Longitudinal sarcomere dynamics of skeletal muscle
The issue of variation in sarcomere behaviour during contraction is important to skeletal muscle mechanics; however, few studies have measured sarcomere length throughout a contracting muscle. This project examined longitudinal variation in sarcomere lengths in skeletal muscle. The behaviour of sarcomeres was investigated under three conditions: isometric, concentric, and lengthening contractions in five regions (distal, mid-distal, middle, mid-proximal and proximal) along the length of sartorius muscles. Sarcomere mechanics was quantified by measuring sarcomere work using the work loop technique. In isometric contractions at seven muscle lengths (between 95% and 105% Lo) and three stimulus durations (50, 100 and 400 ms), the sarcomeres in the distal region lengthened, while the sarcomeres in the other regions shortened. During concentric contractions at three excursion amplitudes (2%, 5% and 10% Lo) and two frequencies (1 Hz and 2 Hz) sarcomeres in the middle three regions of the muscles performed roughly twice the work of those in the distal and proximal regions. Sarcomere work was inversely related to frequency, and increased with excursion amplitude. Sarcomere work showed significant variation between regions and across initial contraction velocities during lengthening contractions. Generally, sarcomeres in the middle regions of the muscle (mid-distal, middle, mid-proximal) showed an increase in work (positive, net and total in the respective regions) with increasing stretch velocity, while the proximal and distal showed an increase in the amount of negative work, a small increase in positive work, and a decrease in net and total work. The work performed by distal sarcomeres tended to respond oppositely to the sarcomeres in other regions of the muscle - that is, if distal sarcomeres displayed an increase in negative work with increasing stretch velocity, then other areas would show an increase. My findings suggest that sarcomeres in various regions of skeletal muscle do not behave uniformly under different conditions. These differences in behaviour have implications for studies that attribute muscle phenomena to sarcomere behaviour measured at only one point on a muscle. The distal region of the muscle may be more prone to injury as it shows more lengthening of sarcomeres during contraction than other regions of the muscle.