Creating your own line: reflections from early career scientists.

The changing landscape of academia can be difficult to navigate for anyone at any point throughout their career. One thing is certainly clear: effective mentorship is key to ensuring success, fueling scientific curiosity, and creating a sense of community. This article is a collection of personal reflections and stories, offering advice directed to aspiring and junior graduate trainees; it is written by Ph.D. students, postdoctoral researchers, early-stage assistant professors, and life-long educators. The objective of this article is to inform, empower, and inspire the next generation of physiologists.NEW & NOTEWORTHY This article is a collection of personal reflections and stories, offering advice directed to aspiring and junior graduate trainees that is written by Ph.D. students, postdoctoral researchers, early-stage assistant professors, and life-long educators. The objective of this article is to inform, empower, and inspire the next generation of physiologists.

Voluntary activation does not differ when using two different methods to determine transcranial magnetic stimulator output.

According to current guidelines, when measuring voluntary activation (VA) using transcranial magnetic stimulation (TMS), stimulator output (SO) should not exceed the intensity that, during a maximal voluntary contraction (MVC), elicits a motor evoked potential (MEP) from the antagonist muscle >15%-20% of its maximal M-wave amplitude. However, VA is based on agonist evoked-torque responses [i.e., superimposed twitch (SIT) and estimated resting twitch (ERT)], which means limiting SO based on electromyographic (EMG) responses will often lead to a submaximal SIT and ERT, possibly underestimating VA. Therefore, the purpose of this study was to compare elbow flexor VA calculated using the original method (i.e., intensity based on MEP size; SOMEP) and a method based solely on eliciting the largest SIT at 50% MVC torque (SOSIT), regardless of triceps brachii MEP size. Fifteen healthy, young participants performed 10 sets of brief contractions at 100%, 75%, and 50% MVC torque, with TMS delivered at SOMEP (73.0 ± 13.5%) or SOSIT (92.0 ± 10.8%) for five sets each. Although the mean ERT torque was greater using SOSIT (15.2 ± 4.8 Nm) compared with SOMEP (13.0 ± 3.7 Nm; P = 0.031), the SIT amplitude at 100% MVC torque was not different (SOMEP: 0.69 ± 0.49 Nm vs. SOSIT: 0.74 ± 0.52 Nm; P = 0.604). Despite the ERT disparity, VA scores were not different between SOMEP (94.6 ± 3.5%) and SOSIT (95.0 ± 3.3%; P = 0.572). Even though SOSIT did not lead to a higher VA score than the SOMEP method, it has the benefit of yielding the same result without the need to record antagonist EMG or perform MVCs when determining SO, which can induce fatigue before measuring VA.NEW & NOTEWORTHY When using transcranial magnetic stimulation (TMS) to determine voluntary activation (VA) of the elbow flexors, we hypothesized that a stimulator output designed to limit antagonist muscle activity would evoke submaximal agonist superimposed twitch amplitudes, thus underestimating VA. Contrary to our hypothesis, VA was not greater with an output based on maximal superimposed twitch amplitude. Nevertheless, our findings advance methodological practices by simplifying the equipment and minimizing the time required to determine VA using TMS.

Normobaric hypoxia does not influence the sural nerve cutaneous reflex during standing.

Hypoxia increases postural sway compared to normoxia, but the underlying sensorimotor factors remain unclear. An important contributor to balance control is cutaneous feedback arising from the feet, which can be partially characterized by electrically evoking a reflex from a purely cutaneous nerve (i.e., sural) and sampling the subsequent motor activity of a muscle. The purpose of the present study was to determine how normobaric hypoxia influences sural nerve reflex parameters during a standing posture. It was hypothesized that normobaric hypoxia would reduce cutaneous reflex area compared to normoxia. Participants (n = 16; 5 females, 11 males) stood with their feet together while receiving two trials of 50 sural nerve stimulations (200-Hz, 5-pulse train, presented randomly every 3-6 s) at baseline (BL; normoxia), and at 2 (H2) and 4 (H4) h of normobaric hypoxia (~ 0.11 fraction of inspired oxygen in a hypoxic chamber). The sural nerve reflex was recorded using surface electromyography from the left medial gastrocnemius, and characterized by area and duration of the initial positive and negative peaks of the response. When normalized to pre-stimulus electromyography, the area of the peak-to-peak cutaneous reflex was not different than BL (p ≥ 0.14) for up to 4 h of normobaric hypoxia (BL: 0.26 ± 0.22, H2: 0.19 ± 0.19, H4: 0.22 ± 0.20 A.U.). Furthermore, the duration of the response was not different during hypoxia (BL: 73.2 ± 42.4; H2: 75.2 ± 47.0; H4: 77.6 ± 54.6 ms; p ≥ 0.13) than BL. Thus, reflexes arising from cutaneous afferents of the lateral border of the foot are resilient to at least 4 h of normobaric hypoxia.

