Exercise training induces thrombogenic benefits in recent but not late postmenopausal females.

Although regular physical activity is known to improve cardiovascular health in men, evidence for its beneficial effects in postmenopausal females is less convincing and it remains unclear whether initiation of exercise training soon after, rather than many years after menopause impacts the magnitude of training-induced adaptations. We evaluated exercise-induced changes in markers of thrombotic risk and conduit artery function in recent≤5yr compared with late≥10yr postmenopausal females. Fourteen recent≤5yr and 13 late≥10yr healthy postmenopausal females completed 8 wk of regular intensive exercise training, consisting of floorball and cycling. Markers of thrombotic risk and vascular health were assessed before and after the intervention, and data were analyzed using a linear mixed model. Exercise training reduced markers of thrombotic risk, including an 11% reduction (P = 0.007) in agonist-induced platelet reactivity and a reduction (P = 0.027) in incipient clot microstructure (∼40% reduction in clot mass) in the recent≤5yr but not the late≥10yr (P = 0.380; P = 0.739, respectively) postmenopausal females. There was no change in conduit artery function, as measured by brachial (recent≤5yr, P = 0.804; late≥10yr, P = 0.311) and popliteal artery (recent≤5yr, P = 0.130; late≥10yr, P = 0.434) flow-mediated dilation. Only the late≥10yr postmenopausal females exhibited an increase (by 9.6%, P = 0.022) in intracellular adhesion molecule-1 levels after training, which may have impacted the thrombogenic adaptation in this group. These findings suggest that 8 wk of high-intensity exercise training reduces thrombotic risk in recent≤5yr, but not late≥10yr postmenopausal females. Thus, regular physical activity initiated soon after, rather than many years after menopause and at a higher age, may be more efficient for reducing thrombogenic risk.NEW & NOTEWORTHY Eight weeks of high-intensity exercise training reduces platelet reactivity as well as blood clot density and strength in females ≤5 yr past menopause but not in females ≥10 yr past menopause. The divergent response in the late postmenopausal females may be explained by training-induced low-grade systemic inflammation. These findings suggest that regular physical activity initiated soon after menopause, compared with many years after menopause, may be more efficient for reducing the risk of blood clots.

The influence of reduced foot dorsum cutaneous sensitivity on the vestibular control of balance.

PURPOSE: Foot sole cooling increases vestibular-evoked balance responses, but less is known about foot dorsum temperature alterations. The purpose was to determine whether decreasing cutaneous receptor sensitivity via foot dorsum cooling modulates the vestibular control of balance. METHODS: Eighteen participants (9 males; 9 females) stood quietly on a force plate with feet together, eyes closed, and head rotated leftward during 4, 90-s trials (2 control; 2 cooled) of continuous electrical vestibular stimulation (EVS). Icepacks placed on the dorsum of both feet for 15 min induced cooling and remained throughout the EVS trials. Monofilament testing was performed at multiple locations before and after cooling to determine tactile detection thresholds. T-type thermocouples monitored skin temperature over the tibialis anterior, soleus, foot dorsum and arch of the right leg. Vestibular-evoked balance responses were characterized using time (cumulant density) and frequency (coherence and gain) domain analyses to determine the relationship between the EVS input and motor output (anteroposterior force-AP force; right medial gastrocnemius electromyography-MG EMG). RESULTS: Skin temperature of the foot dorsum and arch decreased ~ 70 and 15%, respectively during cooling (p < 0.05), but was unaltered at other locations (p ≥ 0.10). Detection thresholds for the foot dorsum increased following cooling (p < 0.05). Surprisingly, cooling reduced EVS-AP force and EVS-MG EMG coherence and gain at multiple frequencies, and peak-to-peak amplitude compared to control (p < 0.05). CONCLUSION: Our results indicate that vestibular-driven balance responses are reduced following foot dorsum cooling, likely owing to alterations in cutaneous mechanoreceptor sensitivity and subsequent alterations in the transformation of vestibular cues for balance control.

Central versus peripheral mechanisms of cold-induced vasodilation: a study in the fingers and toes of people with paraplegia.

