[The isometric knee extension strength threshold for maintaining walking speed and step length in older male inpatients].

AIM: To clarify the minimum knee extension muscle strength needed to maintain walking speed and step length in older male inpatients. METHOD: The participants were 786 male inpatients of ≥65 years of age without cerebrovascular disorder, orthopedic disease, malignancy, or dementia. We investigated the participants' isometric knee extension muscle force (kgf/kg), maximum walking speed (m/s) and step length, based on their medical records. The relationship of walking speed and step length to isometric knee extension muscle force was fitted to linear and nonlinear models, and the respective R2 values were compared. Next, the muscle force data were divided into two groups, and two linear functions were calculated. Then, the muscle force value that minimized the sum of the residual sum of squares of the two linear function expressions was obtained. RESULTS: The R2 values of each equation in the nonlinear model were higher than those in the linear model. Among all participants, the muscle force values that minimized the sum of the residual sum of squares for walking speed and step length were 0.33 kgf/kg and 0.43 kgf/kg, respectively. Among participants of ≤74 years of age, the muscle force value that minimized the sum of the residual sum of squares was 0.30 kgf/kg for both walking speed and step length, whereas the values were 0.32 kgf/kg and 0.43 kgf/kg, respectively, in participants of ≥75 years of age. CONCLUSION: Walking speed and step length were significantly decreased in male inpatients of 65-74 years of age when the isometric knee extension force values for both were <0.30 kgf/kg. In contrast, among male inpatients of ≥75 years of age, these values were significantly decreased when the respective isometric knee extension muscle force values were <0.32 kgf/kg and <0.43 kgf/kg.