Cerebrovascular and blood pressure responses during voluntary apneas are larger than rebreathing.

Both voluntary rebreathing (RB) of expired air and voluntary apneas (VA) elicit changes in arterial carbon dioxide and oxygen (CO2 and O2) chemostimuli. These chemostimuli elicit synergistic increases in cerebral blood flow (CBF) and sympathetic nervous system activation, with the latter increasing systemic blood pressure. The extent that simultaneous and inverse changes in arterial CO2 and O2 and associated increases in blood pressure affect the CBF responses during RB versus VAs are unclear. We instrumented 21 healthy participants with a finometer (beat-by-beat mean arterial blood pressure; MAP), transcranial Doppler ultrasound (middle and posterior cerebral artery velocity; MCAv, PCAv) and a mouthpiece with sample line attached to a dual gas analyzer to assess pressure of end-tidal (PET)CO2 and PETO2. Participants performed two protocols: RB and a maximal end-inspiratory VA. A second-by-second stimulus index (SI) was calculated as PETCO2/PETO2 during RB. For VA, where PETCO2 and PETO2 could not be measured throughout, SI values were calculated using interpolated end-tidal gas values before and at the end of the apneas. MAP reactivity (MAPR) was calculated as the slope of the MAP/SI, and cerebrovascular reactivity (CVR) was calculated as the slope of MCAv or PCAv/SI. We found that compared to RB, VA elicited ~ fourfold increases in MAPR slope (P < 0.001), translating to larger anterior and posterior CVR (P ≤ 0.01). However, cerebrovascular conductance (MCAv or PCAv/MAP) was unchanged between interventions (P ≥ 0.2). MAP responses during VAs are larger than those during RB across similar chemostimuli, and differential CVR may be driven by increases in perfusion pressure.

What Is the Point of the Peak? Assessing Steady-State Respiratory Chemoreflex Drive in High Altitude Field Studies.

Measurements of central and peripheral respiratory chemoreflexes are important in the context of high altitude as indices of ventilatory acclimatization. However, respiratory chemoreflex tests have many caveats in the field, including considerations of safety, portability and consistency. This overview will (a) outline commonly utilized tests of the hypoxic ventilatory response (HVR) in humans, (b) outline the caveats associated with a variety of peak response HVR tests in the laboratory and in high altitude fieldwork contexts, and (c) advance a novel index of steady-state chemoreflex drive (SS-CD) that addresses the many limitations of other chemoreflex tests. The SS-CD takes into account the contribution of central and peripheral respiratory chemoreceptors, and eliminates the need for complex equipment and transient respiratory gas perturbation tests. To quantify the SS-CD, steady-state measurements of the pressure of end-tidal (PET)CO2 (Torr) and peripheral oxygen saturation (SpO2; %) are used to quantify a stimulus index (SI; PETCO2/SpO2). The SS-CD is then calculated by indexing resting ventilation (L/min) against the SI. SS-CD data are subsequently reported from 13 participants during incremental ascent to high altitude (5160 m) in the Nepal Himalaya. The mean SS-CD magnitude increased approximately 96% over 10 days of incremental exposure to hypobaric hypoxia, suggesting that the SS-CD tracks ventilatory acclimatization. This novel SS-CD may have future utility in fieldwork studies assessing ventilatory acclimatization during incremental or prolonged stays at altitude, and may replace the use of complex and potentially confounded transient peak response tests of the HVR in humans.

