Residual force enhancement is not altered while force depression is amplified at the cellular level in old age.

Residual force enhancement (rFE) and residual force depression (rFD) are history-dependent properties of muscle which refer to increased and decreased isometric force following a lengthening or shortening contraction, respectively. The history-dependence of force is greater in older than younger human adults when assessed at the joint level. However, it is unclear whether this amplification of the history-dependence of force in old age is owing to cellular mechanisms or a consequence of age-related remodeling of muscle architecture. Single muscle fibres from the psoas major of old and young F344BN rats were dissected and chemically permeabilized. Single muscle fibres were mounted between a force transducer and length controller, then maximally activated (pCa 4.5). To assess rFD, fibers were actively shortened from 3.1 to 2.5µm at both a slow (0.15Lo/s) and fast (0.6Lo/s) speed, with a fixed-end isometric reference contraction at 2.5µm. To assess rFE, fibers were activated and stretched at 0.3Lo/s from a sarcomere length of 2.2 to 2.5µm, and 2.7 to 3.0µm, and compared to fixed-end isometric reference contractions at 2.5 and 3.0µm, respectively. Isometric force (2.5 µm) was ≈19% lower in old as compared with young (p<0.001). Upon normalizing to fibre cross-sectional area, there was no age-related difference in specific force (p>0.05). rFD was ≈33% greater in old as compared with young (p<0.05), while rFE did not differ between groups (p>0.05). rFD is amplified in old age at the cellular level, while rFE appears to be unchanged, thus previously reported age-related modification of rFE occurs upstream from the cellular level.