570 research outputs found
Passive hyperthermia reduces maximal but not explosive torque production
No full textIntroduction: hyperthermia can reduce maximal voluntary force/torque (MVT) of skeletal muscle due to reduced neural activation (Todd et al., 2005). Rate of force/torque development (RTD) may be a more functionally relevant measure of neuromuscular performance than MVT; however, the effect of hyperthermia on RTD has received little attention. Primary determinants of RTD include neural activation and electrically evoked involuntary RTD (Maffiuletti et al., 2016), and whilst neural activation may decrease with hyperthermia, increased muscle temperature improves involuntary RTD, so it is unclear how voluntary RTD will be affected by hyperthermia. This study aimed to investigate the effects of progressive passive whole-body hyperthermia on MVT and voluntary RTD, as well as the neural and contractile factors contributing to any effects.Methods: nine male participants completed two trials; one involving passive heating to a rectal temperature (Tre) of 39.5˚C (HOT) and the other (CON) where Tre was maintained at ~37˚C. The same neuromuscular measurements of the knee extensors were completed when participants first entered the chamber (Tre ~37˚C; Base), and at Tre=38.5˚C (ModHyp) and Tre=39.5˚C (SevHyp), in the HOT trial, and at equivalent time points into the CON trial. MVT, voluntary activation (VA) determined via interpolated twitch, and EMG at MVT normalised to maximal M-wave (EMGMVT) were measured during MVCs. Explosive voluntary RTD and normalised EMG were measured over 0-50 (RTD50; EMG50) and 0-100 (RTD100; EMG100) from torque/EMG onsets during explosive contractions. Involuntary RTD50 was measured during evoked octets at 300Hz.Results: in the HOT trial, MVT declined throughout and was 15% lower at SevHyp than Base (P=0.016), which coincided with declines in EMGMVT (-44%; P<0.005) and VA (-16%; P<0.005) from Base to SevHyp. In contrast, voluntary RTD50 and RTD100 remained unaffected in the HOT trial (P>0.05), despite 23-31% decreases in EMG50 and EMG100 from Base to SevHyp (P<0.001). Involuntary RTD50 however, increased from Base to ModHyp (10%; P<0.001) and from ModHyp to SevHyp (4%; P<0.001). There were no changes in any dependent variables throughout the CON trial.Conclusion: as expected MVT declined as a result of passive hyperthermia which was largely due to reduced neural activation (i.e., EMGMVT and VA). In contrast, this study provides novel evidence that voluntary RTD was largely unaffected by passive hyperthermia, despite considerable reductions in explosive neural activation, which likely occurred due to the increased involuntary RTD, and thus cancelling out of neural and contractile effects.<br/
Zora Neale Hurston Author and Anthropologist
No full textLike many artists before her, Zora Neale Hurston received virtually no recognition for her work until after her death. Hurston began her career as an anthropologist, observing and documenting the tension of race relations in the American South. She strove to expose the horrific practice of "paramour rights," wherein white men sexually exploited black women in their employment. But this work and her later fiction, including the now famous Their Eyes Were Watching God, would end up in relative obscurity as her fictional portrayal of African American dialect was criticized as offensive and her political views were often less progressive than those of her contemporaries. With engaging, accessible text, this biography gives readers a fuller picture of this complicated writer and woman.Like many artists before her, Zora Neale Hurston received virtually no recognition for her work until after her death. Hurston began her career as an anthropologist, observing and documenting the tension of race relations in the American South. She strove to expose the horrific practice of "paramour rights," wherein white men sexually exploited black women in their employment. But this work and her later fiction, including the now famous Their Eyes Were Watching God, would end up in relative obscurity as her fictional portrayal of African American dialect was criticized as offensive and her political views were often less progressive than those of her contemporaries. With engaging, accessible text, this biography gives readers a fuller picture of this complicated writer and woman.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
Head and neck cooling does not improve maximal voluntary torque or rate or torque development during brief maximal voluntary contractions in the heat
No full textIntroduction: maximal voluntary torque (MVT) is impaired when hyperthermic due to a reduction in the central nervous system’s capacity to voluntarily drive the available force capacity of muscle. Rate of torque development (RTD) is considered more functionally relevant than MVT in some situations and neural drive is a key determinant of RTD. Head and neck cooling can improve endurance performance when hyperthermic, but its effects on neural drive are unclear. The purpose of this study was to investigate head and neck cooling on thermal perception during whole-body hyperthermia on MVT, RTD, neural drive and the contractile properties of the muscle.Method: 9 participants completed two trials in HOT conditions (50°C, 40% RH), involving light exercise before passive heating to a rectal temperature (Tre) of 39.5°C. During one trial, the head and neck