Background Precooling has been shown to enhance performance in repeated sprint exercise in able-bodied subjects in a hot environment. Spinal cord injury causes thermoregulatory impairment with a detrimental effect on performance. This study assessed whether cooling strategies before and during exercise in the heat enhances sprint performance in athletes with tetraplegia.
Methods Eight male athletes with tetraplegia performed intermittent arm crank exercise in the heat (32.0°C (0.1°C); humidity, 50% (0.1%)) for a maximum of 60 min or until exhaustion. Trials involved a no-cooling control (CON), precooling (PRE) or cooling during exercise (DUR). Each intermittent sprint protocol consisted of varied periods of passive rest, maximal sprinting and active recovery.
Results Both PRE and DUR cooling strategies improved the ability of the athletes to repeatedly perform high-intensity sprints, with times to exhaustion (TTE), whereas during the CON trial, athletes demonstrated a reduction in the total number of sprints and TTE (47.2 (10.8), 52.8 (5.8) and 36.2 (9.6) min for CON, PRE and DUR, respectively). Core temperature was significantly higher for CON (37.3°C (0.3°C)) when compared with both PRE and DUR (36.5°C (0.6°C) and 37.0°C (0.5°C), respectively, p<0.01). Ratings of perceived exertion and thermal sensation upon exhaustion or completion were not different.
Conclusions Athletes with tetraplegia should use a precooling or during-exercise cooling strategy specific to the characteristics of their sport when exercising in hot conditions.
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There are a variety of considerations for athletes with disabilities exercising in the heat including the thermoregulatory impairment that accompanies a spinal cord injury (SCI).1 The extent of the thermoregulatory impairment relates to the level of the SCI with only reflex spinal sweating occurring below the level of the spinal lesion.2 The most susceptible athletes are those with tetraplegia.3
Any strategies that delay or reduce the increase in body temperature during exercise may enhance performance in the heat. The concept of reducing skin or core temperature before the endurance competition has been investigated since the 1980s.4 Lower skin temperatures enable a greater temperature gradient for dissipating heat from deeper regions of the body.5 Cooler skin temperatures also mean that less of the total cardiac output is directed towards the skin, possibly allowing more blood to be directed to active skeletal muscle. Lower skin and core temperatures can also delay the onset of sweating and decrease sweat rate, resulting in a conservation of body water during a prolonged competitive event thus aiding performance.5 A recent model of fatigue proposes that activity and exhaustion is controlled by the brain as part of a pacing strategy integrating both internal sensory information and external signals from the environment.6 Athletes with a SCI have an impaired sensory input,7 and this may influence their ability to “pace” their activity appropriately and result in reduced performance.
Studies of able-bodied athletes using precooling have shown increases in performance in endurance and power sports, for example, rowing, but few researchers have looked at intermittent sprint exercise.8 In the Paralympic Games, the majority of quadriplegic athletes are within the sports of wheelchair rugby and wheelchair tennis where intermittent sprints are required. A study of non-elite participants with tetraplegia and paraplegia using foot cooling during exercise attenuated the rise of tympanic temperature in subjects performing continuous arm-cranking at 66% of V02 peak for 45 min.9 However, the device used would not be practical during a competition, and the exercise was not representative of repeated sprint activity. Cooling vests have shown to reduce thermal strain in athletes with tetraplegia when used during a 20-min precooling period and during exercise.10 The purpose of this study was to assess whether precooling or cooling during exercise enhanced repeated sprinting performance in the heat of athletes with tetraplegia.
Eight male trained wheelchair athletes with tetraplegia (age 28.9 (3.0) years; body mass, 71.6 (10.8) kg; C5/C6-C6/C7, two incomplete lesions), volunteered to participate in this study (table 1). All participants gave written informed consent before any involvement. Approval for the study procedures was obtained from the University Research Ethics Committee. All subjects were members of the national wheelchair tennis (n = 4) and rugby (n = 4) squads.
Exercise testing was performed on a modified cycle ergometer (Monark, Ergomedic 620, Varberg, Sweden) adapted for upper body exercise. Power-measuring cranks (SRM, Walldorf, Germany) were fitted to the ergometer to record power output (W) and work done (J) continuously at a sampling rate of 0.5 Hz.
During the first visit to the laboratory, all participants completed a force velocity test and an incremental arm-crank test to volitional exhaustion as described previously.10 The force velocity test was performed to determine the optimal resistance for generation of peak power output (PPO) in each sprint, whereas the incremental exercise test was performed to determine peak oxygen consumption (Vo2peak; l min−1) and peak aerobic power. The remaining three visits involved an intermittent sprint protocol (ISP) performed inside an environmental chamber (32.0°C (0.3°C) and 50.0% (0.3%) relative humidity) on separate days, at least 24 h apart, with two different cooling procedures and a no-cooling control (CON) condition. Trials were conducted in a randomised order and consisted of a repeated measures design where subjects served as their own controls.
