Muscle spindle responses following fatigue and ischemia
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The purpose of this study was to determine whether ischemia would enhance muscle spindle responses to tendon tap and vibration during submaximal fatiguing contractions in the soleus muscle of able-bodied individuals. Nine healthy adults attended two experimental sessions approximately 48 hours apart. Both sessions were identical except that the fatigue task in one was performed with a pressure cuff placed above the knee and inflated to 180 mm Hg. Three 5s maximum voluntary contractions (MVCs) were performed prior to and after the fatigue task. Each participant held a target force of 20% MVC until endurance time (peak-to-peak tremor amplitude exceeded 5% MVC or target force dropped by 2% for 3s). Muscle spindle responses were evaluated using the peak-to-peak EMG amplitude of tendon taps (delivered by a custom-made tapper) and the Motor Unit Firing Rates (MUFR) during 15 s of vibration, recorded with fine-wire intramuscular electrodes. H reflex responses were measured before and after fatigue for each condition, to measure the net excitability of the spinal cord. There were no significant differences (p>0.05) in the P-P EMG of tendon taps or the MUFR across any conditions. The post-fatigue Maximal Voluntary Contraction forces were measured and were less than the pre-fatigue values under both conditions (and significantly different in the non-ischemic condition (p=0.01)). Absence of significant differences in the Hmax:Mmax ratios (p=0.94 in non-ischemic/fatigue and p=0.43 in ischemic condition) indicated that the spinal excitability was relatively unchanged across the conditions. Therefore, we could not conclude that ischemia enhanced the muscle spindle response.