Is the force-length relationship a useful indicator of contractile element damage following eccentric exercise?

Eccentric exercise has been shown to have a measurable effect on the force–length relationship (FLR), as peak force is shifted to longer muscle lengths following exercise. Recently, this shift in the FLR has been proposed as a “simple, reliable indicator” for assessing contractile element damage following eccentric exercise. However, eccentric exercise causes fatigue and damage, and there is evidence that fatigue alone may also cause a shift in the FLR. The purpose of this paper was to assess the role of fatigue on the FLR (as measured by a torque–joint angle relationship) following isometric and eccentric exercise in the New Zealand white (NZW) rabbit. Six NZW rabbits were divided into two groups for eccentric or isometric contractions of the hindlimb dorsiflexor muscles. Pre- and post-exercise torque–joint angle relationships were measured, and the shift from the pre- to the post-exercise relationship was measured as the change in joint angle at which peak torque was produced. Eccentric exercise resulted in a rightward shift of seven degrees; isometric exercise, which is thought to not cause damage, resulted in a shift of four degrees. Furthermore, torque production was reduced to a greater extent at short compared to long muscle lengths for the eccentric and isometric exercise, resulting in a post-exercise torque–joint angle relationship that was altered in shape. We conclude from these results, that the shift in peak torque may not be a simple and reliable indicator of muscle damage, but is caused by a combination of damage and post-exercise fatigue.