PURPOSE: This study examined physiological and perceptual parameters related to cold-induced vasodilation (CIVD) in the fingers and toes of people with paraplegia and compared them with responses observed in able-bodied individuals. METHODS: Seven participants with paraplegia and seven able-bodied individuals participated in a randomized matched-controlled study involving left-hand and -foot immersion in cold water (8 ± 1 °C) for 40 min during exposure to cool (16 ± 1 °C), thermoneutral (23 ± 1 °C), and hot (34 ± 1 °C) ambient conditions. RESULTS: Similar CIVD occurrence was observed in the fingers in the two groups. In toes, three of the seven participants with paraplegia revealed CIVDs: one in cool, two in thermoneutral, and three in hot conditions. No able-bodied participants revealed CIVDs in cool and thermoneutral conditions, while four revealed CIVDs in hot conditions. The toe CIVDs of paraplegic participants were counterintuitive in several respects: they were more frequent in cool and thermoneutral conditions (compared to the able-bodied participants), emerged in these conditions despite lower core and skin temperatures of these participants, and were evident only in cases of thoracic level lesions (instead of lesions at lower spinal levels). CONCLUSION: Our findings demonstrated considerable inter-individual variability in CIVD responses in both the paraplegic and able-bodied groups. While we observed vasodilatory responses in the toes of participants with paraplegia that technically fulfilled the criteria for CIVD, it is unlikely that they reflect the CIVD phenomenon observed in able-bodied individuals. Taken together, our findings favor the contribution of central over peripheral factors in relation to the origin and/or control of CIVD.

Effects of sensory nerve blockade on cutaneous microvascular responses to ischemia-reperfusion injury.

OBJECTIVE: Examine the effects of sensory nerve blockade on cutaneous post-occlusive reactive hyperemia (PORH) and local thermal hyperemia (LTH) following prolonged upper limb ischemia. MATERIALS AND METHODS: In nine males [28 years (standard deviation:6)], volar forearm skin blood flux normalized to maximum vasodilation (%SkBFmax) was assessed at control (CTRL) and sensory nerve blockade (EMLA) treated sites during the PORH response following 20-min of complete arm ischemia and during rapid LTH (33-42 °C, 1 °C·20 s-1, held for ~30-min + 20-min at 44 °C) before and after ischemia-reperfusion (IR) injury. RESULTS: EMLA increased mean [95 % confidence-interval] PORH amplitude by 21%SkBFmax ([9,33]; p = 0.003), delayed time to peak by 111 s ([40,182]; p = 0.007) and increased area under the curve by 19,462%SkBFmax·s ([11,346,27,579]; p < 0.001) compared to CTRL. For LTH, EMLA delayed onset time by 76 s ([46,106]; p < 0.001) Pre-IR and by 46 s ([27,65]; p < 0.001) Post-IR compared to CTRL. Post-IR onset time was delayed for CTRL by 26 s ([8,43]; p = 0.007), but was not different for EMLA (p > 0.050) compared to Pre-IR. EMLA delayed time to initial peak by 24 s ([4,43]; p = 0.022, Main time effect) and it attenuated the initial peak by 27%SkBFmax ([12,43]; p = 0.002) Pre-IR and by 16%SkBFmax ([3,29]; p = 0.020) post-IR compared to CTRL. Post-IR, the initial peak was not different for CTRL (p > 0.050), but it was increased by 16%SkBFmax ([5,26]; p = 0.005) for EMLA compared to Pre-IR. Neither EMLA nor IR altered the steady-state heating plateau (all p > 0.050). CONCLUSION: For the current model of IR injury, sensory nerves appear to have a negligible influence on the LTH response in non-glabrous forearm skin once vasodilation has been initiated.

The effects of local forearm heating and cooling on motor unit properties during submaximal contractions.

