Waterproof, electronics-enabled, epidermal microfluidic devices for sweat collection, biomarker analysis, and thermography in aquatic settings.

Noninvasive, in situ biochemical monitoring of physiological status, via the use of sweat, could enable new forms of health care diagnostics and personalized hydration strategies. Recent advances in sweat collection and sensing technologies offer powerful capabilities, but they are not effective for use in extreme situations such as aquatic or arid environments, because of unique challenges in eliminating interference/contamination from surrounding water, maintaining robust adhesion in the presence of viscous drag forces and/or vigorous motion, and preventing evaporation of collected sweat. This paper introduces materials and designs for waterproof, epidermal, microfluidic and electronic systems that adhere to the skin to enable capture, storage, and analysis of sweat, even while fully underwater. Field trials demonstrate the ability of these devices to collect quantitative in situ measurements of local sweat chloride concentration, local sweat loss (and sweat rate), and skin temperature during vigorous physical activity in controlled, indoor conditions and in open-ocean swimming.

Non-invasive estimation of hydration status changes through tear fluid osmolarity during exercise and post-exercise rehydration.

PURPOSE: To determine if tear fluid osmolarity (Tosm) can track changes in hydration status during exercise and post-exercise rehydration. METHODS: Nineteen male athletes (18-37 years, 74.6 ± 7.9 kg) completed two randomized, counterbalanced trials; cycling (~95 min) with water intake to replace fluid losses or water restriction to progressively dehydrate to 3 % body mass loss (BML). After exercise, subjects drank water to maintain body mass (water intake trials) or progressively rehydrate to pre-exercise body mass (water restriction trials) over a 90-min recovery period. Plasma osmolality (Posm) and Tosm measurements (mean of right and left eyes) were taken pre-exercise, during rest periods between exercise bouts corresponding to 1, 2, and 3 % BML, and rehydration at 2, 1, and 0 % BML. RESULTS: During exercise mean (± SD) Tosm was significantly higher in water restriction vs. water intake trials at 1 % BML (299 ± 9 vs. 293 ± 9 mmol/L), 2 % BML (301 ± 9 vs. 294 ± 9 mmol/L), and 3 % BML (302 ± 9 vs. 292 ± 8 mmol/L). Mean Tosm progressively decreased during post-exercise rehydration and was not different between trials at 1 % BML (291 ± 8 vs. 290 ± 7 mmol/L) and 0 % BML (288 ± 7 vs. 289 ± 8 mmol/L). Mean Tosm tracked changes in hydration status similar to that of mean Posm; however, the individual responses in Tosm to water restriction and water intake was considerably more variable than that of Posm. CONCLUSION: Tosm is a valid indicator of changes in hydration status when looking at the group mean; however, large differences among subjects in the Tosm response to hydration changes limit its validity for individual recommendations.

Impact of polyphenol antioxidants on cycling performance and cardiovascular function.

This investigation sought to determine if supplementation with polyphenol antioxidant (PA) improves exercise performance in the heat (31.5 °C, 55% RH) by altering the cardiovascular and thermoregulatory responses to exercise. Twelve endurance trained athletes ingested PA or placebo (PLAC) for 7 days. Consecutive days of exercise testing were performed at the end of the supplementation periods. Cardiovascular and thermoregulatory measures were made during exercise. Performance, as measured by a 10 min time trial (TT) following 50 min of moderate intensity cycling, was not different between treatments (PLAC: 292 ± 33 W and PA: 279 ± 38 W, p = 0.12). Gross efficiency, blood lactate, maximal neuromuscular power, and ratings of perceived exertion were also not different between treatments. Similarly, performance on the second day of testing, as assessed by time to fatigue at maximal oxygen consumption, was not different between treatments (PLAC; 377 ± 117 s vs. PA; 364 ± 128 s, p = 0.61). Cardiovascular and thermoregulatory responses to exercise were not different between treatments on either day of exercise testing. Polyphenol antioxidant supplementation had no impact on exercise performance and did not alter the cardiovascular or thermoregulatory responses to exercise in the heat.

Interaction of hyperthermia and heart rate on stroke volume during prolonged exercise.

