DEVITA, P. and W. A. SKELLY. Effect of landing stiffness on joint kinetics and energetics in the lower extremity. Med. Sci. Sports Exerc., Vol. 24, No. 1, pp. 108–115, 1992. 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 tall of 59 cm. Soft and still‘ 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 < 0.01) and knee flexor (–0.010 vs –0.013 N · m · s · kg-1; P < 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 < 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 < 0.01; knee, –0.89 vs –0.61 W · kg-1; P < 0.01), while the ankle muscles absorbed more in the stiff landing (–0.88 vs –1.00 W · kg-1; P < 0.05). Overall, the muscular system absorbed 19% more of the body's kinetic energy in the soft landing compared with the stiff landing, reducing the impact stress on other body tissues. The ankle plantarflexors provided the major energy absorption function in both conditions, averaging 44% of the total muscular work done followed by the knee (34%) and hip (22%) extensors.