was continuously cooled (HOTcool) using a towel soaked in ice water. At Tre=39.5°C neuromuscular measurements were completed to assess MVT, voluntary activation and EMG at MVT normalised to maximal M-wave. Voluntary RTD and normalised EMG were measured over 0-50, 0-100, 0-150 and 0-200 ms. Involuntary RTD at 0-50 ms was measured during evoked octets at 300 Hz. Thermoregulatory and perceptual variables were measured throughout.Results: MVT and RTD, and their neuromuscular determinants were unaffected by cooling (P > 0.05). Neck (-20%) and head (-12%) temperature were lower in HOTcool, as were thermal sensation of the head (-36%) and body (-12%) and thermal comfort (body) (-23%). Time to target Tre was increased (71%) in HOTcool, and not all participants were able to reach 39.5°C. Tre (-0.3%) and skin temperature (-6%) were lower in HOTcool (P < 0.05), but heart rate was similar (P > 0.05).Conclusions: head and neck cooling did not affect MVT, RTD, or the neuromuscular determinants of these functional variables despite improving perceptions of thermal strain.<br/
The effect of head and neck per-cooling on neuromuscular fatigue following exercise in the heat
Full text linkThe effect of localised head and neck per-cooling on central and peripheral fatigue during high thermal strain was investigated. Fourteen participants cycled for 60 min at 50% peak oxygen uptake on 3 occasions: thermoneutral control (CON; 18 °C), hot (HOT; 35 °C), and HOT with head and neck cooling (HOTcooling). Maximal voluntary force (MVF) and central activation ratio (CAR) of the knee extensors were measured every 30 s during a sustained maximal voluntary contraction (MVC). Triplet peak force was measured following cycling, before and after the MVC. Rectal temperatures were higher in HOTcooling (39.2 ± 0.6 °C) and HOT (39.3 ± 0.5 °C) than CON (38.1 ± 0.3 °C; P < 0.05). Head and neck thermal sensation was similar in HOTcooling (4.2 ± 1.4) and CON (4.4 ± 0.9; P > 0.05) but lower than HOT (5.9 ± 1.5; P < 0.05). MVF and CAR were lower in HOT than CON throughout the MVC (P < 0.05). MVF and CAR were also lower in HOTcooling than CON at 5, 60, and 120 s, but similar at 30 and 90 s into the MVC (P > 0.05). Furthermore, they were greater in HOTcooling than HOT at 30 s, whilst triplet peak force was preserved in HOT after MVC. These results provide evidence that central fatigue following exercise in the heat is partially attenuated with head and neck cooling, which may be at the expense of greater peripheral fatigue. Novelty Central fatigue was greatest during hyperthermia. Head and neck cooling partially attenuated the greater central fatigue in the heat. Per-cooling led to more voluntary force production and more peripheral fatigue.© 2020, The Author(s). This is an author produced version of a paper published in APPLIED PHYSIOLOGY NUTRITION AND METABOLISM uploaded in accordance with the publisher’s self- archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.</p
Progressive hyperthermia elicits distinct responses in maximum and rapid torque production
Full text linkObjectivesTo investigate the effect of progressive whole-body hyperthermia on maximal, and rapid voluntary torque production, and their neuromuscular determinants.DesignRepeated measures, randomised.MethodsNine participants performed sets of neuromuscular assessments in HOT conditions (~50°C, ~35% relative humidity) at rectal temperatures (Tre) of 37, 38.5 and 39.5°C and in CON conditions (~22°C, ~5% relative humidity) at a Tre of ~37°C and pre-determined comparative time-points. Electrically evoked twitch (single impulse) and octet (8 impulses at 300 Hz) responses were measured at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were measured during 3-5 s isometric maximal voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary isometric contractions from rest.ResultsAll neuromuscular variables were unaffected by time in CON. In HOT, MVT, normalised EMG at MVT and VA were lower at 39.5°C compared to 37°C (p<0.05). Early- (0-50 ms) and middle- (50-100 ms) phase voluntary RTD were unaffected by increased Tre (p>0.05), despite lower normalised EMG at Tre 39.5°C (p<0.05) in rapid contractions. In contrast, late-phase (100-150 ms) voluntary RTD was lower at 38.5°C and 39.5°C compared to 37°C (p<0.05) in HOT. Evoked twitch and octet RTD increased with increased Tre (p<0.05). ConclusionsHyperthermia reduced late-phase voluntary RTD, likely due to reduced neural drive and the reduction in MVT. In contrast, early- and middle-phase voluntary RTD were unaffected by hyperthermia, likely due to the conflicting effects of reduced neural drive but faster intrinsic contractile properties.© 2021, Sports Medicine Australia. Published by Elsevier Ltd. The attached document (embargoed until 19/03/2023) is an author produced version of a paper published in JOURNAL OF SCIENCE AND MEDICINE IN SPORT uploaded in accordance with the publisher’s self-archiving policy. The final published version (version of record) is available online at the link. Some minor differences between this version and the final published version may remain. We suggest you refer to the final published version should you wish to cite from it.</p
The effect of hyperthermia with localised head and neck cooling on neuromuscular function
No full textMaximal voluntary torque and rate of torque development are not effected by whole-body hyperthermia or ten consecutive days of isothermic heat acclimation.