NEW FINDINGS: What is the central question of this study? How do temperature manipulations affect motor unit (MU) properties during submaximal contractions to the same relative percentage of maximal force? What is the main finding and its importance? MU recruitment patterns are affected by temperature manipulations at the forearm. However, the relationship between MU potential amplitude and recruitment threshold indicates no change to the order or recruitment. Additionally, the MU potential amplitude and firing rate relationship was affected by temperature, suggesting that smaller MUs are more affected by temperature changes than larger MUs. ABSTRACT: Temperature impacts muscle contractile properties, such that experiments with workloads based on thermoneutral values will produce different relative intensities if maximal force changes due to muscle temperature. We investigated how temperature affected motor unit (MU) properties with contractions performed at the same normalized percentage of maximal force. Twenty participants (10 females) completed evoked, maximal, and trapezoidal voluntary contractions during thermoneutral-, hot-, and cold-temperature conditions. Forearm temperature was established using 25 min of neutral (∼32°C), hot (∼44°C) or cold (∼13°C) water circulated through a tube-lined sleeve. Flexor carpi radialis MU properties were assessed with contractions at 30% and 60% MVC relative to each temperature using surface electromyography decomposition. Changes to contractile properties and electromechanical delay from the evoked twitch suggest that muscle contractility was changed from the thermal manipulations (effect size (d) ≥ 0.42, P < 0.05). Maximal force was not different between neutral and hot conditions (d = 0.16, P > 0.05) but decreased in the cold (d ≥ 0.34, P < 0.05). For both contraction intensities, MU potential (MUP) amplitude was larger and duration was longer in the cold compared to neutral and hot conditions (d ≥ 1.24, P < 0.05). Cumulative probability density for the number of MUs recruited revealed differences in MU recruitment patterns among temperature conditions. The relationship between MU recruitment threshold and firing rate or MUP amplitude was not different among temperature conditions (P > 0.05); however, the relationship between MUP amplitude and firing rate was (P < 0.05). Local temperature manipulations appear to affect MU recruitment patterns, which may act as compensatory mechanisms to the changes in muscle viscosity and contractile properties due to local temperature changes.

Sex differences in the physiological adaptations to heat acclimation: a state-of-the-art review.

Over the last few decades, females have significantly increased their participation in athletic competitions and occupations (e.g. military, firefighters) in hot and thermally challenging environments. Heat acclimation, which involves repeated passive or active heat exposures that lead to physiological adaptations, is a tool commonly used to optimize performance in the heat. However, the scientific community's understanding of adaptations to heat acclimation are largely based on male data, complicating the generalizability to female populations. Though limited, current evidence suggests that females may require a greater number of heat acclimation sessions or greater thermal stress to achieve the same magnitude of physiological adaptations as males. The underlying mechanisms explaining the temporal sex differences in the physiological adaptations to heat acclimation are currently unclear. Therefore, the aims of this state-of-the-art review are to: (i) present a brief yet comprehensive synthesis of the current female and sex difference literature, (ii) highlight sex-dependent (e.g. anthropometric, menstrual cycle) and sex-independent factors (e.g. environmental conditions, fitness) influencing the physiological and performance adaptations to heat acclimation, and (iii) address key avenues for future research.

Keeping environmental physiology education up and running during the COVID-19 pandemic.

The COVID-19 pandemic provoked a need for rapid adaptation of teaching strategies and learning environments. Thus novel approaches, predominantly based on online/virtual platforms are needed to minimize the negative effects of the pandemic on teaching (and learning). Herein we describe our recent web-based symposium series on environmental physiology and ergonomics initiative as an example of such a strategy. We outline the ideas behind this series and its implementation, which could serve as an example of a useful joint interactive virtual educational environment that could be applied to any physiology subspecialty. Based on the feedback received from all stakeholders involved in the process, we strongly believe that such an approach can provide an excellent platform for all educational levels from undergraduate students up to seasoned academics. Importantly, the unrestricted availability (free registration and publication of recordings and student handouts) is an important consideration for the democratization of science and the inclusion of financially less well-supported students and academics.

Noninvasive assessment of increases in microvascular endothelial function following repeated bouts of hyperaemia.