Before baseline measures being recorded in all conditions, subjects rested for 15 min in temperate conditions (20.0°C (0.3°C) and 45.0% (0.3%) relative humidity). For the no-cooling CON condition, subjects remained resting in the same conditions for a further 20 min before entering the environmental chamber. For the precooling condition (PRE), after the baseline period, subjects wore an ice vest (Arctic Heat Products, Burleigh Heads, Queensland, Australia) for 20 min, after which subjects removed the vest and entered the environmental chamber. The ice vest was frozen in a similar manner to that reported previously.10 For the cooling during exercise (DUR) condition, once baseline measures had been recorded, subjects rested for 20 min as in CON before putting on the ice vest and immediately entering the environmental chamber. Before all testing protocols, a standardised 7-min warm-up was completed.10 The ISP involved up to thirty 2-min periods consisting of 10 s of passive rest, a 5-s maximal sprint from a stationary start against the optimal resistance from the force velocity test, followed by 105 s of active recovery at 35% Vo2peak. No fluid intake was permitted during any of the testing. Including baseline and precooling/non-cooling periods, exercise trials consequently lasted a maximum of 95 min.
Subjects ingested a telemetry pill (Tcore; HQ, Palmetto, Florida) for the measurement of core body temperature, 8.0 (2.3) h before each testing condition. Body mass in minimal clothing was determined before and after testing for an indication of non-urine fluid loss. Surface thermistors (Grant Instruments, Cambridge, UK) were positioned for measures of skin temperature at standard positions on the chest, upper arm, thigh and calf. Values were recorded by a Grant Squirrel meter logger (1000 Series; Grant Instruments). As specific mean skin temperature (MST) formulae have not been developed for individuals with SCI. MST was estimated using the formula of Ramanathan.11 Although MST has been shown to mask local skin temperature differences at rest in cool conditions,12 increases in skin temperature, at the sites measured in this study, have been noted below the level of lesion for athletes with tetraplegia during exercise in warm conditions.3 As all sites used in the Ramanathan formula are likely to demonstrate increases in temperature in this population, the increase in thermal strain contributing to heat storage should be reflected via MST data. Heart rate was continually monitored (Polar Sports Tester; Polar Electro, Kempele, Finland). Subjective measures for rating of perceived exertion13 and thermal sensation14 were also recorded. Baseline measures were recorded after a 15 min period to allow stabilisation of thermistors. During the 20 min precooling manoeuvre and time-matched period for the CON and DUR conditions, all variables were recorded at 2 min intervals. For logistical reasons, during the ISP, all variables were recorded at 1 min of each 2 min exercise block to gauge the responses during the active recovery section of the protocol. PPO was recorded as the highest single value during each 5 s sprint. On completion of each ISP, when subjects felt they could no longer continue or the safety limit of a high core temperature was reached (39.3°C or a 2°C increase from rest), the trial was terminated.
Paired data from each ISP were analysed using a two-way analysis of variance (ANOVA) with repeated measures (condition × time interaction). TTE and total work done during the sprints were analyzed with one-way ANOVA. Because of the differences in TTE, paired comparisons between conditions were made for 28 min (14 sprints), as all subjects completed this duration of exercise. Where TTE differed between conditions, pairwise comparisons were made for as long as possible. Where significance was obtained, Tukey's HSD post hoc test was undertaken. To establish the relationship among core temperature and indicators of exhaustion, two-tailed Pearson correlations were calculated. Significance was accepted at the p<0.05 level. All data were analyzed using a standard statistical package (SPSS V.1 1.0) and are reported as mean (SD).
The precooling manoeuvre reduced Tcore from rest (36.6°C (0.3°C)) by 0.3°C (0.2°C)) (p<0.05) and MST by 1.7°C (1.1°C) (p<0.01).
In CON, all subjects completed at least 14 sprints (28 min), with only two subjects completing the full exercise duration. In PRE, four of the subjects managed to complete the full duration, with all subjects completing 16 sprints (32 min). All subjects in DUR were able to sprint longer than the other conditions, completing 22 sprints (44 min). Two subjects were stopped for safety reasons of elevated core temperature during this trial, whereas all other non-completions were because of voluntarily self-withdrawal from the sprints. Mean exercise duration was, therefore, improved by both PRE and DUR when compared with CON (p<0.05), as shown in table 2.
PPO for the first 14 sprints (28 min) of the ISP was found to be similar for CON and PRE but higher for DUR (main effect, table 2; p<0.01). There were significant reductions in PPO across time during both the CON and the PRE conditions (fig 1; p<0.01). No significant reduction in PPO was present throughout the ISP for DUR, although at minutes 1, 3 and 7 (sprints 1, 2 and 4), PPO was significantly lower than CON and PRE (p<0.05; fig 1). Total work done was greater in PRE (2963 (52) J) when compared with CON (2664 (52) J) and greater during DUR (3790 (46) J) when compared with both PRE and CON (p<0.05).
