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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?
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.
The purpose of this study was to investigate the acute effects of progressive fatigue on the running mechanics parameters previously associated with tibial stress fracture (TSF) risk.
Twenty one male trained distance runners performed three sets (Pre, Mid & Post) of six overground running trials at 4.5 m·s (± 5%). Kinematic and kinetic data were collected during each trial using a twelve-camera motion capture system, force platform, and head and leg accelerometers. Between tests, each runner ran on a treadmill for 20 minutes at their corresponding lactate threshold (LT) speed. Perceived exertion levels (RPE) were recorded at the 3 and last minute of each treadmill run.
RPE scores increased from 11.8 (± 1.3) to 14.4 (± 1.5) at the end of the first LT run and then further to 17.4 (± 1.6) by the end of the second LT run. Peak rear foot eversion, peak axial head acceleration, peak free moment and vertical loading rates were shown to increase (p < 0.05) with moderate-large effect sizes during the progression from Pre to Post tests, although vertical impact peak and peak axial tibial acceleration were not significantly affected by the high intensity running bouts.
Previously identified risk factors for impact-related injuries (such as TSF) are modified with fatigue. As fatigue is associated with a reduced tolerance for impact, these findings lend support to the importance of those measures to identify individuals at risk of injury from lower limb impact loading during running.