Quantifying cerebrovascular reactivity in anterior and posterior cerebral circulations during voluntary breath holding.

NEW FINDINGS: What is the central question of this study? We developed and validated a 'stimulus index' (SI; ratio of end-tidal partial pressures of CO2 and O2 ) method to quantify cerebrovascular reactivity (CVR) in anterior and posterior cerebral circulations during breath holding. We aimed to determine whether the magnitude of CVR is correlated with breath-hold duration. What is the main finding and its importance? Using the SI method and transcranial Doppler ultrasound, we found that the magnitude of CVR of the anterior and posterior cerebral circulations is not positively correlated with physiological or psychological break-point during end-inspiratory breath holding. Our study expands the ability to quantify CVR during breath holding and elucidates factors that affect break-point. The central respiratory chemoreflex contributes to blood gas homeostasis, particularly in response to accumulation of brainstem CO2 . Cerebrovascular reactivity (CVR) affects chemoreceptor stimulation inversely through CO2 washout from brainstem tissue. Voluntary breath holding imposes alterations in blood gases, eliciting respiratory chemoreflexes, potentially contributing to breath-hold duration (i.e. break-point). However, the effects of cerebrovascular reactivity on break-point have yet to be determined. We tested the hypothesis that the magnitude of CVR contributes directly to breath-hold duration in 23 healthy human participants. We developed and validated a cerebrovascular stimulus index methodology [SI; ratio of end-tidal partial pressures of CO2 and O2 (P ET ,CO2/P ET ,O2)] to quantify CVR by correlating measured and interpolated values of P ET ,CO2 (r = 0.95, P < 0.0001), P ET ,O2 (r = 0.98, P < 0.0001) and SI (r = 0.94, P < 0.0001) during rebreathing. Using transcranial Doppler ultrasound, we then quantified the CVR of the middle (MCAv) and posterior (PCAv) cerebral arteries by plotting cerebral blood velocity against interpolated SI during a maximal end-inspiratory breath hold. The MCAv CVR magnitude was larger than PCAv (P = 0.001; +70%) during breath holding. We then correlated MCAv and PCAv CVR with the physiological (involuntary diaphragmatic contractions) and psychological (end-point) break-point, within individuals. There were significant inverse but modest relationships between both MCAv and PCAv CVR and both physiological and psychological break-points (r < -0.53, P < 0.03). However, these relationships were absent when MCAv and PCAv cerebrovascular conductance reactivity was correlated with both physiological and psychological break-points (r > -0.42; P > 0.06). Although central chemoreceptor activation is likely to be contributing to break-point, our data suggest that CVR-mediated CO2 washout from central chemoreceptors plays no role in determining break-point, probably because of a reduced arterial-to-tissue CO2 gradient during breath holding.

Comparing and characterizing transient and steady-state tests of the peripheral chemoreflex in humans.

NEW FINDINGS: What is the central question of this study? We aimed to characterize the cardiorespiratory and cerebrovascular responses to transient and steady-state tests of the peripheral chemoreflex and to compare the hypoxic ventilatory responses (HVRs) between these tests. What is the main finding and its importance? The cardiovascular and cerebrovascular responses to transient tests were small in magnitude and short in duration. The steady-state isocapnic hypoxia test elicited a larger HVR than the transient 100% N(2) test, but the response magnitudes were correlated within individuals. The transient test of the HVR elicits fewer systemic effects than steady-state techniques and may have greater experimental utility than previously appreciated. Carotid chemoreceptors detect changes in arterial PO(2) and PCO(2), eliciting a peripheral chemoreflex (PCR). Steady-state (SS) hypoxia tests using dynamic end-tidal forcing (DEF) have been used to assess the hypoxic ventilatory response (HVR) but may be confounded by concomitant systemic effects. Transient tests of the PCR have also been developed but are not widely used, nor have the cardiovascular and cerebrovascular responses been characterized. We characterized the cardiorespiratory and cerebrovascular responses to transient tests of the PCR and compared the HVR between transient and SS-DEF tests. We hypothesized that the cardiovascular and cerebrovascular responses to the transient tests would be minimal and that the respiratory responses elicited from the transient and SS-DEF tests would be different in magnitude and not well correlated within individuals. Participants underwent five consecutive trials of two transient tests [three-breath 100% N(2) (TT-N(2)) and a single-breath 13% CO(2), in air] and two 10 min SS-DEF tests [isocapnic (SS-ISO) and poikilocapnic (SS-POI) hypoxia]. In response to the transient tests, heart rate, mean arterial pressure and the middle and posterior cerebral artery blood velocity increased (all P < 0.01), but responses were small (all <10%) and transient. Although the TT-N(2) and SS-POI tests elicited similar HVR magnitudes, they were not well correlated within individuals (r = 0.064, P = 0.79). The TT-N(2) test elicited a smaller HVR than the SS-ISO test, but they were correlated within individuals (r = 0.57, P = 0.008). Given that the transient tests exploit the temporal domain of the peripheral chemoreceptors and have minimal cardiovascular and cerebrovascular confounders, we suggest that they may have broader utility than previously appreciated.

