Fig. 6. Tetanic force-length relationships for muscles 178 and 179. Different symbol shapes represent different individuals. The solid blue arrow indicates optimal length for muscle 178 (filled, blue symbols), while the broken red arrow indicates optimal length for muscle 179 (open, red symbols). The region of grey shading represents the range of strains used by the muscles when cyclically oscillated at a strain amplitude of 15%. The filled and open horizontal bars span the ranges of strain experienced by the muscles under in vivo running conditions.