Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 359

15.4.3.10 SSC-Induced Injury
A viable method to study eccentric and concentric muscle performance simultaneously in the context
of muscle injury is via SSCs. SSCs (reciprocal eccentric shortening contractions) have been studied in

the context of human locomotion and athletic performance 14 and have been shown to produce
muscle injury due to the eccentric component of the cycle. 97,125 Natural muscle function involves
SSCs and this model provides a sound physiological foundation in which to study muscle mechanics
and injury. 149 Both the change in concentric and eccentric muscle function before and after an injury
protocol (that includes eccentric contractions), and the change in concentric and eccentric muscle func-
tion during the injury protocol can be investigated using SSCs. Recently, the relationship between
changes in negative and positive work and the isometric force deficit after an injurious exposure has
been investigated and the study showed a positive correlation between the change in negative work
and the isometric force deficit resultant from injury. 62 Also, changes in real-time muscle function
during SSCs (Figure 15.7) are also positively correlated with isometric force deficit and the degree of
myofiber injury after an injurious exposure. 97 Specifically, the decay in peak eccentric forces and the
decay in force enhancement during each stretch during an injury protocol were positively correlated
with isometric force deficit and the degree of fiber injury. This study also demonstrated that changes
in real-time eccentric force production during SSCs are indicative of resultant performance decrement
(Figure 15.8) and myofiber damage days later (Figure 15.9). Exposure to equivalent isometric contrac-
tions did not result in myofiber injury (Figure 15.9). Increasing the number of repetitions of SSCs
results in an increase in the degree of myofiber injury (Figure 15.10).

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15.4.3.11 Chronic Injury
It is well understood that acute exposure to unaccustomed eccentric muscle actions can result in injury,
but an initial exposure can be beneficial to reduce injury resulting from a subsequent exposure. 58,60 This
protective effect from an initial exposure has been shown to reduce the effects of a subsequent exposure
1 week upto 6 months from the initial exposure (with no intervening exercise). 59,179 Thus, there is a

FIGURE 15.7 Fifteen sets of injurious SSCs performed on active dorsiflexor muscles at 1-min intervals. The curves
are the force response (N) of the dorsi flexor muscles during sets 1, 3, 5, 7, and 15. The SSCs were conducted at a range
of motion of 70-120-70 8 in a reciprocal fashion at 500 deg sec.

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