Figure 5. Sarcomere length-muscle length relation.

Relation between sarcomere length and muscle length for theoretical muscle fibres comprising different numbers of sarcomeres and different lengths of tendon. It is assumed that optimum sarcomere length is 2.5 μm. For a muscle fibre with 10 000 sarcomeres and 20 mm of tendon (fibre no. 1) tension begins to rise at 35 mm and the optimum is reached at 45 mm (dashed line). A shift in optimum length for active tension by 5 mm in the direction of longer lengths (dashed line) can be achieved by increasing the length of tendon to 25 mm (fibre no. 2). The drawback is that active tension is not developed until the muscle is stretched to 40 mm, that is, the working range of muscle lengths has been reduced. Increasing the number of sarcomeres from 10 000 to 12 000, and leaving tendon length at 20 mm (fibre no. 3) produces the required increase in optimum length and leads to a smaller reduction in the muscle's working range, where tension begins to rise at 39 mm. Increasing sarcomere number further to 14 000, while at the same time reducing the length of tendon to 15 mm (fibre no. 4) produces the most satisfactory result, the required shift in optimum length by 5 mm and reduction of the working range by only 1 mm.