Thermoregulation and marathon running: biological and environmental influences

The downed runner

Race coverage can be a rewarding experience for the sports medicine clinician. Several conditions are likely to present to the medical tent, and accurate diagnosis is critical to proper treatment. An algorithm approach as outlined in this article can provide a starting point for the assessment of the downed runner. Recognition of the primary causes for collapse can help to instigate the correct treatment approach. A proper history and physical examination often can help to differentiate significant cardiac events from the more innocuous EAC. Furthermore, avoiding immediate i.v. fluids in the downed runner is prudent, at least until an appropriate diagnosis is made. This will help to prevent iatrogenic hyponatremia. In sum, proper preparation and knowledge of the ailments that affect long distance runners will help to maintain an effective medical tent on race day.

Can firefighter instructors perform a simulated rescue after a live fire training exercise?

Two studies were undertaken to determine whether firefighter instructors are capable of performing a simulated rescue task after undertaking a live fire training exercise (LFTE) lasting approximately 40 min. In the first study, ten instructors performed two simulated rescue tasks in air at 19 degrees C, involving dragging an 81-kg dummy for 15 m along a corridor and down two flights of stairs. The first rescue acted as a control (Rcontrol) and was conducted when they were euhydrated and normothermic. The second task was undertaken 10.4 (3.3) min [mean (SD)] after a LFTE resulting in an average rectal temperature of 38.1 (0.4) degrees C (Rhot). All instructors were able to successfully complete Rcontrol and Rhot in 90.1 (28.6) s and 78.7 (15.6) s respectively. Heart rate (HR) and rating of perceived exertion (RPE) were higher after the LFTE [162 (16) beats min(-1) versus 180 (15) beats min(-1); and 13.3 (2.4) versus 15.7 (2.1), respectively, P<0.001]. In the second study, six instructors (one instructor participated twice giving seven trials) undertook a simulated rescue task in 16 degrees C involving dragging an 85-kg dummy along a flat surface 79 (65) s after a LFTE that increased rectal temperature to 38.3 (0.7) degrees C. On six occasions the instructor was able to successfully complete the full 30-m drag in 41.7 (6.9) s and one instructor dragged the dummy for 20 m before stopping through exhaustion. HR during the rescue task reached 173 (19) beats min(-1) and RPE was 16.3 (2.4). In conclusion, most of the instructors were able to perform a rescue task after the LFTE, however they were close to their physical limit.

The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment

AIM

The purposes of this study were to determine (i) the effects of different facing air velocities on body temperature and heat storage during exercise in hot environmental conditions and (ii) the effects of ingesting fluids at two different rates on thermoregulation during exercise in hot conditions with higher air velocities.

METHODS

On five occasions nine subjects cycled for 2 h at 33.0 +/- 0.4 degrees C with a relative humidity of 59 +/- 3%. Air velocity was maintained at 0.2 km h(-1) (0 WS), 9.9 +/- 0.3 km h(-1) (10 WS), 33.3 +/-2.2 km h(-1) (100 WS) and 50.1 +/- 3.2 km h(-1) (150 WS) while subjects replaced 58.8 +/- 6.8% of sweat losses. In the fifth condition, air velocity was maintained at 33.7 +/- 2.2 km h(-1) and subjects replaced 80.0 +/- 6.8% of sweat losses (100.80 WS).

RESULTS

Heat storage, body temperature and rating of perceived exertion were higher in 0 and 10 WS compared with all other conditions. There were no differences in any measured variable between 100 and 150 WS, or between 100 and 100.80 WS. Thus, the evaporative capacity of the environment is increased with higher air velocities, reducing heat storage and body temperature. At higher air velocities, a higher rate of fluid ingestion did not influence heat storage, body temperature or sweat rate.

CONCLUSION

The finding of previous laboratory studies showing a beneficial effect of high rates of fluid ingestion on thermoregulation during exercise in hot, humid, windstill conditions cannot be extrapolated to out-of-doors exercise in which facing air velocities are seldom lower than the athlete's rate of forward progression.

Weight changes, medical complications, and performance during an Ironman triathlon

BACKGROUND

Subjects exercising without fluid ingestion in desert heat terminated exercise when the total loss in body weight exceeded 7%. It is not known if athletes competing in cooler conditions with free access to fluid terminate exercise at similar levels of weight loss.

