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.

Residual force enhancement decreases when scaling from the single muscle fiber to joint level in humans.

Background: Residual force enhancement (rFE), defined as increased isometric force following active lengthening compared to a fixed-end isometric contraction at the same muscle length and level of activation, is present across all scales of muscle. While rFE is always present at the cellular level, often rFE "non-responders" are observed during joint-level voluntary contractions.

Methods: We compared rFE between the joint level and single fiber level (vastus lateralis biopsies) in 16 young males.

Submaximal eccentric resistance training increases serial sarcomere number and improves dynamic muscle performance in old rats.

The age-related loss of muscle mass is partly accounted for by the loss of sarcomeres in series, contributing to declines in muscle mechanical performance. Resistance training biased to eccentric contractions increases serial sarcomere number (SSN) in young muscle, however, maximal eccentric training in old rats previously did not alter SSN and worsened performance. A submaximal eccentric training stimulus may be more conducive to adaptation for aged muscle.

Age-related differences in the loss and recovery of serial sarcomere number following disuse atrophy in rats.

Background: Older adults exhibit a slower recovery of muscle mass following disuse atrophy than young adults. At a smaller scale, muscle fibre cross-sectional area (i.e.