OBJECTIVE: Spectral analyses of laser-Doppler signal can delineate underlying mechanisms in response to pharmacological agents and in cross-sectional studies of healthy and clinical populations. We tested whether spectral analyses can detect acute changes in endothelial function in response to a 6-week intervention of repeated bouts of hyperaemia. METHODS: Eleven males performed forearm occlusion (5 s with 10 s rest) for 30 min, 5 times/week for 6 weeks on one arm; the other was an untreated control. Skin blood flow was measured using laser-Doppler fluxmetry (LDF), and endothelial function was assessed with and without nitric oxide (NO) synthase-inhibition with L-NAME in response to local heating (42 °C and 44 °C) and acetylcholine. A wavelet transform was used for spectral analysis of frequency intervals associated with physiological functions. RESULTS: Basal measures were all unaffected by the hyperaemia intervention (all P > 0.05). In response to local skin heating to 42 °C, the 6 weeks hyperaemia intervention increased LDF, endothelial NO-independent and NO-dependent activity (all P ≤ 0.038). In response to peak local heating (44 °C) endothelial NO-independent and NO-dependent activity increased (both P ≤ 0.01); however, LDF did not (P > 0.2). In response to acetylcholine, LDF, endothelial NO-independent and NO-dependent activity all increased (all P ≤ 0.003) post-intervention. CONCLUSIONS: Spectral analysis appears sufficiently sensitive to measure changes over time in cutaneous endothelial activity that are consistent with standard physiological (local heating) and pharmacological (acetylcholine) interventions of assessing cutaneous endothelial function, and may be useful not only in research but also clinical diagnosis and treatment.

Comparison of laser speckle contrast imaging and laser-Doppler fluxmetry in boys and men.

OBJECTIVE: We compare microvascular reactivity assessed by laser-Doppler fluxmetry (LDF) and laser speckle contrast imaging (LSCI) of boys and men during rest, post-occlusive reactive hyperaemia (PORH), and cycling exercise. METHODS: 19 boys (9 ±â€¯1 y) and 18 men (22 ±â€¯2 y) participated. LDF and LSCI measures were taken of the forearm during rest, PORH, and exercise. RESULTS: For all 3 assessments, the LSCI presented with higher flux values than the LDF for both boys and men (p < 0.001). Bland-Altman analyses indicated that there was a positive linear bias between LSCI and LDF measurements in both boys and men. Regression analyses showed that the responses for the two methods were variable, depending on the particular assessment. For instance, at rest in boys there was no relationship between LDF and LSCI (r2 = 0.002), while in men there was a strong relationship (r2 = 0.86). CONCLUSIONS: LSCI presented with higher values than LDF during rest, PORH, and exercise; the disparity between the two measures was larger as blood flow increased. The assessments were generally consistent, both methods appear to provide usable data for the assessment of microvascular reactivity in both boys and men. There are biases to each method and the data are not interchangeable between LDF and LSCI.

The reliability of cutaneous low-frequency oscillations in young healthy males.

OBJECTIVE: Spectral analyses of laser-Doppler flowmetry measures enable a simple and non-invasive method to investigate mechanisms regulating skin blood flow. We assessed within-day and day-to-day variability of cutaneous spectral analyses. METHODS: Eleven young, healthy males were tested twice in three identical sessions, with 19 to 24 days between visits, for a total of six tests. Wavelet data were analyzed at rest, in response to local skin heating to 42 and 44°C, and during 5-minutes PORH. We did this for six frequency bands commonly associated with physiological functions. To assess reliability, we calculated CV and ICC scores. RESULTS: At rest, mean CV for the wavelet data ranged from 21% to 24% and ICC scores ranged from 0.67 to 0.91. During local heating, mean CV scores ranged from 17% to 22% and mean ICC scores ranged from 0.71 to 0.95. For peak PORH, CV ranged from 14% to 23% and the ICC range was 0.88 to 0.97. For the area under the curve of the PORH, CV range was 12% to 21% and ICC range was 0.81 to 0.92. CONCLUSIONS: These analyses indicate good-to-excellent reliability of the wavelet data in healthy young males.

Glabrous and non-glabrous vascular responses to mild hypothermia.