OBJECTIVES

To determine any associations between percentage weight losses during a 224 km Ironman triathlon, serum sodium concentrations and rectal temperatures after the race, and prevalence of medical diagnoses.

METHODS

Athletes competing in the 2000 and 2001 South African Ironman triathlon were weighed on the day of registration and again immediately before and immediately after the race. Blood pressure and serum sodium concentrations were measured at registration and immediately after the race. Rectal temperatures were also measured after the race, at which time all athletes were medically examined. Athletes were assigned to one of three groups according to percentage weight loss during the race.

RESULTS

Body weight was significantly (p<0.0001) reduced after the race in all three groups. Serum sodium concentrations were significantly (p<0.001) higher in athletes with the greatest percentage weight loss. Rectal temperatures were the same in all groups, with only a weak inverse association between temperature and percentage weight loss. There were no significant differences in diagnostic indices of high weight loss or incidence of medical diagnoses between groups.

CONCLUSIONS

Large changes in body weight during a triathlon were not associated with a greater prevalence of medical complications or higher rectal temperatures but were associated with higher serum sodium concentrations.

Running Performance Differences between Men and Women

More than a decade ago it was reported in the journal Nature that the slope of improvement in the men's and women's running records, extrapolated from mean running velocity plotted against historical time, would eventually result in a performance intersection of the sexes across a variety of running distances. The first of these intersections was to occur for 42 000 m before the 21st century. Most of the error in this prediction is probably explained by the linear mathematical treatment and extrapolation of limited performance data, since including world record-setting running performances for women before and after 1985 results in a non-linear data fit. The reality of early, disproportionate improvements in women's running that gave the appearance of an impending convergence with men is best explained by an historical social sports bias. Women's times have now reached a plateau similar to that observed for men at comparative performance milestones in the marathon. Sex differences at distances from 100 to 10 000 m show similar trends. The remaining sex gaps in performance appear biological in origin. Success in distance running and sprinting is determined largely by aerobic capacity and muscular strength, respectively. Because men possess a larger aerobic capacity and greater muscular strength, the gap in running performances between men and women is unlikely to narrow naturally.

Heat loss and continuous renal replacement therapy

Because of the devastating consequences of thermal imbalance, it is imperative that nurses understand these concepts and apply them to the daily care of their patients. Heat loss, heat conservation, and heat generation interplay to maintain the narrow range that is considered optimal for human cellular function. These concepts factor into patients who are critically ill but are especially important for patients undergoing continuous renal replacement therapy. Many of these types of dialysis expose the individual patient's blood to room temperature dialysate via an extracorporeal circuit 24-hours a day, sometimes for several weeks at a time. Critical care and advanced practice nurses must understand the interplay of the processes of heat loss, conservation, and heat generation to ensure patients undergoing this therapy achieve maximum benefit with the fewest complications possible.

Comparison of sweat loss estimates for women during prolonged high-intensity running

PURPOSE

This study evaluated the error produced by four commonly used field estimates and two prediction equations of total body sweat loss.

METHODS

Eight women distance runners were studied during a 30-km treadmill run (approximately 70% .VO(2max)) in a warm (30 degrees C T(db)) and a cool (14 degrees C T(db)) environment. Total sweat loss (TSL) was determined from changes in body mass corrected for fluid intake (FI), urine losses (UL), clothing (trapped sweat, TS), CO(2)-O(2) exchange (metabolic mass loss, MML), and respiratory water loss (RWL). TSL was compared with four estimates of sweat losses (often employed in the field) from body mass changes corrected for: a) FI only (F-1); b) FI and TS (F-2); c) FI and UL (F-3); or d) FI, TS, and UL (F-4). Two prediction equations were used also for comparison to TSL values.

RESULTS

In the warm environment, F-1, F-3, and F-4 accurately estimated (0.2-6.9%; P > 0.05) TSL, whereas F-2 produced a large error (15.3%; P < 0.05). In the cool environment, all four estimates produced large errors (14-41%; P < 0.05). Both prediction equations markedly underestimated (20-22%) TSL in the warm environment and underestimated (41%) or overestimated (20%) TSL in the cool environment.

CONCLUSION

TSL can be accurately estimated from changes in body mass using F-1, F-3, or F-4 methods in hot environments; however, none of the methods accurately estimated actual TSL values in a cool environment. Neither prediction equation provided accurate estimates of TSL in warm or cool conditions for women runners. These results illustrate the difficulty of accurately estimating and predicting sweat losses in the field.