The ins and outs of breath holding: simple demonstrations of complex respiratory physiology.

The physiology of breath holding is complex, and voluntary breath-hold duration is affected by many factors, including practice, psychology, respiratory chemoreflexes, and lung stretch. In this activity, we outline a number of simple laboratory activities or classroom demonstrations that illustrate the complexity of the integrative physiology behind breath-hold duration. These activities require minimal equipment and are easily adapted to small-group demonstrations or a larger-group inquiry format where students can design a protocol and collect and analyze data from their classmates. Specifically, breath-hold duration is measured during a number of maneuvers, including after end expiration, end inspiration, voluntary prior hyperventilation, and inspired hyperoxia. Further activities illustrate the potential contribution of chemoreflexes through rebreathing and repeated rebreathing after a maximum breath hold. The outcome measures resulting from each intervention are easily visualized and plotted and can comprise a comprehensive data set to illustrate and discuss complex and integrated cardiorespiratory physiology.

Acute normobaric hypoxia causes a rightward shift in the torque-frequency relationship, but has no effect on post-activation potentiation.

Low fractions of inspired oxygen (FIO2; i.e., hypoxia) affect aspects of skeletal muscle contractility in humans, but it remains unclear if post-activation potentiation (PAP) and the torque-frequency (T-F) relationship are altered. We investigated the effects of two (H2) and four hours (H4) of normobaric hypoxia (FIO2=0.11±0.47) on the magnitude of PAP of the knee extensors (KE) and the T-F relationship of the dorsiflexors (DF) in 13 and 12 healthy participants, respectively. To assess PAP, a resting twitch was evoked via femoral nerve stimulation before, and 2-300 s following a 10-s maximal voluntary contraction (MVC). A T-F relationship was obtained by stimulating the common fibular nerve with a single pulse and 1-s trains between 5-100 Hz. During hypoxia, peripheral oxygen saturation decreased by ~18% from 98.0±0.8% at baseline (P<0.001). MVC force and voluntary activation (VA) of the KE were lower than baseline throughout hypoxia (e.g., ~8 and ~5%, respectively at H2; P≤0.027); however, the magnitude of PAP was not altered by hypoxia (P≥0.711). Surprisingly, PAP did increase with time across the control day (P≤0.012). MVC torque and VA of the DF were unaffected by hypoxia (P≥0.127), but the estimated frequency required to evoke 50 % of 100-Hz torque increased by ~1.2 Hz at H2 (P≤0.021). These results imply that two hours of normobaric hypoxia were sufficient to: i) impair neural drive to the KE but not the mechanism(s) responsible for PAP, and ii) lead to a rightward shift of the T-F relationship for the DF.

Steady-state chemoreflex drive captures ventilatory acclimatization during incremental ascent to high altitude: Effect of acetazolamide.

