Leg power and hopping stiffness: relationship with sprint running performance SOUHAIEL M. CHELLY and CHRISTIAN DENIS Laboratoire de Physiologie–GIP Exercice, Groupe PPeh, Faculté de Médecine, Saint Etienne, FRANCE ABSTRACT CHELLY, S. M., and C. DENIS. Leg power and hopping stiffness: relationship with sprint running performance. Med. Sci. Sports Exerc., Vol. 33, No. 2, 2001, pp. 326 –333. Purpose: Although sprint performance undoubtedly involves muscle power, the stiffness of the leg also determines sprint performance while running at maximal velocity. Results that include both of these characteristics have not been directly obtained in previous studies on human runners. We have therefore studied the link between leg power, leg stiffness, and sprint performance. Methods: The acceleration and maximal running velocity developed by 11 subjects (age 16  1) during a 40-m sprint were measured by radar. Their leg muscle volumes were estimated anthropometrically. Leg power was measured by an ergometric treadmill test and by a hopping test. Each subject executed a maximal sprint acceleration on the treadmill equipped with force and speed transducers, from which forward power was calculated. A hopping jump test was executed at 2 Hz on a force platform. Leg stiffness was calculated using the flight and contact times of the hopping test. Results: The treadmill forward leg power was correlated with both the initial acceleration (r = 0.80, P  0.01) and the maximal running velocity (r = 0.73, P  0.05) during track sprinting. The leg stiffness calculated from hopping was significantly correlated with the maximal velocity but not with acceleration. Conclusion: Although muscle power is needed for acceleration and maintaining a maximal velocity in sprint performance, high leg stiffness may be needed for high running speed. The ability to produce a stiff rebound during the maximal running velocity could be explored by measuring the stiffness of a rebound during a vertical jump. Key Words: ERGOMETRIC TREADMILL, HOPPING, MUSCLE VOLUME, RUNNING, SPRING-MASS MODEL T he production of external power during the acceler- ation phase of sprinting has been studied in humans for many decades (17). Cavagna et al. (7) studied the power output at each step during sprint running from the start to the maximal running velocity (9.4 ms -1 ). They found that the power generated by the contractile compo- nent of the leg muscles increased in parallel with speed up to submaximal values (approximately 5 ms -1 ). They pro- posed that the elastic component of the leg muscles provide the additional power required to sustain higher speeds up to maximal values (V max ). They finally suggested that a sprint runner uses the work absorbed in his leg muscles (negative work) at high speed to release further positive work and thus increase power output. Cavagna et al. (8) also recognized that a runner bounces more quickly and stiffly (equivalent to increased elasticity of the support leg) at high running speed than at low running speeds. The role of limb stiffness in maximal running velocity has been clearly outlined in other animals, particularly in lizards (12). Luthanen and Komi (1980) (24) divided the stance phase during running into two apparent spring constants: the first was labeled the apparent spring constant during the eccen- tric phase, and the second was labeled the apparent spring constant during the concentric phase. They found that the apparent spring constant during the eccentric phase in- creased with speed. Mero and Komi (28) showed an abrupt increase in the apparent spring constant during the eccentric phase, between 90% and 100% of maximal running speed in sprinters. The best sprint runners had the greatest apparent spring constant during the eccentric phase. These findings for trained sprinters may be due to specific characteristics and training-induced adaptations. The question of whether leg power and leg stiffness may act together to determine the sprinting ability of teenage runners has never been verified. We have therefore investigated this relationship. The studies cited above all used high-speed cameras and long force platforms to measure the external power. How- ever, simpler ergometric tools can be used to measure the power generated during sprinting. Ergometric treadmills (9 –11,22,23), in which the runner moves a running belt and pulls a strain gauge-equipped rod joining his/her waist to a fixed point at the rear of the treadmill, have been employed to evaluate mechanical power during acceleration. The mean external power output during acceleration, calculated from the speed of the belt and from the pulling force, was 500 –900 W for adult women and men (9 –11,22). Maximal power can also be measured in the laboratory by jumping tests (5). Mero et al. (30) demonstrated functional links between jumping and sprinting performance. The mean power developed during hopping in place can be estimated from the time the foot is in contact with the ground and from the flight time (6). The stiffness of the legs can be evaluated with the spring-mass model using these same variables of time, especially for hopping in place (26). Hopping in place has basic mechanical features similar to the spring-mass model used during forward hopping or running (13). 0195-9131/01/3302-0326/$3.00/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE ® Copyright © 2001 by the American College of Sports Medicine Received for publication November 1999. Accepted for publication May 2000. 326