Journal of applied physiology最新文献

This review follows two previous papers (Farina et al., 2004, 2014) in which we reflected on the use of surface EMG in the study of the neural control of movement. This series of papers began with an analysis of the indirect approaches of EMG processing to infer the neural control strategies and then closely followed the progress in EMG technology. In this third paper, we focus on three main areas: surface EMG modelling; surface EMG processing, with an emphasis on decomposition; and interfacing applications of surface EMG recordings. We highlight the latest advances in EMG models that allow fast generation of simulated signals from realistic volume conductors, with applications ranging from validation of algorithms to identification of non-measurable parameters by inverse modelling. Surface EMG decomposition is currently an established state-of-the-art tool for physiological investigations of motor units. It is now possible to identify large samples of motor units, to track motor units over multiple sessions, to partially compensate for the non-stationarities in dynamic contractions, and to decompose signals in real-time. The latter achievement has facilitated advances in myocontrol, by using the online decoded neural drive as a control signal, such as in the interfacing of prostheses. Looking back over the 20 years since our first review, we conclude that the recording and analysis of surface EMG signals has seen breakthrough advances in this period. Although challenges in its application and interpretation remain, surface EMG is now a solid and unique tool for the study of the neural control of movement.

The influence of peripheral antitussive drugs on spatiotemporal features of coughing have not been reported. We hypothesized that this class of compounds would alter the cough motor pattern, in part, by lengthening cough phases. Peripherally acting antitussives 3-Aminopropylphosphinic Acid (3APPi, 5mg/kg) and levodropropizine (Levo, 3mg/kg) were injected i.v. in anesthetized spontaneously breathing cats (13 males, 2 females; 4.38 ± 0.19 kg). Spatio-temporal analysis of cough induced by mechanical stimulation of the trachea showed significant reductions in cough number and expiratory cough efforts after administration of each drug. A significant reduction in inspiratory cough efforts occurred after Levo. Both drugs induced temporal changes in the cough motor pattern, including prolongations of inspiratory phase, inspiratory-expiratory transition, total cough diaphragm activity and total cough cycle duration. Levo also significantly lengthened the expiratory phase of cough. A shortening of the overlap between diaphragm and abdominal activity and cough abdominal EMG activity was observed after the administration of 3APPi. No significant changes in cardiorespiratory data were seen, with the exception of prolonged expiratory phase after 3APPi and lower blood pressure after Levo. Peripherally induced cough suppression is accompanied with changes in cough temporal characteristics that are not observed after administration of centrally acting antitussives. The motor output produced by the cough central pattern generator differs significantly when coughing is perturbed by peripherally and centrally acting antitussives.

Maximal static dry, i.e., on land, apneas (breath-holds) result in severe hypoxemia and hypercapnia and have easy-going and struggle phases. During the struggle phase, the respiratory muscles involuntarily contract against the closed glottis in increasing frequency and magnitude, i.e., involuntary breathing movements (IBMs). IBMs during a maximal static apnea have been suggested to fatigue respiratory muscles, but this has yet to be measured. Thus, the purpose of this study was to quantify respiratory muscle strength pre- and post-apneas in an elite, world champion, world record holding apneist. To do so, maximal inspiratory and expiratory pressure maneuvers (MIP and MEP, respectively) were performed pre- and post-apnea protocol which included 3 preparatory apneas with 2.5 min rest. All preparatory apneas were ended after the participant reported 7-10 IBMs. Next, he performed 3 maximal static dry apneas with 5 min rest in between. The participant had maximal apneas lasting 363, 408, and 460 seconds. Including preparatory apneas, the participant's total apnea duration was 33.4 min in 57.0 min. Following the apnea protocol, i.e., pre vs. post, there was no change in MIP (-124.2 vs. -123.6 cmH₂O) or MEP (259.4 vs. 262.5 cmH₂O). These data, albeit in a single individual, suggest that respiratory muscle strength is not impacted by maximal static breath-holds. This could be the result of training and/or be a feature of this individual that allows him to excel in this sport.

Elite distance runners have exhibited race time improvements since the 2020 introduction of advanced footwear technology (AFT) for track and field, also known as "super" spikes. The observed performance improvements may be due to changes in midsole compliance, which could affect leg stiffness while wearing AFT spikes. Since increased leg stiffness has been associated with running speeds greater than 6 m/s, race time improvements for elite distance runners wearing AFT spikes may be reduced at faster running speeds. To investigate the relationship between use of AFT spikes, running speed, and race performance, we conducted a statistical analysis of the race times from the top 100 male and female elite runners for 800-m to 10,000-m events from 2001 to 2023. We calculated race performance improvement (RPI) as the percentage difference between the pre-AFT spike (2001 to 2019) regression equation predicted average race times from the top 100 athletes and actual average race times from the top 100 athletes for 2021, 2022, and 2023. Overall, RPI after the introduction of AFT spikes was 0.89 ± 0.58% (range: 0.22 to 2.03%; p < 0.001) or roughly 1.2 seconds faster per kilometer than predicted by the regression equations. Additionally, average running speed is faster as race distance decreases and we found that RPI was negatively associated with running speed from 10,000-m to 800-m (p < 0.001). Although overall race performances have improved since the introduction of AFT spikes, the use of AFT spikes may disproportionately improve race performance based on running speed.

