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Stress fracture is a common musculoskeletal problem affecting athletes and soldiers. Repetitive high bone strains and strain rates are considered to be its etiology. The strain level necessary to cause fatigue failure of bone ex vivo is higher than the strains recorded in humans during vigorous physical activity. We hypothesized that during fatiguing exercises, bone strains may increase and reach levels exceeding those measured in the non-fatigued state. To test this hypothesis, we measured in vivo tibial strains, the maximum gastrocnemius isokinetic torque and ground reaction forces in four subjects before and after two fatiguing levels of exercise: a 2km run and a 30km desert march. Strains were measured using strain-gauged staples inserted percutaneously in the medial aspect of their mid-tibial diaphysis. There was a decrease in the peak gastrocnemius isokinetic torque of all four subjects’ post-march as compared to pre-run (p=0.0001), indicating the presence of gastrocnemius muscle fatigue. Tension strains increased 26% post-run (p=0.002, 95 % confidence interval (CI) and 29% post-march (p=0.0002, 95% CI) as compared to the pre-run phase. Tension strain rates increased 13% post-run (p=0.001, 95% CI) and 11% post-march (p=0.009, 95% CI) and the compression strain rates increased 9% post-run (p=0.0004, 95% CI) and 17% post-march (p=0.0001, 95% CI). The fatigue state increases bone strains well above those recorded in rested individuals and may be a major factor in the stress fracture etiology.
Muscle fatigue affects a lot more than tibial strain. I have seen data from a couple of projects in which lower limb and foot biomechanics are different when muscles are fatigued....sort of raises a question as to when should we being doing kinematic and kinetic studies of foot orthoses --- should subjects be fresh or fatigued?
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Craig Payne
Department of Podiatry
La Trobe University
Melbourne, Australia http://www.latrobe.edu.au/podiatry
__________________________________________________ ___________________________________ God put me on this earth to accomplish a certain number of things - right now I am so far behind, I will never die.
The views expressed above are those of the author and not that of La Trobe University This is where I am, where are you?
If the research grant is accepted, I will be doing some research with Penn State in the coming years regarding how foot orthoses affect tibial strains and whether foot orthoses can reduce tibial strains in simulated running in cadaver foot-leg preparations.
Nice to see that research is increasingly starting to directly measure internal strains on the structural components of the foot and lower extremity to give us a better idea of how the nebulous concept of "fatigue" exactly is played out within the human body during and after exercise.
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Sincerely,
Kevin
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Kevin A. Kirby, DPM
Adjunct Associate Professor
Department of Applied Biomechanics
California School of Podiatric Medicine at Samuel Merritt College
Craig
I coudn't agree more re when we should be doing foot orthoses/ mechanics research. I always advise that practitioners should assess sports patients etc post sport. Work by Gefen and a few Japanese researchers showed that both foot and postural mechanics were changed significantly post - fatigue.
I have also seen significant foot posture index scores and navicular drop in patients pre-and post sport.
Muscle fatigue affects tibial strain
Linear Mode
