Figure 1.
Isokinetic eccentric stretch contractions in (a) cardiac and (b) skeletal muscle. Blue solid lines indicate mean stress responses during an active isokinetic eccentric stretch. The shaded regions indicate the standard deviations. For comparison, the total isometric stress–length relation (black dashed line) and passive isometric stress–length relation (grey dotted line) are shown. Diamonds and crosses express the mean values of total and passive isometric muscle stresses, respectively. (a) Bars indicate corresponding standard deviations for lengths from 0.75 to 1.0 L/L0, except for the mean values at 1.0 L0 of total isometric stress–length dependency, as the stress is normalized to maximum isometric stress (P/P0). The length is normalized to optimum muscle length (L/L0, lower abscissa) or given as SL (µm) (upper abscissa), respectively. The mean values of cardiac muscles from the total isometric contraction (diamonds) were fitted to a third-order polynomial function (black dashed line), whereas those from the passive isometric contraction (crosses) were fitted using an exponential function (grey dotted line). A total of n=11 cardiac trabeculae were examined for all measurements. In all ramp experiments, the stretch velocity was 10% vmax yielding the blue solid line. The observed nonlinear stress response (blue solid line) in cardiac muscle was not statistically different (marked as ‘n.s.’) from the corresponding total isometric stress values at distinct lengths of 0.75 L0, 0.8 L0, 0.85 L0, 0.9 L0 and 0.95 L0 (table 1). (b) For systematic comparison of contractile behaviour between cardiac and skeletal muscles during isokinetic eccentric stretching, measurements, obtained under similar experimental conditions as in the cardiac experiments for skinned skeletal fibres from EDL muscles are shown. In contrast with cardiac muscle, the characteristic linear spring behaviour (blue solid line) in skeletal muscle statistically exceeds (p < 0.001, as indicated by asterisks) the maximum total stresses over nearly the entire physiological working range (inset; unshaded region). Data reproduced from [5].