Note: Master’s thesis work.
Overview:
Sarcopenia, the loss of muscle mass with age, is a common problem in the elderly population. Although exercise is a common prescription, older adults have a decreased ability to gain muscle compared to young adults. Evidence also suggests muscles in older adults are stiffer, due to increased glycation in the extracellular matrix. Strain is a mechanical signal for hypertrophy and increased stiffness may cause muscle cells to experience less strain for any given load. Therefore, we hypothesized that response to exercise is impaired in older adults because of increased stiffness.
For this study, 19 rats were split into young (12 months) and old (32-33 months) groups. Each rat underwent 3 sets of 10 maximum eccentric dorsiflexions utilizing electrical stimulation and a dynamometer. Following sacrifice, two dorsiflexor muscles were harvesed - the tibialis anterior (TA) of both legs and extensor digitorum longus (EDL) of the non-exercised leg. The EDL underwent stiffness testing to determine the Young’s modulus of the muscle. The TAs were used in a Western blot analysis to determine cellular response. Focal adhesison kinase (FAK) is a protein that is phosphorylated with stretch, making it a good indicator of exercise response. The ratio of phosphorylated to total FAK was used a measure of cellular to response to exercise and termed FAK activity.
As a result, we found smaller and stiffer muscles in the old rats. There was also greater variability in these measures in the old rats. A negative relationship was also seen between muscle stiffness and size. In older rats, a negative relationship was seen between FAK activity and muscle stiffness, while a positive relationship was seen between FAK activity and muscle size.
Our results describe relationships between muscle size, muscle stiffness, and response to exercise in old age. This study implicates increased muscle stiffness as possible mechanism behind the development of sarcopenia. However, while results suggest the possibility of a relationship between sarcopenia and muscle stiffness, this study lacks the ability to establish a causal link.
