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 in 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 is a consequence of age-related remodelling 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, fibres were actively shortened from 3.1 to 2.5 µm at both a slow (0.15 Lo s-1) and fast (0.6 Lo s-1) speed, with a fixed-end isometric reference contraction at 2.5 µm. To assess rFE, fibres were activated and stretched at 0.3 Lo s-1 from a sarcomere length of 2.2 to 2.5 µm, and 2.7 to 3.0 µm, and compared with fixed-end isometric reference contractions at 2.5 and 3.0 µm, respectively. Isometric force (2.5 µm) was ∼19% lower in muscle fibres from old as compared with young rats (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 muscle fibres from old as compared with young rats (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.