Relationship between passive properties of the calf muscles and plantarflexion concentric isokinetic torque characteristics

The purpose of this study was to use a model of aging to examine the relationships between passive properties of the calf muscles and plantarflexion concentric isokinetic torque characteristics. Eighty-one active women 20-84 years of age were tested using a Kin-Com isokinetic dynamometer interfaced with electromyography (EMG). The passive properties were tested by stretching the muscles from relaxed plantarflexion to a maximal dorsiflexion (DF) angle at a rate of 5degrees.s(-1) (0.087 rad.s(-1)) with minimal raw EMG activity (<0.05 mV). The maximal concentric torque was tested from maximal passive DF into plantarflexion at four randomly ordered velocities of 30, 60, 120, and 180degrees.s(-1). Pearson correlation coefficients (Bonferroni adjusted) indicated a hierarchical order of high to moderate positive correlations between four passive properties and the peak and mean concentric torque for all test velocities. Correlation coefficients for the four passive properties ranged from 0.50 to 0.78 (P<0.001), and the coefficients of determination (r(2)) from higher to lower were: (1) maximal DF passive resistive torque (r(2) : 0.50-0.62), (2) length extensibility (r(2) : 0.40-0.49), (3) maximal muscle length (r(2) : 0.28-0.41), and (4) passive elastic stiffness in the last half of the full-stretch range of motion (r(2) : 0.25-0.31). The maximal DF passive resistive torque and the length extensibility accounted for 50-62% and 40-49% of the variability in the concentric torque, respectively. The results indicate that the concentrically stronger calf muscles of active women were positively correlated with passively stronger, longer, and stiffer calf muscles, which are characteristics of the calf muscles of younger women. Further studies are needed to examine whether therapeutic interventions, such as stretching and strengthening, can promote adaptations in the calf muscles of older women to attain these more youthful characteristics.