Normalized force, activation, and coactivation in the arm muscles of youngand old men

The purpose of this study was to determine whether the loss of muscle stren gth in the elderly could be explained entirely by a decline in the physiolo gical cross-sectional area (PCSA) of muscle. Isometric force, muscle activa tion (twitch interpolation), and coactivation (surface electromyograph) wer e measured during maximal voluntary contractions (MVCs) of the elbow flexor s (EFs) and extensors (EEs) in 20 young (23 +/- 3 yr) and 13 older (81 +/- 6 yr) healthy men. PCSA was determined using magnetic resonance imaging, an d normalized force (NF) was calculated as the MVC/PCSA ratio. The PCSA was smaller in the old compared with the young men, more so in the EEs (28%) co mpared with the EFs (19%) (P< 0.001); however, the decline in MVC (<similar to>30%) with age was similar in the two muscle groups. Muscle activation w as not different between the groups, but coactivation was greater (5%) (P< 0.001) in the old men for both muscles. NF was less (11%) in the EFs (P< 0. 01) and tended to be unchanged in the EEs of the old compared with young su bjects. The relative maintenance of NF in the EEs compared with the EFs may be related to age-associated changes in the architecture of the triceps br achii muscle. In conclusion, although the decline in PCSA explained the maj ority of strength loss in the old men, additional factors such as greater c oactivation or reduced specific tension also may have contributed to the ag e-related loss of isometric strength.