This study examined cutaneous vasoconstriction to whole-body hypothermia, specifically contributions of neural and endothelial vasomotor responses in glabrous and non-glabrous skin. Eleven participants were semi-recumbent at an ambient temperature of 22 °C for 30 min, after which ambient temperature was decreased to 0 °C until rectal temperature (Tre) had decreased by 0.5 °C. Laser-Doppler fluxmetry was measured at the forehead and thigh for measures of glabrous and non-glabrous skin, respectively; wavelet analysis was performed on the laser-Doppler signal to determine endothelial and neural activities. Hypothermia took on average 97 ±â€¯7 min and caused marked decreases at glabrous (42 ±â€¯5%baseline, p < 0.001) and non-glabrous (69 ±â€¯4%baseline, p < 0.001) skin. In glabrous skin, neural activity increased from 11 ±â€¯1% at thermoneutral to 18 ±â€¯1% (p < 0.001). In non-glabrous skin there was an initial decrease (p = 0.001) in neural activity from 13 ±â€¯2% to 9 ±â€¯1% (-0.2 °C decrease in Tre) and then increased (p = 0.002) to 21 ±â€¯2% baseline at -0.5 °C Tre. Endothelial activity decreased in both glabrous (16 ±â€¯3% to 6 ±â€¯1%, p < 0.001) and non-glabrous (15 ±â€¯1% to 7 ±â€¯1%, p = 0.003) skin. Hypothermia elicits large decreases in skin blood flow in both glabrous and non-glabrous skin that are related to increases in neural activity and a reduction of endothelial activity.

The effects of local muscle temperature on force variability.

PURPOSE: Force variability is affected by environmental temperature, but whether the changes are from altered muscle temperature or proprioception are unclear. We tested how forearm muscle warming and cooling affected a force tracking task. METHODS: Twelve males and four females completed evoked, maximal, and isometric wrist flexion contractions (0-30% maximal) during thermoneutral-, warm-, and cold-muscle conditions. Forearm muscle temperature was manipulated using neutral (~ 33 °C), hot (~ 44 °C), or cold (~ 13 °C) water circulated through a tube-lined sleeve. Evoked and voluntary contractions were performed before and after thermal manipulations. RESULTS: Thermal manipulations altered contractile properties as evident in the twitch half-relaxation time, rate of force development, and duration (all P < 0.05), suggesting that muscle temperature was successfully altered. Changes in surface electromyography of the flexor carpi radialis root-mean-square amplitude and mean power frequency between temperature conditions (all P < 0.05) also indicate muscle temperature changes. No changes to root-mean-square error or variance ratio of the force trace were observed with muscle temperature changes (both P > 0.05). Muscle temperature changes did not have a consistent effect on coefficient of variation during each plateau of the staircase contraction. CONCLUSIONS: Our results suggest that the ability to perform a multi-plateaued isometric force task is not affected by changes to forearm muscle temperature. As the thermal manipulation was limited to the forearm, changes to hand temperature would be minimal, thus, proprioception in the wrist and hand was preserved allowing performance to be maintained. Therefore, modest changes to forearm muscle temperature are not likely to affect force variability if proprioception is maintained.

Comparison of different wheelchair seating on thermoregulation and perceptual responses in thermoneutral and hot conditions in children.

We examined the effects of 4 different wheelchair seatings on physiological and perceptual measures in 21 healthy, pre-pubertal children (9 ±â€¯2 years). Participants were able-bodied and did not regularly use a wheelchair. Participants sat for 2 h in Neutral (∼22.5 °C, ∼40%RH) and Hot (∼35 °C, ∼37%RH) conditions. Four seating technologies were: standard incontinent cover and cushion (SEAT1); standard incontinent cover with new cushion (SEAT2) were tested in Neutral and Hot; new non-incontinent cover with new cushion (SEAT3); new incontinent cover and new cushion (SEAT4) were tested in Neutral only. Measurements included skin blood flow (SkBF), sweating rate (SR) and leg skin temperature (TlegB) on the bottom of the leg (i.e. skin-seat interface), heart rate (HR), mean skin temperature, tympanic temperature, thermal comfort, and thermal sensation. During Neutral, SkBF and TlegB were lower (∼50% and ∼1 °C, respectively) and SR higher (∼0.5 mg cm-2·min-1) (p < 0.05) with SEAT3 compared to all other seats. SkBF was ∼30% lower (p < 0.05) for SEAT2 and SEAT4 compared to SEAT1. No other differences were observed between SEATs (all p > 0.05). During Hot, HR and temperatures were higher than in Neutral but there were no differences (p > 0.05) between SEATs. New cover and cushion improved thermoregulatory responses during Neutral but not Hot. An impermeable incontinent cover negated improvements from cushion design. Seat cover appears more important than seat cushion during typical room conditions.

The effect of repeated bouts of hyperaemia on sensory nerve-mediated cutaneous vasodilatation in humans.

