Pj. Sparto et M. Parnianpour, An electromyography-assisted model to estimate trunk muscle forces during fatiguing repetitive trunk exertions, J SPINAL D, 12(6), 1999, pp. 509-518
During submaximal shortening muscle contraction, fatigue characteristically
results in an increase in measured surface electromyography, whereas the m
aximum force that can be produced by muscle is reduced. This finding compro
mises researchers' ability to estimate muscle stress in a joint system such
as the spine, which is composed of more muscles than degrees of freedom of
the joint. A three-dimensional, electromyography-assisted, dynamic biomech
anical model of spinal loading was developed and validated for use during f
atiguing repetitive trunk extension exertions. A time-varying maximum muscl
e stress was included to model the effect of a change in the maximum force-
producing capacity of the erector spinae muscle. Sixteen men performed subm
aximal isokinetic trunk extension endurance tests at 15 degrees per second.
The exertion level (35% and 70% of their maximum dynamic extension torque)
and repetition rate (5 and 10 repetitions per minute) of the tests were va
ried during four testing sessions. Using trunk muscle electromyography and
the measured torque as input, the model predicted significant linear reduct
ions in the maximum muscle stress in 78% of the endurance tests, which resu
lted in an estimated decrease in erector spinae force in 75% of the tests.
Conversely, if the maximum muscle stress was assumed to be constant, the er
ector spinae force would have been predicted to increase in 73% of the test
s. The magnitude of the change in predicted erector spinae maximum muscle s
tress and force depended on the exertion level and repetition rate. This mo
del will allow researchers to assess the effects of changes in recruitment
patterns of trunk muscles during dynamic trunk extension on the estimated s
pinal loading of the lumbar spine.