Age-related differences in the force generation capabilities and tendon extensibilities of knee extensors and plantar flexors in men

Background. Recently, the number of elderly individuals who participate in sports has increased, thus injuries from overuse are now becoming recognized in the elderly population. Therefore, it is important to determine which muscle groups and tendons are most affected with aging to plan appropriate exercise interventions for elderly individuals. In particular, muscles and tendons in knee extensors and plantar flexors play an important role during locomotion. The purpose of this study was to compare the knee extensor and plantar flexor muscles and tendons. Methods. Young (n = 19) and elderly (n = 17) men performed isometric voluntary knee extension and plantar flexion contractions. Muscle thickness and elongation of tendon structures in knee extensors and plantar flexors were measured by ultrasonogaphy. Results. Relative muscle thickness (to limb length) in the elderly group was significantly lower than that in the young group in knee extensors (p < .001), although no significant difference was found between the two groups in plantar flexors (p = .063). Relative muscle strength (to body mass) in the elderly group was significantly lower than that in the young group in both sites (all p < .001). Ratio of muscle strength to muscle thickness in the elderly group was significantly lower than that in the young group in plantar flexors, but not in knee extensors. The elderly group had significantly lower maximal elongation and strain of tendon structures in both sites than the young group had. Conclusion. These results suggest that the age-related weakness in knee extensors may be attributed to muscle atrophy, whereas that in plantar flexors is not, and that elderly persons have less extensible tendon structures in both sites.