Investigation into the relationship between the passive flexibility and active stiffness of the ankle plantar-flexor muscles

Abstract

Objective. The purpose of the study was to investigate the relationship between measurements of passive flexibility and active stiffness of the ankle plantar-flexor muscles. Design. The study was a correlation design. Background. Flexibility has passive and active components. Little information is available regarding the relationship of these measurements in terms of the information that they yield on the state of the muscle–tendon unit. Methods. Free oscillation data representing active stiffness was obtained using applied masses equivalent to 30%, 60% and 90% of the subject’s maximal voluntary contraction. The angle of dorsiflexion, representing passive flexibility, was measured in standing using computer digitisation to obtain the angle. Results. Mean active stiffness values were 14280 N/m (30% maximal voluntary contraction), 22260 N/m (60% maximal voluntary contraction) and 28010 N/m (90% maximal voluntary contraction). Dorsiflexion measurement gave a mean of 34.3 degrees (S.D. 4.8). Correlation’s obtained for the association between range of motion and active stiffness were r=0.01 (30% maximal voluntary contraction), r=0.09 (60% maximal voluntary contraction) and r=0.04 (90% maximal voluntary contraction). Moderate reliability coefficients of 0.71 (30% maximal voluntary contraction), 0.78 (60% maximal voluntary contraction) and 0.68 (90% maximal voluntary contraction) were obtained. Conclusions. The results imply that measurements of passive flexibility and active stiffness of the lower leg musculature are independent measures of components of muscle–tendon unit flexibility. Relevance Flexibility is a construct with different components of measurement. Etiological studies typically relate static flexibility measurements to injury with conflicting outcomes. This study suggests that static and active flexibility measures yield different information about the muscle tendon unit of the ankle plantar flexors, and that researchers should consider this point in the design of etiological studies.