Measurement of sarcomere length during fast contraction of muscle fibers by digital image analysis

A low-cost, high-resolution (spatial and temporal) image analysis system was developed to measure sarcomere length (Sl) during fast twitch of isolated striated muscle fibers at different temperatures. Fiber images were examined during twitch with an imaging rate of 220 Hz. To increase temporal resolution beyond 220 Hz, consecutive temporally shifted image sequences (N sequences) were acquired. Individual or average Sl was directly measured from a horizontal profile without spatial-frequency assessment. Measurement precision (E) was determined and expressed as: E(%)=100×Ps/(Is×Sl), where Ps is the pixel size and Is the involved sarcomere number. At 18°C during isometric twitch, Sls were measured with 220 Hz temporal and 0.2% spatial resolutions. Sl shortened in the central region (0.21±0.12 μm) as tension developed, reaching a maximal shortening of 8.09±2.05% (at rest, Sl=2.59±0.05 μm, n=4) in 32.5±1.96 ms. At 30°C, Sl variations were examined with 880 Hz temporal resolution, in which case maximal Sl shortening was reached in 15.74±1.99 ms, and then decreased to 5.19±1.97% (at rest, Sl=2.6±0.06 μm). The twitch tension developed by the whole fiber was recorded and compared with sarcomere length behavior. Sarcomere length variations in the central region were representative of overall developed tensions at 18 and 30°C.