Assessing the effects of local hyperthermia on muscle function in younger and older adults

Abstract

A preparatory process aimed at optimising skeletal muscle function prior to maximal muscular contraction is fundamental and commonly observed during physical exercise. Additionally, certain populations may benefit from such preparation before engaging in activities of daily living. A key strategy used to achieve optimal skeletal muscle function is to increase tissue temperature above resting baseline. Skeletal muscle tissue temperature is usually increased prior to exercise requiring high force or high velocity production through an active warm up, consisting of moderate intensity exercise and dynamic contractions. For some populations, for example those with limited physical capacity such as the frail and elderly, an active warm up is not always possible. Increasing muscle temperature passively before exercise is an emerging field that may be an appropriate strategy to prepare those populations for dynamic contractions that are otherwise limiting factors in daily life. The central aim of this thesis was to investigate the effects of localised passive heating during maximal knee extensor exercise to determine the efficacy of passive heating as an ergogenic aid in younger and older adults. The first experimental study (Chapter 4) investigated the effects of heating across a variety of dynamic contractile speeds and aimed to quantify the inter and intraday reliability of isokinetic and isotonic force measurements in younger healthy adults. The reliability across all isokinetic contractile speeds were measured to be “excellent” in the heated limb (>.9 ICC and Cronbach’s alpha) and displayed low variance (<10% Coefficient of variation). In response to the heating intervention, skeletal muscle temperature was increased to levels associated with active warm ups. Heating increased peak torque by 8% during moderate and by 10 % during fast contractions from 30 min onwards relative to the unchanged control leg. Rate of force production at 50ms and early force production was increased during the slow contraction by 14% and 15% respectively from 30 min in the heated leg whilst the control leg was unchanged. Within the second experimental study (Chapter 5) the findings of chapter 4 were expanded upon and included healthy older adults within a comparable testing protocol. It is shown when comparing passive thigh heating responses between younger and older adults it was revealed that the increases in peak force that were observed in younger adults did not increase significantly in older adults. Older adults did however report increases in early force production that were even more pronounced when compared against improvements in younger adults. Both Chapters 4 and 5 report decreases in perceived exertion and improved perceptions in readiness for exercise in response to the passive thigh heating intervention, but the older adults also rated this effect to be more pronounced. The final experimental study (Chapter 6) sought to further investigate how passive thigh heating may improve muscle function beyond maximal force production considerations by investigating the effect upon fatigue resistance during a maximal effort 30 moderate speed isokinetic knee extension task. Whilst peak torque was increased, reproducing the outcomes of chapter 4 and 5, the heating intervention appeared to have no effect on any fatigue related measure with no change in total work, average torque or peak EMG amplitude. This thesis therefore identifies that localised passive heating has the potential to be an effective ergogenic aid, especially for younger adults, by improving muscle function during maximal contractions. The studies reveal that thigh heating increases peak isokinetic force during moderate and fast contractions and enhances early force production and rate of force development in both younger and older adults during slow contractions. These effects are likely due to direct changes in muscle contractile properties in response to localised increases in tissue temperature, not systemic physiological factors. While older adults did not improve peak force, they did show better early force production, indicating passive heating may enhance neuromuscular function with age and facilitate the completion of tasks of daily living that are modulated more by power than absolute strength. Additionally, participants reported feeling more ready to exercise and experienced less perceived exertion, suggesting passive heating could encourage physical activity, particularly among those reluctant to exercise due to discomfort.