The Effects of Muscle Damage on Muscle Spindle Function

Abstract

The effects of eccentric muscle damage on extrafusal muscle fibers and motor function have been well studied. However, very few studies have examined the effects of muscle damage on sensory function, specifically, whether eccentric contractions damage the intrafusal fibers muscle spindles are wrapped around. PURPOSE: To investigate the effects of fatigue and muscle damage on the premotor latency (PML) and reflex magnitude (normalized reflex Torque; nRT) of patellar tendon reflexes. METHODS: Fifteen males (mean ± SD: age = 24.1 ± 2.9 yrs.) and fifteen females (age = 21.67 ± 2.1 yrs.) participated in a familiarization trial followed by two experimental sessions, separated by seven (±1) days. The 2 conditions consisted of either fatiguing (CON) or muscle damaging (ECC/CON) contractions at 60°· s-1 until 70% of peak torque (PT) was achieved. Patellar tendon taps were performed at a knee angle of 90° and a reflex hammer fastened to a swing arm was used to provide uniform impacts. PML was calculated as the time between the strike of the hammer to the onset of myoelectric activity in the rectus femoris, and nRT was normalized [% of Pre maximal voluntary contraction (MVC)] at all time points. Reflex assessments and MVCs were completed before (Pre) and following (Post) both the CON and ECC/CON protocols until recovery of PT to Pre values (Recov). A three-way repeated measures ANOVA [gender (males vs females) × condition (CON vs ECC/CON) × time (Pre vs Post vs Recov)] was used to analyze all reflex data. An alpha value of P ≤ 0.05 was considered statistically significant for all comparisons. RESULTS: A main effect for time (P = 0.003) was observed in which PML was greater at Recov compared to Pre (P = 0.01). Additionally, a two-way gender × time interaction was observed in which nRT was lower at Post and Recov compared to Pre (P = 0.001-0.002) for the females. No differences were observed across time for the males (P = 1.000). CONCLUSION: These findings revealed that changes in PML may not be sensitive to muscle damaging contractions but may have delayed responses as a result of exercise-induced fatigue. Furthermore, reflex magnitude deficits may be evident in females following fatiguing exercise but not likely as a result of muscle damage.