Figure 11. Stiffness during steady lengthening in slow and fast fibres.

A, sample records of force responses to step length changes imposed on the active fibre either in isometric conditions (upper row) or during steady force response to lengthening at about 0.65 μm s⁻¹ hs⁻¹ (lower row) in a slow fibre (left column) and in a fast 2A/2X fibre (right column). In each panel, the upper trace is length change per hs; the lower trace is the force response. Slow fibre: fibre length, 3.35 mm; segment length, 1.07 mm; average sarcomere length in the segment, 2.51 μm; CSA, 4500 μm²; temperature, 12.1°C; T₀, 67 kPa; T_v/T₀, 2.48. 2A/2X fibre: fibre length, 3.4 mm; segment length, 1.2 mm; average sarcomere length in the segment, 2.64 μm; CSA, 12 400 μm²; temperature, 11.6°C; T₀, 170 kPa; T_v/T₀, 1.32. B, relation of stiffness versus lengthening velocity. Stiffness is relative to that measured in isometric conditions. Filled circles are mean values (± s.e.m.) for slow fibres (nine fibres; data contributing to each class between 5 and 15). Open symbols are for fast fibres (four fibres; each symbol refers to a different fibre). C, relation of the stiffness measured in kPa nm⁻¹ hs⁻¹versus lengthening velocity for the same data as in B.