Department of School of Life and Health Sciences, University of Roehampton, London, UK.
School of Sport Science and Physical Activity, University of Bedfordshire, Bedford, UK.
Exp Physiol. 2023 Apr;108(4):607-620. doi: 10.1113/EP090644. Epub 2023 Feb 20.
What is the central question of this study? Hyperthermia reduces the human capacity to produce muscular force, which is associated with decreased neural drive: does mitigating a reduction in neural drive by altering localised thermal sensation help to preserve voluntary force output? What is the main finding and its importance? Altering thermal sensation by cooling and heating the head independent of core temperature did not change neural drive or benefit voluntary force production. Head cooling did slow the rate of rise in core temperature during heating, which may have practical applications in passive settings.
This study investigated altered local head and neck thermal sensation on maximal and rapid torque production during voluntary contractions. Nine participants completed four visits in two environmental conditions: at rectal temperatures ∼39.5°C in hot (HOT; ∼50°C, ∼39% relative humidity) and ∼37°C in thermoneutral (NEU; ∼22°C, ∼46% relative humidity) conditions. Local thermal sensation was manipulated by heating in thermoneutral conditions and cooling in hot conditions. Evoked twitches and octets were delivered at rest. Maximum voluntary torque (MVT), normalised surface electromyography (EMG) and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions of the knee extensors. Rate of torque development (RTD) and EMG were measured during rapid voluntary contractions. MVT (P = 0.463) and RTD (P = 0.061) were similar between environmental conditions despite reduced VA (-6%; P = 0.047) and EMG at MVT (-31%; P = 0.019). EMG in the rapid voluntary contractions was also lower in HOT versus NEU during the initial 100 ms (-24%; P = 0.035) and 150 ms (-26%; P = 0.035). Evoked twitch (+70%; P < 0.001) and octet (+27%; P < 0.001) RTD during the initial 50 ms were greater in the HOT compared to NEU conditions, in addition to a faster relaxation rate of the muscle (-33%; P < 0.001). In conclusion, hyperthermia reduced neural drive without affecting voluntary torque, likely due to the compensatory effects of improved intrinsic contractile function and faster contraction and relaxation rates of the knee extensors. Changes in local thermal perception of the head and neck whilst hyperthermic or normothermic did not affect voluntary torque.
这项研究的核心问题是什么？体温升高会降低人体产生肌肉力量的能力，这与神经驱动的减少有关：通过改变局部热感觉来减轻神经驱动的减少是否有助于维持自愿力输出？主要发现及其重要性是什么？通过冷却和加热头部来改变头部的局部热感觉，而不改变核心温度，不会改变神经驱动或有益于自愿力的产生。头部冷却确实减缓了在加热过程中核心温度的上升速度，这在被动环境中可能具有实际应用。
本研究调查了在自愿收缩过程中改变头部和颈部的局部热感觉对最大和快速扭矩产生的影响。九名参与者在两种环境条件下完成了四次访问：直肠温度约为 39.5°C 的热（HOT；约 50°C，约 39%相对湿度）和直肠温度约为 37°C 的常温中性（NEU；约 22°C，约 46%相对湿度）。在常温中性条件下加热和在热条件下冷却来改变局部热感觉。在休息时给予诱发的 twitch 和 octet。在短暂的最大等长自愿膝关节伸展收缩期间评估最大自愿扭矩（MVT）、表面肌电图（EMG）和自愿激活（VA）的正常化。在快速自愿收缩期间测量扭矩发展速率（RTD）和 EMG。尽管 VA 降低了 6%（P=0.047），MVT 时的 EMG 降低了 31%（P=0.019），但环境条件之间的 MVT（P=0.463）和 RTD（P=0.061）相似。在 HOT 与 NEU 相比，快速自愿收缩的初始 100ms（-24%；P=0.035）和 150ms（-26%；P=0.035）期间的 EMG 也较低。与 NEU 条件相比，在 HOT 条件下，初始 50ms 期间的诱发 twitch（+70%；P<0.001）和 octet（+27%；P<0.001）的 RTD 更大，此外，膝关节伸肌的收缩和放松速度更快（-33%；P<0.001）。结论：体温升高会降低神经驱动，而不会影响自愿扭矩，这可能是由于改善了内在收缩功能以及更快的收缩和放松速度的补偿作用。在热或常温中性时改变头部和颈部的局部热感觉并不会影响自愿扭矩。
