Michael N. Sawka

Exercise associated hyponatraemia: quantitative analysis to understand the aetiology

Background: The development of symptomatic hyponatraemia consequent on participation in marathon and ultraendurance races has led to questions about its aetiology and prevention. Objectives: To evaluate: (a) the assertion that sweat sodium losses cannot contribute to the development of hyponatraemia during endurance exercise; (b) the adequacy of fluid replacement recommendations issued by the International Marathon Medical Directors Association (IMMDA) for races of 42 km or longer; (c) the effectiveness of commercial sports drinks, compared with water, for attenuating plasma sodium reductions. Methods: A mathematical model was used to predict the effects of different drinking behaviours on hydration status and plasma sodium concentration when body mass, body composition, running speed, weather conditions, and sweat sodium concentration were systematically varied. Results: Fluid intake at rates that exceed sweating rate is predicted to be the primary cause of hyponatraemia. However, the model predicts that runners secreting relatively salty sweat can finish ultraendurance exercise both dehydrated and hyponatraemic. Electrolyte-containing beverages are predicted to delay the development of hyponatraemia. The predictions suggest that the IMMDA fluid intake recommendations adequately sustain hydration over the 42 km distance if qualifiers—for example, running pace, body size—are followed. Conclusions: Actions to prevent hyponatraemia should focus on minimising overdrinking relative to sweating rate and attenuating salt depletion in those who excrete salty sweat. This simulation demonstrates the complexity of defining fluid and electrolyte consumption rates during athletic competition.

Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports

Exercise heat acclimation induces physiological adaptations that improve thermoregulation, attenuate physiological strain, reduce the risk of serious heat illness, and improve aerobic performance in warm‐hot environments and potentially in temperate environments. The adaptations include improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain, improved fluid balance, altered metabolism, and enhanced cellular protection. The magnitudes of adaptations are determined by the intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (i.e., dry or humid heat). Evidence is emerging that controlled hyperthermia regimens where a target core temperature is maintained, enable more rapid and complete adaptations relative to the traditional constant work rate exercise heat acclimation regimens. Furthermore, inducing heat acclimation outdoors in a natural field setting may provide more specific adaptations based on direct exposure to the exact environmental and exercise conditions to be encountered during competition. This review initially examines the physiological adaptations associated with heat acclimation induction regimens, and subsequently emphasizes their application to competitive athletes and sports.

Cardiovascular adaptations supporting human exercise-heat acclimation

This review examines the cardiovascular adaptations along with total body water and plasma volume adjustments that occur in parallel with improved heat loss responses during exercise-heat acclimation. The cardiovascular system is well recognized as an important contributor to exercise-heat acclimation that acts to minimize physiological strain, reduce the risk of serious heat illness and better sustain exercise capacity. The upright posture adopted by humans during most physical activities and the large skin surface area contribute to the circulatory and blood pressure regulation challenge of simultaneously supporting skeletal muscle blood flow and dissipating heat via increased skin blood flow and sweat secretion during exercise-heat stress. Although it was traditionally held that cardiac output increased during exercise-heat stress to primarily support elevated skin blood flow requirements, recent evidence suggests that temperature-sensitive mechanisms may also mediate an elevation in skeletal muscle blood flow. The cardiovascular adaptations supporting this challenge include an increase in total body water, plasma volume expansion, better sustainment and/or elevation of stroke volume, reduction in heart rate, improvement in ventricular filling and myocardial efficiency, and enhanced skin blood flow and sweating responses. The magnitude of these adaptations is variable and dependent on several factors such as exercise intensity, duration of exposure, frequency and total number of exposures, as well as the environmental conditions (i.e. dry or humid heat) in which acclimation occurs.

Consensus recommendations on training and competing in the heat

Exercising in the heat induces thermoregulatory and other physiological strain that can lead to impairments in endurance exercise capacity. The purpose of this consensus statement is to provide up‐to‐date recommendations to optimize performance during sporting activities undertaken in hot ambient conditions. The most important intervention one can adopt to reduce physiological strain and optimize performance is to heat acclimatize. Heat acclimatization should comprise repeated exercise‐heat exposures over 1–2 weeks. In addition, athletes should initiate competition and training in a euhydrated state and minimize dehydration during exercise. Following the development of commercial cooling systems (e.g., cooling vest), athletes can implement cooling strategies to facilitate heat loss or increase heat storage capacity before training or competing in the heat. Moreover, event organizers should plan for large shaded areas, along with cooling and rehydration facilities, and schedule events in accordance with minimizing the health risks of athletes, especially in mass participation events and during the first hot days of the year. Following the recent examples of the 2008 Olympics and the 2014 FIFA World Cup, sport governing bodies should consider allowing additional (or longer) recovery periods between and during events for hydration and body cooling opportunities when competitions are held in the heat.

