A simple method for field measurements of leg stiffness in hopping

A new method to measure the leg stiffness in hopping and bouncing, with simple technical equipment and under field conditions, is introduced and validated. The leg stiffness (K (N)) was calculated from only contact and flight times measured by a contact 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 x 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 regulation in the optimization of human locomotion.