Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 360

FIGURE 15.8 Time course of change in isometric force resultant from exposure to SSCs (oscillatory group) and
isometric contractions (isometric control) over a 10-day period.
pronounced reduction in soreness, loss of strength, and release of enzymes, such as creatine kinase
after the second exposure than the initial exposure. 17,58,60 Also, a second exposure of eccentric con-
tractions does not appear to delay the recovery from the first exposure. Typically, recovery from eccentric
contraction-induced injury takes at least 10 days for recovery of performance and remodeling of
muscle fiber disruption. 96 If the second session is administered within 6 days after the first exposure,
the recovery time was unaffected. 194 Thus, the normal recovery process is not affected by at least
one intervening injurious exposure. Clearly there is an adaptation that takes place after one exposure
to injurious eccentric contractions that ameliorates the injurious response to subsequent exposures.
The mechanism by which this takes place is not clearly understood at this time. Exposure to a mild
session of eccentric contractions still provides a protective effect to further sessions of more intense
eccentric exposure. 40,60
Some authors postulate that the adaptive mechanism could be at the level of the central nervous
system where recruitment patterns could be adapting to recruit in a different fashion such that motor
unit recruitment would be appropriately synchronized to reduce asymmetric stresses in the muscle
fibers. 126,194 However, the repeated bout effect has also been found in electrically stimulated animal
models, which would argue against the hypothesis that neural factors provide the adaptive effect. 179
Thus, the mechanism for adaptation must be in the muscle fibers themselves. There could be weak sar-
comeres as a result of deconditioning that are more susceptible to stresses generated during eccentric
contractions. The notion that stress-susceptible fibers exist was first postulated by Armstrong et al. 8 If
fragile fibers were compromised after an initial session of eccentric muscle actions, the target muscle
should lose muscle volume as a result of the loss of those fragile fibers. This hypothesis was supported
by findings in human elbow flexors where there was a loss of muscle volume (approximately 10%) 14
days after an injurious exposure to eccentric muscle actions. In contrast, after a second session performed
8 weeks later, there was no loss of muscle volume. The authors concluded that the fragile fibers were
lost after the first session and are repaired over time. As repaired fibers replace the fragile fibers, the
muscle becomes more resistant to eccentric contraction-induced damage. 85 However, if muscle is not
used, fragile fibers may again develop that can be injured at some juncture. There is also evidence
that one session of injurious eccentric contractions can ameliorate the resultant damage from the