Ventilatory acclimatization (VA) is important to maintain adequate oxygenation with ascent to high altitude (HA). Transient hypoxic ventilatory response tests lack feasibility and fail to capture the integrated steady-state responses to chronic hypoxic exposure in HA fieldwork. We recently characterized a novel index of steady-state respiratory chemoreflex drive (SSCD), accounting for integrated contributions from central and peripheral respiratory chemoreceptors during steady-state breathing at prevailing chemostimuli. Acetazolamide is often utilized during ascent for prevention or treatment of altitude-related illnesses, eliciting metabolic acidosis and stimulating respiratory chemoreceptors. To determine if SSCD reflects VA during ascent to HA, we characterized SSCD in 25 lowlanders during incremental ascent to 4240 m over 7 days. We subsequently compared two separate subgroups: no acetazolamide (NAz; n = 14) and those taking an oral prophylactic dose of acetazolamide (Az; 125 mg BID; n = 11). At 1130/1400 m (day zero) and 4240 m (day seven), steady-state measurements of resting ventilation (V̇I ; L/min), pressure of end-tidal (PET )CO2 (Torr), and peripheral oxygen saturation (SpO2 ; %) were measured. A stimulus index (SI; PET CO2 /SpO2 ) was calculated, and SSCD was calculated by indexing V̇I against SI. We found that (a) both V̇I and SSCD increased with ascent to 4240 m (day seven; V̇I : +39%, p < 0.0001, Hedges' g = 1.52; SSCD: +56.%, p < 0.0001, Hedges' g = 1.65), (b) and these responses were larger in the Az versus NAz subgroup (V̇I : p = 0.02, Hedges' g = 1.04; SSCD: p = 0.02, Hedges' g = 1.05). The SSCD metric may have utility in assessing VA during prolonged stays at altitude, providing a feasible alternative to transient chemoreflex tests.

Maximal results with minimal stimuli: the fewest high-frequency pulses needed to measure or model prolonged low-frequency force depression in the dorsiflexors.

Quantifying prolonged low-frequency force depression (PLFFD) with the gold-standard 1-s trains presents challenges, so paired pulses have been used. Owing to greater impairment of high-frequency doublet than tetanic torque, paired pulses underestimate PLFFD. This study aimed to approximate the minimum number of high-frequency pulses needed to avoid such underestimation and assess the feasibility of modeling PLFFD from a limited number of experimental pulses. In 13 participants, a 1-s 10-Hz train and 100-Hz trains with 2, 4, 7, 12, 15, 25, 50, or 100 pulses were evoked before and after (15 min, 2, 4, and 7 days) eccentric exercise of the dorsiflexors. With ≤12 pulses, impairment of 100-Hz torque was greater than the 1-s train (P ≤ 0.05; e.g., 12 vs. 100 pulses at 4 days: 97.8 ± 8.5% vs. 100.5 ± 8.2% baseline). Consequently, with ≤12 pulses, PLFFD was underestimated compared with the gold-standard measure (P ≤ 0.05; e.g., 12 vs. 100 pulse 10:100-Hz torque ratio at 4 days: 86.8 ± 12.8% vs. 84.6 ± 13.5% baseline). Modeling reproduced 10:100-Hz ratios (PLFFD) with 95% limits of agreement of -13.6% to 16.7% of experimental values with ≥12 pulses. Our results indicate that a minimum of 13-25 pulses of 100 Hz are needed to accurately quantify PLFFD in the dorsiflexors. Although this may not be the minimum range for other muscles, a similar relationship with pulse number likely exists. Modeling may eventually provide an option to estimate PLFFD from experimental trains with relatively few pulses; however, further development is imperative to reduce variability.NEW & NOTEWORTHY Ideally, prolonged low-frequency force depression (PLFFD) is measured with 1-s trains of supramaximal stimuli; however, this induces considerable discomfort. We tested briefer trains to approximate the minimum number of high-frequency pulses needed to accurately determine PLFFD and the feasibility of modeling 1-s tetani with relatively few pulses. After eccentric exercise, 13-25 high-frequency pulses were needed to accurately measure PLFFD. Modeling reproduced mean experimental values but had considerable variability.

Females and males do not differ for fatigability, muscle damage and magnitude of the repeated bout effect following maximal eccentric contractions.