The aim of the present study was to quantify the time-course of changes in maximum skin wettedness (ω_max) - i.e., the proportion of skin surface area covered in sweat at the point of uncompensable heat stress, throughout 7 consecutive days of heat acclimation. Nine adults (6M, 3F) completed a humidity-ramp protocol (RAMP) on days 1, 3, 5 and 7 of seven consecutive days of heat acclimation. In each RAMP trial, participants cycled continuously at 275 W·m^-2 for 120 min at 37°C: 60-min at a vapour pressure of 2.05 kPa followed by 60-min with vapour pressure increased by 0.045 kPa·min^-1. An upward inflection in esophageal temperature (T_eso) signaled a transition to uncompensable heat stress with the critical water vapour pressure at that point used to calculate ω_max. In days between RAMP assessments participants cycled for 90-min at 75% HRmax at 37°C, 60% RH. T_eso, whole-body sweat rate (WBSR), local sweat rate (LSR_back, LSR_arm) and activated sweat gland density (AGSD) were measured throughout. ω_max was progressively and significantly greater from Day 1 (0.68±0.10) to Day 3 (0.75±0.10;P=0.002), to Day 5 (0.79±0.10;P=0.004), to Day 7 (0.87±0.06;P=0.009). WBSR was higher on Day 5 (1.11±0.30 L·h^-1;P=0.01) and Day 7 (1.12±0.19 L·h^-1;P<0.001) compared to Day 1 (0.94±0.21 L·h^-1). ASGD was higher on Day 5 (78±15 glands·cm^-2;P<0.001), and Day 7 (81±17 glands·cm^-2;P=0.001) compared to Day 1 (65±12 glands·cm^-2). There were no observed differences in sweat gland output (P=0.21). In conclusion, ω_max significantly increased throughout 7 days of heat acclimation. These progressive increases in ω_max were predominantly mediated by an increase in the number of active sweat glands, not the output per gland.

Eccentric contractions (ECC) are accompanied by accumulation of intracellular calcium ions ([Ca²⁺]_i) and induce skeletal muscle damage. Suppressed muscle damage in repeated bouts of ECC is well characterized, however, whether it is mediated by altered Ca²⁺ profiles remains unknown. PURPOSE: We tested the hypothesis that repeated ECC suppresses Ca²⁺ accumulation via adaptions in Ca²⁺ regulation. METHODS: Male Wistar rats were divided into two groups: ECC single bout (ECC-SB) and repeated bout (ECC-RB). Tibialis anterior (TA) muscles were subjected to ECC (40 times, 5 sets) once (ECC-SB), or twice 14 days apart (ECC-RB). Under anesthesia, the TA muscle was loaded with Ca²⁺ indicator Fura-2 AM and the 340/380 nm ratio was evaluated as [Ca²⁺]_i. Ca²⁺ handling proteins were measured by western blots. RESULTS: ECC induced [Ca²⁺]_i increase in both groups, but ECC-RB evinced a markedly suppressed [Ca²⁺]_i (Time: P < 0.01, Group: P = 0.0357). 5 hours post-ECC, in contrast to the localized [Ca²⁺]_i accumulation in ECC-SB, ECC-RB exhibited lower and more uniform [Ca²⁺]_i (P < 0.01). In ECC-RB mitochondria Ca²⁺ uniporter complex components, MCU and MICU2, were significantly increased pre-second ECC bout (P < 0.01) and both SERCA1 and MICU1 were better preserved after contractions (P < 0.01). CONCLUSION: 14 days after novel ECC skeletal muscle mitochondrial Ca²⁺ regulating proteins were elevated. Following subsequent ECC [Ca²⁺]i accumulation and muscle damage were suppressed and SERCA1 and MICU1 preserved. These findings suggest that tolerance to a subsequent ECC bout is driven, at least in part, by enhanced mitochondrial and SR Ca²⁺ regulation.

Rising global temperatures, driven by climate change, pose a threat to human health and regional livability. Empirical data and biophysical model derived estimates suggest that the critical environmental limits (CEL) for livability are dependent on ambient temperature and humidity. We use a well-validated, physiology-based, six-cylinder thermoregulatory model (SCTM) to independently derive CELs during sustained minimal, light, and moderate activity across a broad range of ambient temperatures and humidity levels and compare to published data. The activity and environments were considered livable if predicted core temperatures did not reach 38±0.25°C within six hours. The outcomes for minimal activity revealed CELs ranging from 34°C/95% RH to 50°C/5% RH. Corresponding dry heat losses ranged from 14 W·m^-2 and -72 W·m^-2 (negative = heat gain) and evaporative heat losses ranged from 39 W·m^-2 to 104 W·m^-2. The wet bulb temperature (Twb) at the CELs ranged from 33.3 to 20.9°C. Activity shifted CELs toward lower temperatures and humidities. Importantly, our predicted CELs largely agree with observed livability CELs from physiology and those from a biophysical model. The physiology-grounded SCTM has utility for assessing the impact of climate change on regional livability.

