Passive elastic properties of the calf muscle-tendon unit of distance runners

Studies have suggested that distance runners have decreased calf muscle-tendon unit (MTU) flexibility, which is related to improved running economy. This study examined the passive elastic properties of the calf MTU of distance runners to test the hypothesis that they have increased passive elastic properties measured in vivo. Ten distance runners (aged 18-21 years) and 10 untrained men (aged 23-28 years) were tested. A Kin-Com (R) dynamometer stretched the right calf MTU from plantarflexion (PF) to maximal dorsiflexion (DF) at 5 degrees center dot s(-1) (0.087 rad center dot s(-1)) with minimal surface electromyogram activity in the soleus, gastrocnemius, and tibialis anterior muscles. ANOVA analyses indicated that the maximal DF ROM did not differ between groups, but the maximal passive DF torque (Nm), total absorbed passive elastic energy (area under full stretch curve [degrees center dot Nm]) and passive elastic stiffness (Nm center dot(degrees-1)) within the last 10 degrees of the stretch were greater for the distance runners when controlled for body mass (P <= 0.007). The maximal passive DF torque and the passive elastic stiffness were greater when controlled for PF maximal voluntary isometric contraction torque (P <= 0.028). The increased passive resistive properties suggested structural adaptations in the calf MTU, possibly the result of eccentric loading and stretching, which would contribute to improved running economy.