Introduction In literature [5, 24], human locomotion has been studied by using a spring-mass model; the mass representing the total body mass and the spring the behaviour of the support leg. Then, the leg stiffness is the ratio of the peak force and the maximal deforma- tion of the spring. Previous literature has shown that leg stiffness regulation could play an important role in the optimization of human locomotion. For instance it has been evidenced that leg stiffness is increased with running speed [1] and with stride fre- quency [15]. Moreover, at given velocity, inter-individual varia- tions of the energy cost of running were also related to leg stiff- ness differences [12] i.e. the higher the leg stiffness, the lower the energy cost of running. In addition the capability to produce and maintain high level stiffness has been considered as an important factor of maximal performance measured during explosive exercise [28,30] and more specifically during sprint running [8,11, 23,29]. In laboratory conditions, lower limb stiffness is already measur- ed and calculated in bouncing, hopping [2, 6,14,18] or running [1,6,16,24] by means of force plates or treadmill ergometers [14,16,19, 20, 22]. Different methods have been developed to cal- culate limb stiffness: some authors used the slope of the linear regression ground-reaction force versus centre of mass (CM) dis- placement relationship [2,10,12] or used the ratio between the maximal force and the CM displacement [15], others measured the time when the vertical ground reaction force is greater than A Simple Method for Field Measurements of Leg Stiffness in Hopping G. Dalleau 1,2 A. Belli 3 F. Viale 1,2 J.-R. Lacour 2 M. Bourdin 2 Affiliation 1 Centre Universitaire de Recherche en ActivitØs Physiques et Sportives, FacultØ des Sciences et Technologies, UniversitØ de la RØunion, Le Tampon, France 2 Laboratoire de Physiologie de lExercice, FacultØ de MØdecine Lyon-Sud, UniversitØ Claude-Bernard, Oullins, France 3 Laboratoire de Physiologie ± GIP Exercice, DØpartement STAPS, UniversitØ de Saint Etienne, France Correspondence G. Dalleau ´ Centre Universitaire de Recherche en ActivitØs Physiques et Sportives ´ FacultØ des Sciences et Technologies ´ UniversitØ de la RØunion ´ 117 Rue du GØnØral Ailleret ´ F97430 Le Tampon ´ France ´ Fax: +262 57 95 71 ´ E-mail: georges.dalleau@univ-reunion.fr Accepted after revision: June 23, 2003 Bibliography Int J Sports Med 2004; 25: 170±176  Georg Thieme Verlag Stuttgart ´ New York ISSN 0172-4622 ´ DOI 10.1055/s-2003-45252 Abstract A new method to measure the leg stiffness in hopping and bouncing, with simple technical equipment and under field con- ditions, is introduced and validated. The leg stiffness (K N ) was calculated from only contact and flight times measured by a con- tact mat. It was compared to the reference stiffness (K R ) obtained from force platform measurements. Eight subjects performed, first, submaximal hopping movements at different frequencies (1.8 to 4 Hz, by step 0.2 Hz) and, second, maximal hopping. In sub maximal hopping K N was significantly correlated with K R (r = 0.94; p < 0.001) and the difference between K N and K R ranged from ±7.2 % to 6.9 % (at 1.8 and 3.6 Hz respectively) with a limit of agreement of ±1.5 kN  m ±1 . In maximal hopping K N was also related to K R (r = 0.98, p < 0.001) and the inter individual rank order was respected (R = 0.87). It was concluded that the new method could be applied to study extensively intra individual and inter individual variations of leg stiffness in respectively sub maximal and maximal hopping and thus to simplify further investigations in field conditions of the role of stiffness regula- tion in the optimization of human locomotion. Key words Spring-mass model ´ leg stiffness ´ hopping ´ field testing Training & Testing 170