National Avenue, Springfield, MO 65897, USA.
authors.forename: Rachel L
authors.initials: RL
authors.lastname: Clay
authors.affiliation: Department of Physical Therapy, Missouri State University, 901 South National Avenue, Springfield, MO 65897, USA.
authors.forename: Meredith
authors.initials: M
authors.lastname: James
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D035002:Lower Extremity / Q000502:physiology*; D008297:Male; D013177:Sports / Q000502:physiology*; D055815:Young Adult
keywords: Damping; Drop jumps; Drop landings; Force return