Eccentric Contractions: Testing the predictions.

Non-uniform sarcomeres

This has been tested in two ways; electron-microscopy and disorder measurement.

Using rapid fixation of whole toad sartorius muscles and electron-microscopy of surface myofibrils of surface fibres, we found that over-extended half-sarcomeres were present in muscles that had undergone stretch at long length, but were absent in a muscle that had undergone an isometric contraction at the final length, and present in much smaller numbers in a muscle that had undergone a stretch at short length. A fourth muscle was fixed after relaxing from a single contraction that included a stretch at long length, and showed a very small number of disrupted sarcomeres. These experiments used a fixative that has been shown to fix a fibre rapidly, without allowing it to relax. This was confirmed by recording tension during the fixation process.

The number of overstretched sarcomeres found in the first muscle accounted for about two thirds of the applied stretch. Along with the small number found in the muscle stretched at short length, this suggests that the variability of sarcomere lengths may have been sufficient to have some sarcomeres in both regions of the length-tension curve.

It was also found that, when one half of a sarcomere was overstretched, the other half was shortened. This shortening probably occurred during the decay of tension between the end of the stretch and fixation, and may also partly account for the apparent shortfall in stretch. It also implies that the restriction of extension of the popped sarcomere was not entirely due to intra-myofibrillar structures, but includes some component to to extra-myofibrillar structures as well.

This work is reported in:

•  J. A. Talbot and D. L. Morgan (1996). Quantitative analysis of sarcomere non-uniformities in active muscle following a stretch. Journal of Muscle Research and Cell Motility, 17: 261-268.

In recent work, the existence of popped sarcomeres was sought indirectly, by monitoring the degree of striation disorder in optical micrographs of living single frog fibres. This showed that fibres decame disordered during activation, and more so again during stretch, but not significantly more so during the decay of tension that followed the end of the stretch. This demonstration that disorder was fully developed at the end of stretch could not be accomplished witht e electron microscopy. This work is being prepared for publication.

Shift in optimum

The shift in optimum has now been shown in a number of preparations.

Toad sartorius

The shift has been shown to occur without shift in the passive curve, to depend on the amplitude of the stretch, to be relatively independent of the velocity of the stretch, and to depend strongly on the range of sarcomere lengths, increasing greatly as more of the stretch occupies the descending limb of the length-tension curve. It has also been shown to spontaneously reverse after 4 to 6 hours, without a reversal of the accompanying fall in isometric tension, which is often taken as a measure of damage.

 This work has been reported in:

•  S. A. Wood, D. L. Morgan and U. Proske (1993). Effects of repeated eccentric contractions on structure and mechanical properties of toad sartorius muscle. American Journal of Physiology, 265: C792-C800.
•  C. Jones, T. Allen, J. A. Talbot, D. L. Morgan and U. Proske (1997). Changes in the mechanical properties of human and amphibian muscle after eccentric exercise. European Journal of Applied Physiology and Occupational Physiology, 76: 21-31. 
•  J. A. Talbot and D. L. Morgan (1998). The effects of stretch parameters on eccentric exercise induced damage to toad skeletal muscle. Journal of Muscle Research and Cell Motility, 19: 237-245.

 Rat vastus intermedius

The vastus intermedius (VI) in the rat is the deep, red, slow knee extensor, that has been shown to be damaged and trained by decline running. Our preparation for testing it involves plotting angle torque curves with it attached to its origin and insertion. This ensure that no tendon slippage can occur in clamping the muscle, as is always a concern in isolated muscles. As the moment arm of VI is almost constant, the angle-torque curve accurately represents the length-tension curve.

The shift has been shown to occur and depend strongly on the sarcomere length range involved, being almost zero for stretches confined to the descending limb of the angle torque curve, and increasing as more of the movement occurred on the descending limb.

These results have been reported in:

•  R. Lynn, J. A. Talbot and D. L. Morgan (1998). Differences in rat skeletal muscles after incline and decline running. Journal of Applied Physiology, 85: 98-104.

 Human triceps surae

The method use to generate eccentric contraction of the human triceps surae has been "walking backwards downhill on a treadmill". This causes the subject to land on the toe of the leading leg, and stretch the triceps surae as the heel is lowered onto the treadmill surface. If alternate steps lead with alternate legs, both legs undergo ecentric exercise. In a variant, each step cycle consists of stepping out with the eccentric leg, and then bringing the control leg to meet it. In this way, only one leg undergoe eccentric exercise.