Purpose: The purpose of this study was to determine age-related differences in the human plantar flexor muscles and tendon.
Methods: Four age groups-a 20-yr group (20-27 yr, N = 19), 30-yr group (31-38 yr, N = 15), 50-yr group (46-57 yr, N = 10) and 70-yr group (62-77 yr, N = 15)-volunteered to take part in the present study. Muscle thickness, strength, and activation level (using twitch-interpolation technique) of plantar flexor muscles were measured. Elongation of the Achilles tendon was determined using ultrasonography while subjects performed ramp isometric plantar flexion up to the voluntary maximum.
Results: No significant difference in relative muscle thickness (to limb length) was observed among the four age groups. Muscle strength and activation level of the 20-yr group were significantly higher than those of the 50- and 70-yr groups (activation levels were not measured in the 70-yr group), and maximal strain (elongation/initial tendon length) of the Achilles tendon decreased with aging. Although there were no differences in muscle strength and activation levels between the 20- and 30-yr groups, maximal strain of the Achilles tendon of the 30-yr group was already lower than that of the 20-yr group (P = 0.062).
Conclusion: These results suggest that the processes of age-related changes in the muscle and tendon are different. Furthermore, the differences in age-related changes of muscle and tendon might play a role in the frequency of Achilles tendon ruptures among men in their 30s.