PURPOSE: To investigate cutaneous sensory nerve contribution to hyperaemia following chronic shear stress training. METHODS: Eleven males underwent a shear stress intervention (forearm occlusion 5 s, rest 10 s) for 30 min, 5 times·week-1 for 6 weeks on one arm, the other was an untreated control. Skin blood flow was measured using laser-Doppler flowmetry, and sensory nerve function was assessed with and without blockade with EMLA cream in response to 3 levels of local heating (39, 42, and 44 °C) and post-occlusive reactive hyperaemia (PORH). RESULTS: In response to local heating, EMLA treatment significantly delayed the onset of vasodilatation (p < 0.001), time-to-peak (p < 0.001), time to 39 °C (p < 0.02), time to 42 °C (p < 0.006), but not time to 44 °C (p > 0.2). EMLA treatment also increased time-to-peak for PORH (p ≤ 0.01). In the experimental limb after 6 weeks, both onset time and time to peak were shorter in response to local heating at the untreated and EMLA-treated sites (all p < 0.001). There were no changes in time-to-peak for PORH at the untreated and EMLA-treated sites (p ≥ 0.4); however, the peak PORH response was reduced with EMLA treatment (p ≤ 0.03). The 6-week intervention increased the peak PORH at the untreated sites (p < 0.001) but not at EMLA-treated (p > 0.05) sites. Comparing the control limb before and after 6 weeks, no differences in responses occurred at either the untreated skin sites (p ≥ 0.9) or the EMLA-treated sites (p ≥ 0.9). CONCLUSIONS: Sensory nerve blockade attenuated the improvements in cutaneous vascular responses to thermal hyperaemia and PORH following chronic exposure to shear stress. These data demonstrate an important role for sensory nerve function in the initiation of vasodilatation to both PORH and thermal hyperaemia, in both the time to onset and the magnitude of vasodilatation.

Ischemia-reperfusion injury alters skin microvascular responses to local heating of the index finger.

BACKGROUND: Ischemia-reperfusion (IR) injury impairs microcirculatory function by reducing nitric oxide (NO) bioavailability and increasing sympathetic tone. This study non-invasively examined the effects of acute upper limb IR injury on local thermal hyperemia (LTH) in glabrous and non-glabrous finger skin. MATERIALS AND METHODS: In ten healthy males, LTH was examined twice (~7-10 d apart) for each skin type on the index finger using laser-Doppler flowmetry in a counterbalanced design with either 1) 20 min ischemia, followed by reperfusion (ISCH) or 2) time-matched control (SHAM). LTH tests were performed using a standard heating protocol (33-42 °C at 1 °C·20 s-1 + 20 min at 44 °C) and baseline, initial peak, nadir, delayed plateau and maximal heating phases were identified as well as vasodilatory onset time and time to initial peak. Cutaneous vasomotion was evaluated using spectral analysis and comparing absolute and normalized wavelet amplitudes between conditions for both skin types at baseline and during LTH. RESULTS: In non-glabrous skin, IR injury delayed the vasodilatory onset of local heating by 27.4 [11.3, 43.4] s (p = 0.004) and attenuated cutaneous vasodilation during the initial peak and sustained heating by -44.5 [-73.0, -15.9] PU (p = 0.003) and -34.4 [-62.9, -5.8] PU (p = 0.020), respectively. Analysis of normalized wavelet amplitudes in non-glabrous skin identified impaired microvascular function at baseline via NO-dependent mechanisms (-3.64 [-7.22, -0.05] %, p = 0.047), and during LTH via respiratory influences (-2.83 [-5.39, -0.21] %, p = 0.031). In glabrous skin, IR injury delayed vasodilatory onset time by 24.9 [1.1, 67.6] s (p = 0.042). The vasodilatory response to sustained local skin heating in glabrous skin was increased following IR injury (+56.3 [15.1, 116.5], p = 0.012), however, this was not evident when accounting for differences in blood pressure between conditions. Additionally, no other differences in vasodilatory or vasomotor functions were observed in this skin type between conditions (all, p > 0.05). CONCLUSIONS: The current IR model elicits impaired cutaneous vasodilatory responses to local heating in young males, primarily in non-glabrous skin, and may be useful for exploring mechanisms of IR-injury and for testing potential countermeasures in otherwise healthy humans.

