Effect of landing stiffness on joint kinetics and energetics in the lower extremity.

Ground reaction forces (GRF), joint positions, joint moments, and muscle powers in the lower extremity were compared between soft and stiff landings from a vertical fall of 59 cm. Soft and stiff landings had less than and greater than 90 degrees of knee flexion after floor contact. Ten trials of sagittal plane film and GRF data, sampled at 100 and 1000 Hz, were obtained from each of eight female athletes and two landing conditions. Inverse dynamics were performed on these data to obtain the moments and powers during descent (free fall) and floor contact phases. Angular impulse and work values were calculated from these curves, and the conditions were compared with a correlated t-test. Soft and stiff landings averaged 117 and 77 degrees of knee flexion. Larger hip extensor (0.010 vs 0.019 N.m.s.kg-1; P less than 0.01) and knee flexor (-0.010 vs -0.013 N.m.s.kg-1; P less than 0.01) moments were observed during descent in the stiff landing, which produced a more erect body posture and a flexed knee position at impact. The shapes of the GRF, moment, and power curves were identical between landings. The stiff landing had larger GRFs, but only the ankle plantarflexors produced a larger moment (0.185 vs 0.232 N.m.s.kg-1; P less than 0.01) in this condition. The hip and knee muscles absorbed more energy in the soft landing (hip, -0.60 vs -0.39 W.kg-1; P less than 0.01; knee, -0.89 vs -0.61 W.kg-1; P less than 0.01), while the ankle muscles absorbed more in the stiff landing (-0.88 vs -1.00 W.kg-1; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)