Kalmar J M, Del Balso C, Cafarelli E
Department of Biology, Rm 346 Bethune College, York University, Toronto, ON, Canada.
Exp Brain Res. 2006 Aug;173(3):446-57. doi: 10.1007/s00221-006-0383-0. Epub 2006 Feb 28.
We hypothesized that if reduced spinal excitability contributes to central activation failure, then a caffeine-induced increase in spinal excitability would enhance postfatigue maximal voluntary activation and maximal voluntary contraction (MVC). Ten male volunteer subjects attended two laboratory sessions separated by at least 1 week. Contractile and electrical properties were assessed before, and 1 h after oral administration of caffeine (6 mg/kg) or placebo (all-purpose flour), and again following a fatigue protocol. The slope of the H reflex recruitment curve, normalized to that of the M wave (H(slp)/M(slp)), was used to estimate spinal excitability. Maximal voluntary activation was assessed using maximal EMG (EMG(max)) and twitch interpolation. Postfatigue, MVC torque declined (P<0.05) to 75.2+/-12.7 and 70.2+/-9.3% of the prefatigue values in the placebo (PL) and caffeine (CF) trials, respectively, and remained depressed throughout the recovery period. This was accompanied by a decline in % activation (P<0.05) from 99.6+/-0.3% (PL) and 99.8+/-0.3% (CF) to 94.8+/-3.5% (PL) and 95.3+/-5.0% (CF), indicating the presence of central activation failure. Caffeine offset the decline in H(slp)/M(slp )observed in the placebo trial (P<0.05), but it did not prevent the decline in maximal voluntary activation or MVC torque. Furthermore, although the decline in spinal excitability was correlated to the decline in EMG(max) (r=0.55, P<0.05) it was not correlated with the decline in % activation or MVC torque. Thus a fatigue-induced decline in spinal excitability did not limit maximal activation.
我们推测，如果脊髓兴奋性降低导致中枢激活失败，那么咖啡因引起的脊髓兴奋性增加将增强疲劳后最大自主激活和最大自主收缩（MVC）。10名男性志愿者参加了两个实验室测试，间隔至少1周。在口服咖啡因（6mg/kg）或安慰剂（通用面粉）之前、之后1小时以及疲劳方案后，评估收缩和电特性。H反射募集曲线的斜率（相对于M波斜率进行标准化，即H(slp)/M(slp)）用于估计脊髓兴奋性。使用最大肌电图（EMG(max)）和抽搐插值法评估最大自主激活。疲劳后，在安慰剂（PL）和咖啡因（CF）试验中，MVC扭矩分别下降（P<0.05）至疲劳前值的75.2±12.7%和70.2±9.3%，并且在整个恢复期间一直处于较低水平。这伴随着激活百分比的下降（P<0.05），从99.6±0.3%（PL）和99.8±0.3%（CF）降至94.8±3.5%（PL）和95.3±5.0%（CF），表明存在中枢激活失败。咖啡因抵消了安慰剂试验中观察到的H(slp)/M(slp)的下降（P<0.05），但并未阻止最大自主激活或MVC扭矩的下降。此外，虽然脊髓兴奋性的下降与EMG(max)的下降相关（r = 0.55，P<0.05），但它与激活百分比或MVC扭矩的下降无关。因此，疲劳引起的脊髓兴奋性下降并不限制最大激活。
