The Effects Of Anterior In-flight Perturbation On Lower Extremity Biomechanics During Drop Landings

Landing after an athlete is airborne is a common situation associated with ACL injuries. We surmised that an unanticipated force applied to the trunk (about the longitudinal axis) would cause an undesirable body position and subsequently cause abnormal landing biomechanics. PURPOSE: To determine the effects of in-flight perturbations applied to the trunk about the longitudinal axis of the body. METHODS: Ten recreational female collegiate athletes (age: 20.4±1.3 yr; mass: 64.2±10.3 kg; height: 169.6±8.4 cm) participated in this study and performed two blocks of testing. First, to serve as a baseline (BASE), participants performed five acceptable trials of natural double-leg drop landings (ht = 0.55 m). In the second block of testing, trials consisted of drop landings with either an unexpected anteriorly-directed perturbation (PERT) or no perturbation (SHAM) with the testing sequence of PERT and SHAM trials randomly assigned. During the flight phase for PERT trials, the cable attached to a proprietary perturbing machine pulled on the participants’ dominant side shoulder in an anterior direction. Three-dimensional lower extremity joint kinematic (240 Hz) and ground reaction force (GRFs:1,200 Hz) data were recorded for the landing phase. Paired-samples t-tests were used to detect differences between the BASE and PERT landing conditions (p<.05). RESULTS: During the PERT condition, compared to the BASE condition, the following were greater: peak magnitudes of vertical (2.77±0.65 N·kg-1 and 2.25±0.57 N·kg-1) medial (0.34±0.08 N·kg-1and 0.27±0.08 N·kg-1), and posterior (1.15±0.27 N·kg-1and 0.95±0.23 N·kg-1) GRFs, and knee adduction (0.27±5.9° and -0.92±5.8°) and hip abduction (0.43±1.3° and 1.70±1.9°) displacements. CONCLUSION: We found that an unexpected anteriorly-directed in-flight perturbation about the longitudinal axis of the body leads to an increased peak magnitude of GRFs in all three directions (vertical, posterior, and medial). The greater peak GRFs during PERT suggest an increased risk of injury, as increased peak GRFs are associated with increased ACL loading. However, the anterior in-flight perturbation might induce compensatory frontal plane knee and hip joint angles that are beneficial for avoiding a knee valgus position during the landings.