Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 356

15.4.3.3 The Effect of Repetitions on Muscle Injury
There is clear evidence that the number of eccentric or SSC repetitions has an effect on the amount of
resultant muscle injury and force deficit. 97,116 Models that have induced single stretches in muscle
within the physiological range have not resulted in muscle damage or a pronounced force deficit. 39,129
In other studies, it required more than one stretch within the physiological range to produce muscle
injury. 97,110,284,297,298 Repeated stretches that varied from 225 to 900 at a final length of 110% L o have
resulted in myofiber damage and a resultant force deficit. 35,163,177,178,313 Thus, the amount of loading
does have a graded effect on both changes in muscle performance and the extent of myofiber
injury. 97,116 However, the effect of repetitions on repetitive exposures of either eccentric muscle
actions or SSC has not been studied to date.

15.4.3.4 The Effect of Other Mechanical Factors on Muscle Injury
Dynamic muscle forces and length changes that are measured during eccentric muscle actions can be dis-
sected into components of the dynamic signature. Components such as peak force, 110,178 average force, 39
work during the stretch, 129 and fiber length 110,129 have been found to affect the magnitude of contraction-
induced injury. The force deficit resultant from an injurious exposure has been predicted by: (i) work done
during the stretch when initiated from optimal length, 39,172 (ii) initial length and work during the stretch
when not initiated at optimal length, 129 or by (iii) peak force and initial length. 110 Within a given level of
force output, eccentric muscle actions performed at longer ranges of motion or fiber length have resulted in
larger isometric force deficits in both humans 188 and animals 110,129,305 than stretches performed at a shorter
range of motion. However, in these studies, the change in work (calculated by integration of the force-
muscle displacement curve) during repeated stretches was not reported. 39,110,305 Work during the eccentric
phase or stretch (negative work) has been shown to be well correlated with the isometric force deficit
after a single eccentric contraction. 129,172 However, the length perturbation in these studies was beyond
the normal physiological range of the target muscle. However, muscles stretched within the normal
physiological range have required more than one repetition to produce injury. 284,297,298 Studies of
repetitive eccentric muscle actions in the physiological range may have more external validity than
single stretch models that have been studied outside of the normal physiological range.

15.4.3.5 The Effect of Exposure Duration and Lengthening Velocity
To determine if exposure duration and lengthening velocity affected muscle injury, McCully and
Faulkner 178 used EDL muscles of mice. Although measured for 15 min exercise duration, there was
no change in P o due to muscle fatigue after the initial 5 min. A velocity of 1 L f

sec produced a deficit

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in P o after 3 days while the 0.2 L f sec lengthening velocities did not. A drop in P o was

mostly associated with stretch velocity. It was theorized that loss of peak force after fatigue prevents
further muscle damage. Muscle injury increased with eccentric exercise duration for upto 5 min
(no further force decrements were observed with subsequent eccentric muscle actions), higher velocities
shortened the duration time for injury, and muscle force was a critical component in producing injury.
Warren et al.'s 284 in vitro results in rats also indicated that higher lengthening velocities produced larger
force decrements. The velocity component in muscle injury was investigated by Scifres and Martin 235
using a Kin-Com dynamometer to test human subjects for eccentric leg extension performance. One
leg was tested at 30

sec and 0.5 L f

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sec. Delayed onset muscle soreness
(DOMS) was more pronounced at the higher velocity, which indicated that higher velocity may
produce additional muscle fiber injury. To further examine the effect of velocity on contraction-
induced injury, Lynch and Faulkner 169 used single permeabilized fibers from mice EDL muscles.
The severity of contraction-induced injury was not affected by the velocity of stretch. Controversy still
exists as to the role of lengthening velocity in acute injury to skeletal muscle fibers. Furthermore, the
role of lengthening velocity in chronic contraction-induced injury has not been investigated thus far.
The effect of exposure duration (length of exposure) on repetitive injury has not been investigated to
date, however it may be an important factor in repetitive injury causation.

8 / sec while the other leg was tested at 120 8 /