Novel Methods for Sensing Acoustical Emissions From the Knee for Wearable Joint Health Assessment

Objective: We present the framework for wearable joint rehabilitation assessment following musculoskeletal injury. We propose a multimodal sensing (i.e., contact based and airborne measurement of joint acoustic emission) system for at-home monitoring. Methods: We used three types of microphones - electret, MEMS, and piezoelectric film microphones - to obtain joint sounds in healthy collegiate athletes during unloaded flexion/extension, and we evaluated the robustness of each microphones measurements via: 1) signal quality and 2) within-day consistency. Results: First, air microphones acquired higher quality signals than contact microphones (signal-to-noise-and-interference ratio of 11.7 and 12.4 dB for electret and MEMS, respectively, versus 8.4 dB for piezoelectric). Furthermore, air microphones measured similar acoustic signatures on the skin and 5 cm off the skin (~4.5× smaller amplitude). Second, the main acoustic event during repetitive motions occurred at consistent joint angles (intra-class correlation coefficient ICC(1, 1) = 0.94 and ICC(1, k) = 0.99). Additionally, we found that this angular location was similar between right and left legs, with asymmetry observed in only a few individuals. Conclusion: We recommend using air microphones for wearable joint sound sensing; for practical implementation of contact microphones in a wearable device, interface noise must be reduced. Importantly, we show that airborne signals can be measured consistently and that healthy left and right knees often produce a similar pattern in acoustic emissions. Significance: These proposed methods have the potential for enabling knee joint acoustics measurement outside the clinic/lab and permitting long-term monitoring of knee health for patients rehabilitating an acute knee joint injury.

Novel approaches to measure acoustic emissions as biomarkers for joint health assessment

The ultimate objective of this research is to quantify changes in joint sounds during recovery from musculoskeletal injury, and to then use the characteristics of such sounds as a biomarker for quantifying joint rehabilitation progress. This paper focuses on the robust measurement of joint acoustic emissions using miniature microphones placed on the knee and interfaced to custom hardware. Two types of microphones were investigated: (1) miniature microphones with a sound port for detecting airborne sounds; and (2) piezoelectric film based contact microphones for detecting skin vibrations associated with internal sounds. Additionally, inertial measurements were taken simultaneously with joint sounds to observe the consistency in the acoustic emissions in the context of particular activities: knee flexion / extension (without load) and multi-joint weighted movement involving knee and hip flexion / extension (i.e. sit-to-stand). The preliminary data demonstrated that high quality joint sound measurements can be obtained with unique and repeatable acoustic signatures in healthy and injured joints. Additionally, the results suggest that combining piezoelectric contact microphones (which detect high quality acoustic emission signals directly from the skin vibrations but can be compromised with loss of skin contact) and electret microphones (which measure lower signal-to-noise ratio airborne sounds from the joint but can even measure such sounds at 5 cm distance from the skin) can provide robust measurements for a future wearable system to assess joint health in patients during rehabilitation at home.

Quantitative model of hematologic and plasma volume responses after ascent and acclimation to moderate to high altitudes

