Sarcomere length changes during end-held (isometric) contractions in intact mammalian (rat) fast and slow muscle fibres

The sarcomere length change, within a 2 mm region, during end-held isometri c contractions in intact rat fast and slow muscle fibre bundles was investi gated at 20 degreesC and an initial sarcomere length of 2.68 mum using He-N e laser diffraction. In some experiments, the fibre segment displacement wa s monitored with markers (pieces of human hair) placed at regular intervals on the surface of the muscle fibre bundles. The sarcomere length changes, monitored near the proximal end of the bundle (transducer end), during teta nic contractions were similar to those previously reported in frog muscle f ibres. Thus, throughout the tension plateau, sarcomere length remained cons tant (and shortened) but showed evidence of non-uniform sarcomere behaviour (further shortening) during the rapid tension relaxation phase. Such non-u niform behaviour was not seen during twitch contractions. During a twitch c ontraction, sarcomeres at the proximal end shortened rapidly at first and c ontinued to shorten - or remained shortened - until the tension had relaxed to between 20-23% of its peak value before lengthening back to the origina l length. The maximum twitch sarcomere shortening (mean +/- SEM) was 5.9 +/ - 0.2% (n = 16) in fast and 5.4 +/- 0.3% (n = 14) in slow fibre bundles at 20 degreesC; sarcomere shortening near body temperature (similar to 35 degr eesC) was greater, 8.8 +/- 0.2% (n = 7) in fast and 8.1 +/- 0.2% (n = 5) in slow fibre bundles. Increasing the initial sarcomere length of a preparati on decreased the extent of sarcomere shortening and reducing the amount of sarcomere shortening, by sarcomere length clamping, markedly increased the peak twitch tension without significantly altering the twitch time course. When examined at different positions along muscle fibres, a sarcomere short ening was observed along much of the fibre length in most preparations. How ever, in about a third of the preparations some sarcomere lengthening was r ecorded in the distal end, but its amplitude was too small to accommodate t he fibre shortening elsewhere. Complementary data were obtained using the s urface marker technique. The displacement was largest and in opposite - but fibre shortening - direction in the markers placed similar to0.5-1.0 mm aw ay from the two tendon attachments; the markers placed at or near the centr e of the fibre bundle showed the least amount of displacement. The findings suggest that the compliant region, where lengthening occurs, is at fibre e nds, i.e. near myotendinous junction.