Unaccustomed eccentric (ECC) exercise induces muscle fatigue as well as damage and initiates a protective response to minimize impairments from a subsequent bout (i.e., repeated bout effect; RBE). It is uncertain if the sexes differ for neuromuscular responses to ECC exercise and the ensuing RBE. Twenty-six young adults (13 females) performed 2 bouts (4 weeks apart) of 200 ECC maximal voluntary contractions (MVCs) of the dorsiflexors. Isometric (ISO) MVC torque and the ratio of ISO torque in response to low- versus high-frequency stimulation (10:100 Hz) were compared before and after (2-10 min and 2, 4, and 7 days) exercise. The decline in ECC and ISO MVC torque and the 10:100 Hz ratio following bout 1 did not differ between sexes (P > 0.05), with reductions from baseline of 31.5% ± 12.3%, 24.1% ± 15.4%, and 51.3% ± 12.2%, respectively. After bout 2, the 10:100 Hz ratio declined less (45.0% ± 12.4% from baseline) and ISO MVC torque recovered sooner compared with bout 1 but no differences between sexes were evident for the magnitude of the RBE (P > 0.05). These data suggest that fatigability with ECC exercise does not differ for the sexes and adaptations that mitigate impairments to calcium handling are independent of sex. Novelty: One bout of 200 maximal eccentric dorsiflexor contractions caused equivalent muscle fatigue and damage for females and males. The repeated bout effect observed after a second bout 4 weeks later also had no sex-related differences. Prolonged low-frequency force depression is promoted as an indirect measure of muscle damage in humans.

Prior oxygenation, but not chemoreflex responsiveness, determines breath-hold duration during voluntary apnea.

Central and peripheral respiratory chemoreceptors are stimulated during voluntary breath holding due to chemostimuli (i.e., hypoxia and hypercapnia) accumulating at the metabolic rate. We hypothesized that voluntary breath-hold duration (BHD) would be (a) positively related to the initial pressure of inspired oxygen prior to breath holding, and (b) negatively correlated with respiratory chemoreflex responsiveness. In 16 healthy participants, voluntary breath holds were performed under three conditions: hyperoxia (following five normal tidal breaths of 100% O2 ), normoxia (breathing room air), and hypoxia (following ~30-min of 13.5%-14% inspired O2 ). In addition, the hypoxic ventilatory response (HVR) was tested and steady-state chemoreflex drive (SS-CD) was calculated in room air and during steady-state hypoxia. We found that (a) voluntary BHD was positively related to initial oxygen status in a dose-dependent fashion, (b) the HVR was not correlated with BHD in any oxygen condition, and (c) SS-CD magnitude was not correlated with BHD in normoxia or hypoxia. Although chemoreceptors are likely stimulated during breath holding, they appear to contribute less to BHD compared to other factors such as volitional drive or lung volume.

Prolonged low-frequency force depression is underestimated when assessed with doublets compared with tetani in the dorsiflexors.

Prolonged low-frequency force depression (PLFFD) after damaging eccentric exercise may last for several days. Historically, PLFFD has been calculated from the tetanic force responses to trains of supramaximal stimuli. More recently, for methodological reasons, stimulation has been reduced to two pulses. However, it is unknown whether doublet responses provide a valid measure of PLFFD in the days after eccentric exercise. In 12 participants, doublets and tetani were elicited at 10 and 100 Hz before and after (2, 3, 5 min, 48 and 96 h) 200 eccentric maximal voluntary contractions of the dorsiflexors. Doublet and tetanic torque responses at 10 Hz were similarly depressed throughout recovery (P > 0.05; e.g., 2 min: 58.9 ± 12.8% vs. 57.1 ± 14.5% baseline; 96 h: 85.6 ± 11.04% vs. 85.1 ± 10.8% baseline). At 100 Hz, doublet torque was impaired more than tetanic torque at all time points (P < 0.05; e.g., 2 min: 70.5 ± 14.2% vs. 88.1 ± 11.7% baseline; 96 h: 83.0 ± 14.2% vs. 98.7 ± 9.5% baseline). As a result, the postfatigue reduction of the 10 Hz-to-100 Hz ratio (PLFFD) was markedly greater for tetani than for doublets (P < 0.05; e.g., 2 min: 64.3 ± 15.1% vs. 83.0 ± 5.8% baseline). In addition, the doublet ratio recovered by 48 h (99.2 ± 5.0% baseline), whereas the tetanic ratio was still impaired at 96 h (88.2 ± 9.7% baseline). Our results indicate that doublets are not a valid measure of PLFFD in the minutes and days after eccentric exercise. If study design favors the use of paired stimuli, it should be acknowledged that the true magnitude and duration of PLFFD are likely underestimated. NEW & NOTEWORTHY Prolonged low-frequency force depression (PLFFD) will result from damaging exercise and may last for several days. After 200 eccentric maximal dorsiflexor contractions, we compared the gold-standard measure of PLFFD (calculated using trains of supramaximal stimulation) to the value obtained from an alternative technique that is becoming increasingly common (paired supramaximal stimuli). Doublets underestimated the magnitude and duration of PLFFD compared with tetani, so caution must be used when reporting PLFFD derived from paired stimuli.

