Increased residual force enhancement in older adults is associated with a maintenance of eccentric strength

Abstract

Despite an age-related loss of voluntary isometric and concentric strength, muscle strength is well maintained during lengthening muscle actions (i.e., eccentric strength) in old age. Additionally, in younger adults during lengthening of an activated skeletal muscle, the force level observed following the stretch is greater than the isometric force at the same muscle length. This feature is termed residual force enhancement (RFE) and is believed to be a combination of active and passive components of the contractile apparatus. The purpose of this study was to provide an initial assessment of RFE in older adults and utilize aging as a muscle model to explore RFE in a system in which isometric force production is compromised, but structural mechanisms of eccentric strength are well-maintained. Therefore, we hypothesised that older adults will experience greater RFE compared with young adults. Following a reference maximal voluntary isometric contraction (MVC) of the dorsiflexors in 10 young (26.1 ± 2.7 y) and 10 old (76.0 ± 6.5 y) men, an active stretch was performed at 15°/s over a 30° ankle joint excursion ending at the same muscle length as the reference MVCs (40° of plantar flexion). Any additional torque compared with the reference MVC therefore represented RFE. In older men RFE was ~2.5 times greater compared to young. The passive component of force enhancement contributed ~37% and ~20% to total force enhancement, in old and young respectively. The positive association (R(2) = 0.57) between maintained eccentric strength in old age and RFE indicates age-related mechanisms responsible for the maintenance of eccentric strength likely contributed to the observed elevated RFE. Additionally, as indicated by the greater passive force enhancement, these mechanisms may be related to increased muscle series elastic stiffness in old age.

Figure 1. Raw data depicting the determination of residual force enhancement (RFE) in older and young men.

Passive force enhancement (PFE) was determined as elevated force above rested baseline upon relaxation after stretch.

Figure 2. Eccentric and concentric strength in old (open bars) and young men (closed bars) relative to isometric strength.

Values are Means ± standard error.

Figure 3. Relationship between the ratio of eccentric strength to concentric strength (Ecc:Con) and residual force enhancement (RFE) for old (closed circle) and young (open circle) (A.).

Relationship between the ratio of eccentric strength to concentric strength (Ecc:Con) and passive force enhancement (PFE) for old (closed circle) and young (open circle) (B.). Relationship between the passive force enhancement (PFE) residual force enhancement (RFE) for old (closed circle) and young (open circle) (C.).