Cutaneous neural activity and endothelial involvement in cold-induced vasodilatation.

Whether sympathetic withdrawal or endothelial dilators such as nitric oxide (NO) contributes to cold-induced vasodilation (CIVD) events is unclear. We measured blood flow and finger skin temperature (Tfinger) of the index finger in nine participants during hand immersion in a water bath at 35 °C for 30 min, then at 8 °C for 30 min. Data were binned into 10 s averages for the entire 60 min protocol for laser-Doppler flux (LDF) and Tfinger. At baseline, Tfinger was 35.3 ± 0.2 °C and LDF was 227 ± 28 PU. During hand cooling, minimum Tfinger was 10.9 ± 0.4 °C and LDF was 15 ± 4 PU. All participants exhibited at least one CIVD event (Tfinger increase ≥ 1 °C), with a mean peak Tfinger 13.2 ± 0.8 °C and a corresponding peak LDF of 116 ± 34 PU. A Morlet mother wavelet was then used to perform wavelet analysis on the LDF signal, with frequency ranges of 0.005-0.01 Hz (endothelial NO-independent), 0.01-0.02 Hz (endothelial NO-dependent), and 0.02-0.05 Hz (neurogenic). The synchronicity of wavelet fluctuations with rising LDF coincident with CIVD events was then quantified using Auto-regressive Integrated Moving Average time-series analysis. Fluctuations in neural activity were strongly synchronized in real time with increasing LDF (stationary-r2 = 0.73 and Ljung-box statistic > 0.05), while endothelial activities were only moderately synchronized (NO-independent r2 = 0.15, > 0.05; NO dependent r2 = 0.16, > 0.05). We conclude that there is a direct, real-time correlation of LDF responses with neural activity but not endothelial-mediated mechanisms. Importantly, it seems that neural activity is consistently reduced prior to CIVD, suggesting that sympathetic withdrawal directly contributes to CIVD onset.

Episodic bouts of hyperaemia and shear stress improve arterial blood flow and endothelial function.

INTRODUCTION: Exercise and heat stress lead to systemic improvements in arterial endothelial function, vascular stiffness, and cardiopulmonary capacity. The improvements in endothelial function may be primarily mediated via increases in shear stress. This study examined whether improvements in arterial function may be achieved in the absence of systemic vascular adaptations. Specifically, we hypothesized that repeated bouts of brief occlusion would improve arterial endothelial function via shear stress-dependent mechanisms. METHODS: Eleven healthy males underwent a shear stress intervention (5 s brachial occlusion, 10 s rest) for 30 min, five times weekly for 6 weeks on one arm while the other acted as an untreated control. Ultrasound was used to assess brachial arterial forearm blood flow (FBF) and vascular conductance (FVC), diameter, and shear rate (SR), while endothelial function was assessed by flow-mediated dilatation (FMD). Post-occlusive reactive hyperaemia and pulse wave velocity (PWV) were also measured. RESULTS: There were no changes in any of the measures in the control arm (all d < 0.2, p > 0.05). After 3 weeks of the intervention, FMD was increased from baseline (7.6 ± 0.6 vs. 5.9 ± 0.9%; d = 1.3, p = 0.038) and further increased after 6 weeks to 9.5 ± 2.6% (d = 1.7, p < 0.001). SR was also increased following the 6-week intervention (all d ≥ 0.6, p < 0.001). Resting and peak FBF and FVC were also increased in response to the intervention (all d ≥ 0.6, p < 0.001) and PWV was reduced. CONCLUSIONS: These data demonstrate that episodic increases in shear stress elicit marked increases in arterial endothelial function and vascular reactivity.

The effects of local forearm muscle cooling on motor unit properties.

