Plantar flexor muscle size, composition and strength and the redistribution of joint work in gait of healthy older adults.

Background: Older adults exhibit hallmark changes in gait mechanics that may contribute to mobility loss. These changes are suggested to be precipitated by decreased mechanical output from the ankle plantar flexor muscles. However, the extent of age-related changes within individual triceps surae muscles and the consequences for plantar flexor specific torque (torque per unit muscle) and walking mechanics remains unclear. The study aims were to quantify age-related differences in triceps surae muscle cross-sectional area (CSA), fat fraction and peak absolute and specific plantar flexion torque, and to evaluate their relationships with positive hip and ankle work distribution during walking. Methods: Eleven younger (36.2 +/-3.4 years, 5 female) and 12 older (73.3+/-2.8 years, 6 female) adults completed dynamometry, magnetic resonance fat-water imaging of the shank, and overground gait analysis at a prescribed (1.2 m/s) and fast walking speed. Results: Older adults had a smaller fat-free muscle CSA for the soleus and medial gastrocnemius, resulting in ∽25% smaller triceps surae CSA. There was no difference by age in fat fraction for any muscle. Peak plantar flexor torque was lower in older and proportional to the differences in cross-sectional area, thus specific torque did not differ by age. Older adults generated greater positive hip work at both speeds and less positive ankle work at the fast speed leading to a greater redistribution ratio compared to younger. However, no significant correlations were found between the redistribution ratio and triceps surae muscle morphology or plantar flexion absolute or specific torque in older adults. Conclusions: These results suggest that triceps surae muscle morphology and function may not be a primary source of age-related changes in gait mechanics in healthy older adults.