Rate of force development is Ca-dependent and influenced by Ca-sensitivity in human single muscle fibres from older adults.

Natural adult aging is associated with declines in skeletal muscle performance, including impaired Ca sensitivity and a slowing of rapid force production (rate of force redevelopment; k). The purpose of this study was to investigate the relationship between impaired Ca sensitivity and k of single muscle fibres from young and older adults. Participants included 8 young (22-35 yrs) and 8 older (60-81 yrs) males who were living independently. A percutaneous muscle biopsy of the vastus lateralis of each participant was performed. Single muscle fibre mechanical tests included maximal Ca-activated force (P), force-pCa curves, and k. We showed a decrease in pCa in old type II fibres compared to young, indicating impaired Ca sensitivity in older adults. The k behaved in a Ca-dependent manner such that with increasing [Ca], k increases, to a plateau. Interestingly, k was not different between young and old muscle fibres. Furthermore, we found strong associations between pCa and k in both old type I and type II fibres, such that those fibres with lower Ca sensitivity had a slowed k. This Ca association, combined with impaired Ca handling in older adults suggests a potential Ca-dependent mechanism affecting the transition from weakly- to strongly-bound cross-bridge states, leading to a decline in skeletal muscle performance. Future research is needed to explore the role alterations to Ca sensitivity/handling could be playing in age-related whole muscle performance declines.