PURPOSE: Muscle cooling impairs maximal force. Using needle electromyography (EMG) to assess motor unit properties during muscle cooling, is limited and equivocal. Therefore, we aimed to determine the impact of local muscle cooling on motor unit firing properties using surface EMG decomposition. METHODS: Twenty participants (12 M, 8 F) completed maximal, evoked, and trapezoidal contractions during thermoneutral and cold muscle conditions. Forearm muscle temperature was manipulated using 10-min neutral (~ 32 °C) or 20-min cold (~ 3 °C) water baths. Twitches and maximal voluntary contractions were performed prior to, and after, forearm immersion in neutral or cold water. Motor unit properties were assessed during trapezoidal contractions to 50% baseline force using surface EMG decomposition. RESULTS: Impaired contractile properties from muscle cooling were evident in the twitch amplitude, duration, and rate of force development indicating that the muscle was successfully cooled from the cold water bath (all d ≥ 0.5, P < 0.05). Surface EMG decomposition showed muscle cooling increased the number of motor units (d = 0.7, P = 0.01) and motor unit action potential (MUAP) duration (d = 0.6, P < 0.001), but decreased MUAP amplitude (d = 0.2, P = 0.012). Individually, neither motor unit firing rates (d = 0.1, P = 0.843) nor recruitment threshold (d = 0.1, P = 0.746) changed; however, the relationship between the recruitment threshold and motor unit firing rate was steeper (d = 1.0, P < 0.001) and had an increased y-intercept (d = 0.9, P = 0.007) with muscle cooling. CONCLUSIONS: Since muscle contractility is impaired with muscle cooling, these findings suggest a compensatory increase in the number of active motor units, and small but coupled changes in motor unit firing rates and recruitment threshold to produce the same force.

Effect of passive heat exposure on cardiac autonomic function in healthy children.

PURPOSE: The aim of this study was to examine the effect of passive heat stress on heart rate variability parameters in healthy children. METHOD: Fifteen children (9.3 ± 1.6 years) of both sexes (eight male) participated in two randomized experimental conditions separated by 5-12 days. Children were seated for 2 h in an environmental chamber for two sessions: neutral (22.4 ± 0.1 °C, 40.4 ± 6.5% RH) and hot (34.9 ± 0.3 °C, 36.6 ± 6.2% RH) conditions. Electrocardiogram, mean skin temperature, tympanic temperature, and blood pressure were recorded. Five min epochs were averaged for analysis of cardiac autonomic function over the 2-h protocol. RESULT: Mean skin and tympanic temperatures and heart rate increased during the hot condition (all p < 0.01) while mean arterial pressure decreased (p < 0.01). During the hot condition, root-mean-square difference of successive normal RR intervals (45 ± 9 to 38 ± 7 ms), and low- (LF, 1536 ± 464 vs. 935 ± 154 ms2) and high-frequency power (HF, 1544 ± 693 vs. 866 ± 355 ms2) decreased, whereas LF/HF ratio increased (1.64 ± 0.24 vs. 2.40 ± 0.23 au); all indices were different from neutral (all p < 0.05). These were all unchanged throughout the neutral condition (all p > 0.05), except for LF/HF ratio which decreased during the neutral condition (p < 0.05). CONCLUSION: Mild hyperthermia elicited marked changes in cardiac autonomic control in young children. These data suggest that, in healthy children, vagal withdrawal is responsible for the cardiac autonomic response to hyperthermia.

Between-day reliability of local thermal hyperemia in the forearm and index finger using single-point laser Doppler flowmetry.

OBJECTIVE: To assess between-day reliability for LTH in glabrous and nonglabrous index finger skin and nonglabrous forearm skin, with single-point laser Doppler flowmetry. METHODS: Part-1: In healthy, habitually active males (n=10), LTH was examined twice (~7-10 days apart) for both skin types on the index finger. Part-2: Identical testing was performed on the volar forearm. Local heating (33-42°C at 1°C·20 s-1  + 20 minutes at 44°C) was performed at all skin sites and baseline, initial peak, and plateau phases were identified. Data were expressed as raw CVC (laser-Doppler flux/MAP), and as CVC normalized to baseline (%CVC33°C ) and maximum heating (%CVC44°C ). Reliability was assessed using between-day mean difference, %CV, and ICC. RESULTS: Reliability (%CV) was poor at baseline for all forms of data presentation and for other phases with %CVC33°C . At the initial peak and plateau, reliability was moderate-poor (20%-26%) for CVC and good-moderate (6%-18%) for %CVC44°C . Reliability was good-moderate for vasodilatory onset time (10%-23%) and time to initial peak (6%-13%). CONCLUSIONS: For all sites, LTH reliability was acceptable for the timing, and for the initial peak and plateau using CVC or %CVC44°C .