In lowlanders, high altitude (HA) acclimatization induces hemoconcentration by reducing plasma volume (PV) and increasing total hemoglobin mass (Hb_mass). Conversely, Tibetan highlanders living at HA are reported to have a similar hemoglobin concentration ([Hb]) as lowlanders near sea level and we investigated whether this reflects alterations in the PV or the Hb_mass response to HA. Baseline assessment of PV and Hb_mass was performed by carbon monoxide rebreathing at low altitude (~1,400 m) in Sherpas (an ethnic group of Tibetans living in Nepal) and native lowlanders. Participants then ascended to the Everest Base Camp (5,400 m), where further measurements were performed after ~2 days (EBC 1) and ~6 weeks (EBC 2). While on EBC 1 an increase in [Hb] was observed in lowlanders (p=0.004), but not in Sherpas (p=0.179), marked increases in [Hb] were observed in both groups on EBC 2 (p<0.001). On EBC 1, Hb_mass (Sherpas, p=0.393; lowlanders, p=0.123) and PV (Sherpas, p=0.348; lowlanders, p=0.172) were not different from baseline in either group, whilst circulating erythropoietin was increased in both groups (p<0.001). On EBC 2, large increases in Hb_mass and reductions in PV were observed along with elevated circulating erythropoietin in both groups (all p<0.002). Neither the increases in erythropoietin on EBC 1 (p=0.846) or EBC 2 (p=0.564), nor the expansion of Hb_mass (p=0.771) or reduction in PV (p=0.099) on EBC 2 differed between the groups. We conclude that the hematological response of Sherpas to extended exposure to very high altitude does not fundamentally differ from that of native lowlanders.

Maximum skin wettedness (ω_max) is the proportion of the body covered in sweat at the upper limit of compensable heat stress. It has yet to be determined how ω_max changes with aging. We examined variability in ω_max at the upper limit of compensable heat stress in warm-humid (WH) and hot-dry environments (HD) in young (Y, 18-29 yrs), middle-aged (MA, 40-60 yrs) and older (O, 65-89 yrs) adults during minimal activity (MinAct; ~1.8 METS) and in O subjects at rest. ω_max was calculated using partitional calorimetry for 27 Y (13F), 27 MA (16F), and 32 O (18F) at the previously determined upper limits of compensable heat stress in WH and HD environments. In WH environments, ω_max was greater in Y (0.69 ± 0.12) and MA (0.64 ± 0.20) compared to O (0.47 ± 0.14; both P<0.05), but not different between Y and MA (P=0.85). In HD environments, ω_max was greater in Y (0.52 ± 0.05) compared to O adults (0.40 ± 0.07; P<0.05), but not different between MA (0.48 ± 0.10) and Y or O (both P≥0.15). In O participants at rest, ω_max was lower than MinAct in WH (P<0.001) but not HD environments. These findings indicate that (1) ω_max is lower with advanced age across environments and (2) is lower at rest than during light activity in O in humid conditions. ω_max established herein for unacclimated adults during activities of daily living and older adults at rest may be used to model heat stress responses for these populations and environments.

The mechanisms responsible for increased metabolic cost of walking in older adults are poorly understood. We recently proposed a theoretical premise by which age-related reductions in Achilles tendon stiffness (k_AT) can disrupt the neuromechanics of calf muscle force production and contribute to faster rates of oxygen consumption during walking. The purpose of this study was to objectively evaluate this premise. We quantified k_AT at a range of matched relative activations prescribed using electromyographic biofeedback and walking metabolic cost and ankle joint biomechanics in a group of 15 younger (age: 23±4 yrs) and 15 older adults (age: 72±5 yrs). Older adults averaged 44% lower k_AT than younger adults at matched triceps surae activations during isokinetic dorsiflexion tasks on a dynamometer (p=0.046). Older adults also walked with a 17% higher net metabolic power (p=0.017) but indistinguishable peak Achilles tendon forces than younger adults. Thus, data implicate altered tendon length-tension relations with age more than differences in the operating region of those length-tension relations between younger and older adults. In addition, we discovered empirical evidence that lesser k_AT - likely due to the shorter muscle lengths and thus higher relative activations it imposes - was positively correlated with higher net metabolic power during walking (r=-0.365, p=0.048). These results pave the way for interventions focused on restoring ankle muscle-tendon unit structural stiffness to improve walking energetics in aging.