Testing the Sliding-Filament Model

• When skeletal (or cardiac) muscle contracts, the thin and thick filaments in each sarcomere slide along each other without their shortening, thickening, or folding.
• The strength of the relative motion between the thick and thin filaments is determined by the number of cross-bridges that can form between the two.

• An isolated muscle (e.g., the calf muscle of a frog) placed in this apparatus cannot shorten when stimulated by an electric shock.
• Thus stimulation of the muscle produces an isometric ("same length") contraction.
• The muscle is placed in the apparatus and stretched to the desired length.
• It is then given a series of tetanizing shocks to measure its "active" tension; that is, the tension produced when it is stimulated.
• The strain gauge measures the tension exerted by the muscle.
• (Muscles are elastic, so if we stretch the muscle in putting it in the apparatus, it will already be exerting a "resting" tension.)
• Now let us plot the effect of muscle length on the active tension that is produced.

The results:

• The muscle produces the highest tension when held in the apparatus at the length it normally has in the intact animal.
• If held at longer (or shorter) lengths, the active tension produced is less.

  You may conclude from this that nature knows best. And, in fact she does. If a muscle is surgically reattached to an animal so that its length is changed, the muscle gradually adapts to its new length and, after a few weeks, is able to exert its maximum isometric contractions at the new length.

The interpretation:

• A muscle stretched beyond its normal length has less overlap between the thick and thin filaments.
• Thus fewer cross-bridges can form to slide the filaments against each other.
• In fact, if the muscle is stretched so far that the thin filaments are pulled entirely away from the thick filaments, the muscle exerts no tension at all.
• As for the effect of holding the muscle at shorter than normal length, the thin filaments extend so far across the sarcomere that they interact with cross-bridges exerting force the opposite way — reducing the tension generated.

These electron micrographs (courtesy of Dr. H. E. Huxley) show the pattern of striations in stretched muscle and resting muscle.

Consequently

• The light I bands and the H zone become wider, but
• the width of the dark A band remains unchanged.
• We would not expect this pattern if sarcomere length involved a change in the length of the thick filaments.