Alex Lloyd

The interaction between peripheral and central fatigue at different muscle temperatures during sustained isometric contractions

Changes in central fatigue have been linked to active and passive changes in core temperature, as well as integration of sensory feedback from thermoreceptors in the skin. However, the effects of muscle temperature (Tm), and thereby metaboreceptor and local afferent nerve temperature, on central fatigue (measured using voluntary activation percentage) during sustained, high muscle fatigue exercise remain unexamined. In this study, we investigated Tm across the range of cold to hot, and its effect on voluntary activation percentage during sustained isometric contractions of the knee extensors. The results suggest that contrary to brief contractions, during a sustained fatiguing contraction Tm significantly (P < 0.001) influences force output (-0.7%/°C increase) and central fatigue (-0.5%/°C increase), showing a negative relationship across the Tm continuum in moderately trained individuals. The negative relationship between voluntary activation percentage and Tm indicates muscle temperature may influence central fatigue during sustained and high muscle fatigue exercise. On the basis of on an integrative analysis between the present data and previous literature, the impact of core and muscle temperature on voluntary muscle activation is estimated to show a ratio of 5.5 to 1, respectively. Accordingly, Tm could assume a secondary or tertiary role in the reduction of voluntary muscle activation when body temperature leaves a thermoneutral range.

Interactions in human performance: An individual and combined stressors approach

In many clinical, ergonomic and sporting contexts, humans are exposed to environments that are suboptimal for physical and cognitive performance. This has prompted a substantial body research on the human response to heat, cold, hypoxia, noise, vibration, hypoand hyperbaria, as well as hyperand microgravity. However, working at environmental extremes can expose individuals to more than just a single stressor. Indeed, it is the combination of stressful factors which characterizes the ‘extreme’ nature of environments like high-altitude (e.g. hypobaric hypoxia, cold, solar radiation), deep-sea (e.g., hyperbaria, cold, inspiratory gas toxicity) and space (e.g. heat, cold, hypobaric normoxia, hyperand microgravity).

Anchoring biases affect repeated scores of thermal, moisture, tactile and comfort sensations in transient conditions

In this study, we addressed potential biases which can occur when sensorial scores of temperature, wetness and discomfort are repeatedly reported, in transient exercise conditions. We pointed out that, when repeatedly reported, previous sensorial scores can be set by the participants as reference values and the subsequent score may be given based on the previous point of reference, the latter phenomenon leading to a bias which we defined as ‘anchoring bias’. Indeed, the findings shown that subsequent sensorial scores are prone to anchoring biases and that the bias consisted in a systematically higher magnitude of sensation as compared to when reported a single time only. As such, the study allowed recognition, quantification and mitigation of the identified bias which can improve the methodological rigour of research studies involving assessments of sensorial data in transient conditions.

The effect of winter clothing ensembles and hypoxia on performance and exercise-related sensations during uphill walking: A combined and individual stressors approach [Abstract]

Multiple environmental stressors are present at high altitude, including low oxygen availability, decreased atmospheric pressure, cold ambient temperatures, dry air and solar radiation. Rarely cons...

Localized and systemic variations in central motor drive at different local skin and muscle temperatures

This study investigated the ability to sustain quadriceps central motor drive while subjected to localized heat and metaboreceptive feedback from the contralateral leg. Eight active males each completed two counter-balanced trials, in which muscle temperature (Tm) of a single-leg (TEMP-LEG) was altered to 29.4°C (COOL) or 37.6°C (WARM), while the contralateral leg (CL-LEG) remained thermoneutral: 35.3°C and 35.2°C Tm in COOL and WARM, respectively. To activate metaboreceptive feedback, participants first performed one 120-s isometric maximal voluntary contraction (MVC) of the knee extensors in the TEMP-LEG, immediately followed by postexercise muscle ischemia (PEMI) via femoral blood flow occlusion. To assess central motor drive of a remote muscle group immediately following PEMI, another 120-s MVC was subsequently performed in the CL-LEG. Voluntary muscle activation (VA) was assessed using the twitch interpolation method. Perceived mental effort and limb discomfort were also recorded. In a cooled muscle, a significant increase in mean force output and mean VA (force, P < 0.001; VA, P < 0.05), as well as a significant decrease in limb discomfort (P < 0.05) occurred during the sustained MVC in the TEMP-LEG. However, no differences between Tm were observed in mean force output, mean VA, or limb discomfort during the sustained MVC in the CL-LEG (force, P = 0.33; VA, P > 0.68; and limb discomfort, P = 0.73). The present findings suggest that elevated local skin temperature and Tm can increase limb discomfort and decrease central motor drive, but this does not limit systemic motor activation of a thermoneutral muscle group.

The interaction between cooling and hypoxia on the rate of peripheral and central fatigue development of the knee extensors

High altitude often comprises hypobaric hypoxia and cold ambient temperatures. However, research examining human performance during these stressors in combination is sparse [1]. Previous findings have reported that the rate of fatigue additively increases when hypoxia and cold are combined [2]. However this study investigated small muscle groups (forearm flexors) using a fixed duration (closed) exercise protocol. Thus, the present study sought to examine whether volitional exhaustion or task failure (during an open protocol) of the larger knee extensor muscles would result in a similar additive effect during combined hypoxic-cold exposure.

The use of optimised heating trousers and the role of the blood flow on the reduction in muscle temperature post warm up

Activities that are highly dependent on power output can benefit from increases in muscle temperature (Tm) in terms of work done and skeletal muscle power output. When athletes experience a significant delay between active warm up and performance, Tm declines. Previous studies have demonstrated that using heated trousers during a period of inactivity can attenuate this decline, with a greater peak power output as result [1,2]. However, in these studies, the reduction in Tm was not completely eliminated. Thus, in the current study we aimed to optimise the heating procedure, in order to eliminate the reduction in Tm post-warm up. Furthermore, to understand the reason of this reduction, the effect of the blood flow in the cooling process of the leg was studied.