Soft-Tissue Pathomechanics - Fundamentals and Assessment Tools for Occupational Ergonomics - page 367

to assess contraction-induced damage have been reviewed previously. 289 In humans, these approaches
include examining force production, histology, blood levels of muscle-associated proteins, and reports
of pain.
Force production has commonly been used to assess muscle function because reductions in force are
often correlated with injury. Changes in force are immediate, and maintained for a number of days
following the exposure that caused the injury. 10,61,87,96,291 Although muscle damage is often associated
with a force deficit, force measurements alone should not be used to diagnose injury because muscle
fatigue can also result in reductions in force. 80 - 82
Overuse disorders can also be associated with losses in force, however a better marker for these dis-
orders is muscle fatigue. 30,133 Although injured muscle does demonstrate signs of fatigue, 80,299 fatigue
is also seen prior to the generation of injury, and it is believed that fatigue and the cellular changes
associated with this physiological state, may be in part responsible for generating overuse disorders. 243
Thus, depending on when measurements are taken, fatigue can be used as a biomarker for predicting
the development of a disorder, or for diagnosing a disorder.
Increases in the circulating concentrations of certain muscle-associated proteins, including creatine
kinase, lactate dehydrogenase, and myoglobin are often associated with strain-induced muscle inju-
ries. 54,195,223,227,248 Increases in these circulating proteins do not correlate well with injury-induced force
deficits during the first 24 h of injury, 56,60,186 and thus, the concentrations of these proteins do not serve
as good markers for the early diagnosis of muscle injury. In addition, certain proteins, such as creatine
kinase, fluctuate in response to muscle activity, and not solely in response to muscle injury. 55,187,289
Histological examination of muscle biopsies can be used to diagnose muscle injury. As mentioned pre-
viously, infiltration of immune cells, disruptions of the sarcolemma, and necrosis can be seen in muscles
with strain-induced injuries. 90,163,255 However, these changes are not usually apparent until 24-48 h after
the injury, 90 and thus, they cannot be used to identify injuries early during the process. With overuse
disorders, ragged red fibers are found by histological examination of biopsy tissue. 159 These ragged
red fibers appear to be strongly immunopositive for cytochrome C oxidase (an enzyme involved in mito-
chondrial function), or immunonegative for this marker. 158 Thus, the presence of ragged red fibers, and
their cytochrome C oxidase phenotype, can be used as a marker of muscle disorders.
Pain or muscle soreness is one of the most common symptoms used to determine if there is muscle
damage. However, with strain-induced injuries, muscle soreness is not apparent until 24-48 h after
the injury occurred, 55,90 and it does not correlate well with injury-induced force deficits. 50,69,136 There-
fore, pain is not a good marker to use for early diagnoses of strain injuries. Pain is also common in
overuse disorders and is associated with muscle fatigue, 159,240 the presence of ragged red fibers, 158,159
and reductions in blood flow. 158 Pain has also been correlated with biochemical changes associated
with fatigue. 30,240,242 Because fatigue often precedes actual damage, pain and muscle fatigue may serve
as biomarkers for predicting muscle damage.
Based on our current knowledge the diagnoses of muscle strains can most quickly be done by checking
for force deficits. Reports of muscle soreness, or the presence of myofiber proteins in the blood can be
used to confirm that the force deficit is due to an injury. Depending on the timing of events, reports
of pain and measurements of muscle fatigue can also be used to indicate that injury may occur, or to
diagnose an injury. Although most biomarkers are currently used for diagnosis, research focusing on
pain, fatigue, and their association with biomarkers may provide a means for determining when an indi-
vidual is at risk for developing a muscle strain or disorder.

15.6 Recommendations for Future Work

15.6.1 The Need for More Refined Models

The most refined in vivo models to date have used dynamometry and electrical stimulation to study the
target muscle of interest. However, these models have generally been used to study acute muscle injury.

In Vivo