People who become hyperthermic during exercise display large increases in heart rate (HR) and reductions in stroke volume (SV). It is not clear if the reduction in SV is due primarily to hyperthermia or if it is a secondary effect of an elevation in HR reducing ventricular filling. In the present study, the upward drift of HR during prolonged exercise was prevented by a very small dose of the ß1-adrenoreceptor blocker (atenolol; ßB), thus allowing SV to be compared at a given HR during normothermia and hyperthermia. Eleven men cycled for 60 min at 57% of peak O2 uptake after receiving placebo control (PL) or a low dose (0.2 mg/kg) of ßB. Hyperthermia was induced by reducing heat dissipation during exercise. Four experimental conditions were studied: normothermia-PL, normothermia-ßB, hyperthermia-PL, and hyperthermia-ßB. Hyperthermia increased skin and core temperature by 4.3 degrees C and 0.8 degrees C (P<0.01), respectively. ßB prevented HR elevation with hyperthermia: HR values were similar at minute 60 during normothermia-PL and hyperthermia-ßB (155±11 and 154±13 beats/min, respectively, P=0.82). However, SV was increased by 7% during the final 20 min of exercise during hyperthermia-ßB compared with normothermia-PL (treatment×time interaction, P=0.03). In conclusion, when matched for HR, mild hyperthermia increased SV during exercise. Furthermore, the reduction in SV throughout prolonged exercise under normothermic and mildly hyperthermic conditions appears to be due to the increase in HR.

Serum sodium concentration changes are related to fluid balance and sweat sodium loss.

PURPOSE: This study determined if changes in serum sodium concentration are related to fluid balance as well as sweat sodium losses in triathletes competing in the Hawaii Ironman triathlon. METHODS: Endurance trained athletes (N = 46, age = 24-67 yr) were studied during 30 min of stationary cycling at 70%-75% of HRmax in a warm outdoor laboratory (26.4 degrees C +/- 1.7 degrees C wet bulb globe temperature [WBGT], 28.3 degrees C +/- 1.2 degrees C dry bulb [DB]) 3-7 d before race day. Sweat sodium concentration was measured from absorbent patches on the forearm and scapula, and sweating rate was derived from changes in body mass. Before and after the race, serum sodium concentration, body mass, and nutritional intake during the race were also measured (N = 46). Sweating and race day comparisons and changes in serum sodium concentration were analyzed via Student's t-test, correlation, and multiple regression. RESULTS: In men, the change in serum sodium concentration during the race was correlated with relative sweating rate (mL.kg.h; r = -0.49, P = 0.012), rate of sweat sodium loss (mEq.kg.h; r = -0.44, P = 0.023), and body mass change (kg; r = -0.54, P = 0.005). Together, the rate of sweat sodium loss and body mass change accounted for 46% of the change in serum sodium concentration in men (R = 0.46). In women, body mass change alone was significantly correlated with the change in serum sodium concentration (r = 0.31). The rate of sodium intake (mEq.kg.h) was related to the rate of sweat sodium loss in women (mEq.kg.h; r = 0.64, P = 0.035) but not in men (r = 0.27, P = 0.486). CONCLUSION: Changes in serum sodium concentration during an ultraendurance triathlon are significantly related to interactions of fluid balance, sweat sodium loss, and sodium ingestion.

Maximal mechanical power during a taper in elite swimmers.

INTRODUCTION: Insight regarding the fluctuations in neuromuscular function among athletes during a taper is lacking. PURPOSE: This study examined the time course of changes in maximal mechanical power (Pmax), torque at power maximum (T), velocity at power maximum (V), and swim performance (m x s(-1)) that occur during the taper. METHODS: Using an arm ergometer with inertial loading, measurements were made during the week prior to the initiation of the taper (high volume, HV), during the 2- to 3-wk period of the taper (taper), and during the week of peak competition (peak) in 24 male competitive collegiate swimmers. Subjects were divided into groups that tapered to peak performance at either the conference (CONF, N = 13) or national (NAT, N = 11) championship competitions. RESULTS: CONF increased Pmax 10.2% (P < 0.01) and swim performance 4.4% (P < 0.001). NAT increased Pmax by 11.6% (P < 0.01), T by 7.4% (P < 0.02), and swim performance by 4.7% (P < 0.001). Pmax displayed a biphasic increase with approximately 50, 5, and 45% of the total increase occurring during the first, second, and third weeks of the taper, respectively. The biphasic response was the most common response among individual swimmers. Swimming performance was significantly correlated to both power and torque (P < 0.05). CONCLUSION: In summary, maximal arm power measured using inertial load ergometry increased largely during the first and third weeks after training volume was tapered for peak performance in elite collegiate swimmers.