Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 357

15.4.3.6 The Effect of Loading History on Injury Susceptibility
The understanding of the injury pathophysiology is important in preventing contraction-induced
injury. The predisposing factors, which mitigate injury are also important. Anecdotal observations of
physical activities that result in a high incidence of injury has led to the hypothesis that if physical activi-
ties are repeated with adequate recovery time, muscles will eventually become “trained” and no injury
will occur. Conversely, disuse could increase contraction-induced injury susceptibility 286 (Cutlip et al.,
unpublished observations). Human muscle performance studies assessed the relationship between
muscle strength, soreness, and the release of intracellular proteins into the serum. Komi and
Buskirk 150 used a training protocol of isometric, shortening, and eccentric muscle actions of the
elbow flexors for 4 days

week for 7 weeks. Muscle soreness peaked after the first week and then
disappeared. Maximum force during shortening and eccentric muscle actions increased throughout
the 7-week test period. Newham et al. 186 also studied the effect of eccentric muscle actions of the
elbow flexors performed on three occasions separated by 2 weeks. Muscle soreness was reduced after
the second and third sessions, but was still present. Plasma levels of creatine kinase (a muscle-associated
enzyme) were also greatly elevated after the first session, but were not elevated after the second and third
sessions. Recovery of the maximum force was more rapid and complete after each subsequent session. A
similar study indicated that a pre-training session of 24 eccentric contractions of the elbow flexors
reduced muscle soreness and force deficit due to a 70 eccentric contraction protocol administered 2
weeks later. 60 Serum creatine kinase that showed an increase after the first session, diminished after the
later session, which indicates a temporal relationship between creatine kinase levels and muscle soreness.
Results of studies employing small rodents (mice and rats) have been consistent with human studies.
In one study, 198 eccentric contractions of the dorsi flexors were administered to anesthetized rodents
in vivo once every 7 days for 6 weeks using a dynamometer. 76 The initial exposure to the protocol
produced a 60% force deficit, which returned to 80% of the pretest value at 7 days. By the sixth week,
no decline in force was observed. Muscles demonstrated an adaptive response, with increased whole
muscle mass. Results from human and animal studies indicate that muscles can be trained to perform
maximal eccentric muscle actions without injury; however, training must be continuous. The result
of these studies raises an important issue in the study of muscle injury. Instituting training sessions
containing eccentric contractions produces short-term force deficits and resultant myofiber disruption
and inflammation, which later ceases and the muscle returns to normal function. The transient effects
(temporary soreness and inflammation) are reduced with subsequent sessions. The preceding results
pose an important issue about how the desirable effects of training can be distinguished from patholo-
gical changes that occur due to repetitive motion? Also, what characteristics of the dynamic inputs
(force, strain, strain rate, number of repetitions) produce adaptive versus pathological responses in a
chronic model?

/

15.4.3.7 The Effect of Age
Age can be an important factor in injury susceptibility and recovery. Zerba et al. 313 tested the hypothesis
that muscles of old mice were more susceptible to injury than muscles of young and adult mice by
attaching the distal tendon of the EDL muscle to a servomotor. The left leg was exposed to stretch
while the right leg served as control. Results indicated a 27% deficit in P o , and that old mice (43%
deficit) were more susceptible to injury than young and adult mice. In vitro results of mice EDL
muscles conducted by Faulkner et al. 72 indicated that old mice are more susceptible to injury and
intrinsic differences in single permeabilized fibers account for the difference. Subsequent in situ and in
vitro work by Brooks and Faulkner 37 and Faulkner et al. 72 supports earlier results that initial eccentric
contraction-induced injury increases in old age. Single stretches of whole muscle and single permeabi-
lized fibers (which rules out effects of excitation-contraction coupling, or membrane and extracellular
effects) represented an effective method of focusing on factors that contribute to contraction-induced
injury. Because their in vitro findings supported their in situ work, it suggested that failure has a
mechanical etiology (due to work input) and increased injury susceptibility with age may be due to