Mechanisms of sympathetic regulation during Apnea.

Volitional Apnea produces a robust peak sympathetic response through several interacting mechanisms. However, the specific contribution of each mechanism has not been elucidated. Muscle sympathetic activity was collected in participants (n = 10; 24 ± 3 years) that performed four maximal volitional apneas aimed at isolating lung-stretch (mechanical) and chemoreflex drive: (Ainslie and Duffin ) end-expiratory breath-hold, (Ainslie et al. ) end-inspiratory breath-hold, (Alpher et al. ) prehyperventilation breath-hold, and (Andersson and Schagatay ) prehyperoxia breath-hold. A final repeated rebreathe breath-hold protocol was performed to measure the peak sympathetic response during successive breath-holds at increasing chemoreflex stress. Finally, the influence of dynamic ventilation was assessed through asphyxic rebreathe. Muscle sympathetic activity was calculated as the change in burst frequency (burst/min), burst incidence (burst/100 heart-beats), and amplitude (au) between baseline and prevolitional breakpoint. Rebreathe was analyzed at similar chemoreflex stress as inspiratory breath-hold. All maneuvers increased muscle sympathetic activity compared to baseline (P < 0.01). However, prehyperoxia exhibited a smaller increase (+22.18 ± 9.13 burst/min; +25.52 ± 11.7 burst/100 heart-beats) compared to inspiratory, expiratory, and prehyperventilation breath-holds. At similar chemoreflex strain, rebreathe sympathetic activity was blunted compared to inspiratory breath-hold (P < 0.01). Finally, muscle sympathetic activity was not different between the repeated rebreathe trials, despite elevated chemoreflex stress and lower breath-hold duration with each subsequent breath-hold. We have demonstrated an obligatory role of the peripheral, but not central, chemoreflex (prehyperventilation vs. prehyperoxia) in producing peak sympathetic responses. At similar chemoreflex stresses the act of dynamic ventilation, but not static lung stretch per se, blunts muscle sympathetic activity. Finally, similar peak sympathetic responses during successive repeated breath-holds suggest a sympathetic ceiling may exist.

Assessing chemoreflexes and oxygenation in the context of acute hypoxia: Implications for field studies.

Carotid chemoreceptors detect changes in PO2 and elicit a peripheral respiratory chemoreflex (PCR). The PCR can be tested through a transient hypoxic ventilatory response test (TT-HVR), which may not be safe nor feasible at altitude. We characterized a transient hyperoxic ventilatory withdrawal test in the setting of steady-state normobaric hypoxia (13.5-14% FIO2) and compared it to a TT-HVR and a steady-state poikilocapnic hypoxia test, within-individuals. No PCR test magnitude was correlated with any other test, nor was any test magnitude correlated with oxygenation while in steady-state hypoxia. Due to the heterogeneity between the different PCR test procedures and magnitudes, and the confounding effects of alterations in CO2 acting on both central and peripheral chemoreceptors, we developed a novel method to assess prevailing steady-state chemoreflex drive in the context of hypoxia. Quantifying peak hypoxic/hyperoxic responses at low altitude may have minimal utility in predicting oxygenation during ascent to altitude, and here we advance a novel index of chemoreflex drive.