Protocol

Aim
To demonstrate the relationship between muscle length and passive and active tension. For this experiment the frog sartorius muscle is used.

The muscle is set up at the equilibrium length (this is the flaccid length of the muscle when it is removed from the body). The muscle is then passively stretched, in 1 mm increments, using the micrometer screw gauge and the tension (resistance to stretch) measured at each length. This is known as the 'passive tension' and is due to the elastic components of muscle which resist stretch rather like an elastic band. In order to study the effect of muscle length on contractile tension, the muscle is stimulated tetanically at each length and the contractile tension (i.e. the extra tension generated above the passive tension) measured. This is known as 'active tension'. In this experiment the equilibrium length of the flaccid muscle is 26 mm and it is possible to stretch it, in 1 mm increments, a total of 10 mm.

1. Stretch the muscle, in increments of 1 mm, by 10 mm from the flaccid length;
2. at each length measure the passive tension;
3. at each length deliver a brief tetanic stimulus to the muscle (supramaximal voltage; 100 Hz for 0.5 sec).

Task
Measure passive and active tension at each muscle length. Draw a graph of active and passive tension (y-axis) against muscle length (x-axis).

Then try answering the questions.

Note
total tension = passive + active tension.

Additional Information

The sartorius muscle is a strap-like muscle located in the anterior thigh. It originates in the iliac spine and inserts into the medial head of the tibia in the lower limb. It is innervated by the femoral nerve and adducts the thigh and flexes the knee rotating the leg medially and the thigh laterally.