Despite decades of research, the magnitude and time course of hematologic and plasma volume (PV) changes following rapid ascent and acclimation to various altitudes are not precisely described. To develop a quantitative model, we utilized a comprehensive database and general linear mixed models to analyze 1,055 hemoglobin ([Hb]) and hematocrit (Hct) measurements collected at sea level and repeated time points at various altitudes in 393 unacclimatized men (n = 270) and women (n = 123) who spent between 2 h and 7 days at 2,500-4,500 m under well-controlled and standardized experimental conditions. The PV change (ΔPV) was calculated from [Hb] and Hct measurements during a time period when erythrocyte volume is stable. The results are 1) ΔPV decreases rapidly (~6%) after the 1st day at 2,500 m and [Hb] and Hct values increase by 0.5 g/dl and 1.5 points, respectively; 2) ΔPV decreases an additional 1%, and [Hb] and Hct increase an additional 0.1 g/dl and 0.2 points every 500-m increase in elevation above 2,500 m after the 1st day; 3) ΔPV continues to decrease over time at altitude, but the magnitude of this decrease and subsequent increase in [Hb] and Hct levels is dependent on elevation and sex; and 4) individuals with high initial levels of [Hb] and Hct and older individuals hemoconcentrate less at higher elevations. This study provides the first quantitative delineation of ΔPV and hematological responses during the first week of exposure over a wide range of altitudes and demonstrates that absolute altitude and time at altitude, as well as initial hematologic status, sex, and age impact the response.

Inflammation: sustaining the balance to optimize recovery of skeletal muscle, connective tissue, and exertional injuries.

inflammation has been described as part of the healing process since Hippocrates in ancient Greece ([5][1]). Inflammatory responses are initiated immediately after tissue damage to protect the organism, but a hyperactive inflammatory response may attack healthy tissues. In such situations, anti-

Impact of skin temperature and hydration on plasma volume responses during exercise

Heat stress and hydration may both alter plasma volume (PV) responses during acute exercise; potential interactions have not been fully studied. The purpose of this study was to determine the effect of graded elevations in skin temperature (Tsk) on PV changes during steady-state exercise under conditions of euhydration (EU) and hypohydration (HYPO, -4% of body mass). Thirty-two men (22 ± 4 yr) were divided into four cohorts (n = 8 each) and completed EU and HYPO trials in one environment [ambient temperature (Ta) 10, 20, 30, and 40°C]. Thirty minutes of cycle ergometry (50% V̇o2peak) was performed. Core (Tre) and mean skin (Tsk) temperatures were measured; changes in PV, total circulating protein (TCP), and mean arterial pressure (MAP) were calculated; and skin blood flow (SkBF) was estimated. Hypohydration decreased (P < 0.05) PV by 200 ml (-5.7%) but did not alter TCP. Plasma loss was not different between EU and HYPO during exercise at any Ta. Plasma losses were greater (P < 0.05) with elevated Ta with an average -130, -174, -294, and -445 ml losses during the 10, 20, 30, and 40°C trials, respectively. Significant (P < 0.05) correlations (r = 0.50 to 0.84) were found between ΔTCP and ΔPV during exercise when Tsk was cool/warm (<33°C; Ta 10, 20, and 30°C), but not at 40°C (high Tsk). We conclude that 1) graded skin warming proportionally accentuated plasma loss; 2) plasma loss was associated with plasma protein efflux at lower Tsk and SkBF; 3) at high Tsk, additional plasma loss likely results from increased net filtration at the capillaries; and 4) HYPO did not alter vascular fluid loss during exercise in any environment.

Wearable Technology for Compensatory Reserve to Sense Hypovolemia

Traditional monitoring technologies fail to provide accurate or early indications of hypovolemia-mediated extremis because physiological systems (as measured by vital signs) effectively compensate until circulatory failure occurs. Hypovolemia is the most life-threatening physiological condition associated with circulatory shock in hemorrhage or sepsis, and it impairs ones ability to sustain physical exertion during heat stress. This review focuses on the physiology underlying the development of a novel noninvasive wearable technology that allows for real-time evaluation of the cardiovascular systems ability to compensate to hypovolemia, or its compensatory reserve, which provides an individualized estimate of impending circulatory collapse. Compensatory reserve is assessed by real-time changes (sampled millions of times per second) in specific features (hundreds of features) of arterial waveform analog signals that can be obtained from photoplethysmography using machine learning and feature extraction techniques. Extensive experimental evidence employing acute reductions in central blood volume (using lower-body negative pressure, blood withdrawal, heat stress, dehydration) demonstrate that compensatory reserve provides the best indicator for early and accurate assessment for compromises in blood pressure, tissue perfusion, and oxygenation in resting human subjects. Engineering challenges exist for the development of a ruggedized wearable system that can measure signals from multiple sites, improve signal-to-noise ratios, be customized for use in austere conditions (e.g., battlefield, patient transport), and be worn during strenuous physical activity.