No full textVoluntary torque production is unaffected by changes in local thermal sensation during normothermia and hyperthermia
Full text linkThis 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
The effect of hyperthermia with localised head and neck cooling on neuromuscular function
No full textIntroduction: hyperthermia reduces volitional force production, voluntary muscle activation and agonist-electromyography (EMG) during a sustained maximal voluntary contraction (MVC) [1], [2]. This reduction in neuromuscular function may explain a reduced exercise capacity in the heat. Cooling of the neck has been shown to improve running capacity in the heat [3]; however the mechanism is unknown. The aim of the study was to investigate whether localised cooling of the head and neck during hyperthermia would affect neuromuscular function following 60 min of cycling in the heat. Methods: fourteen male participants exercised on a cycle ergometer for 60 min at 50% ̇V̇O2max in three experimental conditions; hot (35 °C, 50% rh; HOT), hot with head and neck cooling (35 °C, 50% rh; HOTcooling) and control (18 °C, 50% rh; CON). Immediately after the cycling bout, participants performed a 120-s sustained isometric MVC of the knee extensors of their dominant limb. Neuromuscular activation was assessed during the MVC at 5, 30, 90 and 120-s by superimposing supra-maximal triplet (3 impulses at 100 Hz) contractions by electrical stimulation of the femoral nerve, and calculating the central activation ratio (CAR). EMG amplitude (normalised to maximal M-wave) of the 3 superficial quadriceps heads was recorded throughout the MVC. Rectal temperature (Tre) was measured throughout each condition.Results: Tre was raised in both the HOT (39.27 (0.52) °C) and HOTcooling (39.19 (0.56) °C) trials vs CON (38.07 (0.28) °C) immediately post cycling (P <0.001) and remained elevated during the 120-s MVC. Force declined throughout the MVC in all conditions (Fig 1). The decline in force was on average 18 and 13.6% greater in HOT and HOTcooling respectively compared to CON (Fig.1; P <0.001 for both). This was similar for voluntary activation, with significant reductions in HOT vs CON trials across all time points (P <0.001). Normalised agonist EMG showed significant differences between HOT vs CON throughout the first 60-s of contraction; thereafter a reduced neural drive in all conditions was similar.Discussion: cooling had no physiological effect on Tre during hyperthermia trials. HOT appeared to show the greatest decline in voluntary force and was associated with a greater decline in CAR and normalised EMG in comparison to CON. The higher force output for HOTcooling may be explained by improved neural drive of the central nervous system to voluntarily activated muscles.Conclusion: localised head and neck cooling improves neuromuscular function of the knee extensors during a sustained MVC under hyperthermic conditions. <br/
Factors Modulating Post-Activation Potentiation and its Effect on Performance of Subsequent Explosive Activities
No full textPost-activation potentiation (PAP) is induced by a voluntary conditioning contraction (CC), performed typically at a maximal or near-maximal intensity, and has consistently been shown to increase both peak force and rate of force development during subsequent twitch contractions. The proposed mechanisms underlying PAP are associated with phosphorylation of myosin regulatory light chains, increased recruitment of higher order motor units, and a possible change in pennation angle. If PAP could be induced by a CC in humans, and utilized during a subsequent explosive activity (e.g. jump or sprint), it could potentially enhance mechanical power and thus performance and/or the training stimulus of that activity. However, the CC might also induce fatigue, and it is the balance between PAP and fatigue that will determine the net effect on performance of a subsequent explosive activity. The PAP-fatigue relationship is affected by several variables including CC volume and intensity, recovery period following the CC, type of CC, type of subsequent activity, and subject characteristics. These variables have not been standardized across past research, and as a result, evidence of the effects of CC on performance of subsequent explosive activities is equivocal. In order to better inform and direct future research on this topic, this article will highlight and discuss the key variables that may be responsible for the contrasting results observed in the current literature. Future research should aim to better understand the effect of different conditions on the interaction between PAP and fatigue, with an aim of establishing the specific application (if any) of PAP to sport
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