Heart rate increased from minute 1 by minute 13 (p<0.05) during the ISP but was not different across conditions or at exhaustion (table 2). Change in body mass was not different among CON (0.15 (0.3) kg), PRE (0.01 (0.9) kg) and DUR (0.05 (0.1) kg) (p = 0.71).
A main effect for the trial was observed for ratings of perceived exertion that were lower during PRE (12.7 (1.0)) and DUR (12.4 (1.0)) than that in CON (13.8 (1.7); main effect for trial; p<0.01). During the ISP, significant increases in RPE were seen within 11 min for CON, 13 min for PRE and 15 min for DUR (p<0.01). Although final values for RPE and thermal sensation at exhaustion/completion of the protocol were not significantly different, values approached significance (RPE, p = 0.07; thermal sensation, p = 0.09) (table 2).
For the DUR condition, the main effect for the trial observed for thermal sensation was lower (5.0 (0.3)) than that in CON (6.0 (0.3), main effect; p<0.05) but no different from PRE (5.5 (0.3)). When compared with values recorded during the first minute of the ISP, thermal sensation was elevated for the CON and the PRE conditions (5.0 (0.5) and 4.5 (0.7), respectively) at 13 min (6.0 (0.5) and 5.5 (0.9), respectively, p<0.01) and minute 15 (5.3 (1.0)) for DUR (4.4 (0.9), p<0.01).
A main effect for trial (p<0.01) was observed for core temperature with a higher value for CON (37.3°C (0.3°C)) when compared with both PRE (36.5°C (0.6°C)) and DUR (37.0°C (0.5°C)) and also between PRE and DUR. There was a significant increase in core temperature throughout the ISP by minute 7 in DUR (36.8°C (0.4°C)) and minute 9 for CON and PRE (37.1°C (0.3°C) and 36.4°C (0.5°C), respectively, p<0.01; fig 2). Table 2 shows that performance, physiological and perceptual responses at volitional exhaustion/end of the ISP core temperature were not different between conditions. Table 3 shows selected correlations between core temperature and indicators of exhaustion. There was a significant negative correlation between core temperature and PPO and work done per sprint in all conditions to varying levels of significance.
Mean skin temperature
No main effects were observed for MST between CON (34.0°C (0.3°C)) and PRE (33.4°C (0.3°C)), but DUR was lower than CON (32.2°C (0.6°C); main effect; p<0.05). In all conditions, skin temperature increased and remained elevated throughout the ISP (p<0.01). MST for PRE and DUR were lower than CON (p<0.01) with DUR also lower than PRE (p<0.01) during the ISP (fig 3). Final MST values upon exhaustion or completion of the ISP were not different (table 2).
The aim of this study was to establish whether PRE or DUR in the heat enhanced repeated sprinting performance in athletes with tetraplegia. It was evident that both PRE and DUR exercise cooling strategies prolonged the ability of the athletes to repeatedly perform high-intensity sprints when compared with no cooling, however, for different reasons. Both PRE and DUR cooling strategies improved total exercise capacity (total work done) when compared with CON, with subjects completing, on average, six and eight more sprints, respectively.
A number of studies have examined hyperthermia and exhaustion during prolonged continuous exercise15 16 and also during prolonged periods of repeated sprinting.8 17 Trained subjects can reach volitional exhaustion at core (oesophageal) temperatures of 40°C15, which may alter cerebral activity and perceptions of effort because of reduced cerebral blood flow.16 Recently, the effect of elevated core temperature in repeated sprint performance was examined8 suggesting that after 40 min of repeated sprints, impairments in performance were not related to metabolic factors but to the influence of high core temperature on central nervous system function. At exhaustion in the present study, similar Tcore values were observed between trials (∼39.0°C), which are lower than those reported for able-bodied subjects.8 Although Tcore measures are lower than other core temperature measurements at rest and during exercise (0.6–0.9°C),17 Saboisky et al18 observed that exhaustion during exercise in the heat occurred at 38.8°C (0.2°C). It, therefore, seems that a critical core temperature of 40°C15 may not be necessary for volitional exhaustion in athletes with tetraplegia.
Furthermore, MST at exhaustion was similar between trials (34°C) but lower than those tolerated by able-bodied subjects (36°C)19 during the 30-min arm crank ergometry in hot conditions. It is possible that with the reduced afferent information available to the thermoregulatory centres due to SCI, the temperature perceived as tolerable may be reduced.
The relationship between core temperature and MST appear to have reflected the specific cooling strategy used. For example, PRE resulted in both reduced core temperature and MST before exercise when compared with the other trials. However, on the removal of the cold stimulus, MST increased at a greater rate than the other trials because of the greater thermal gradient between the skin and the environment, whereas core temperature increased at a similar rate to CON, but at a lower absolute value. PRE, therefore, provided a greater potential for heat storage. Conversely, responses for DUR demonstrated a reduced rate of increase in core temperature during exercise; as a result, the ice vest cooled the skin temperature and possibly insulated the torso from environmental heat gains.
Although high correlations were generally observed between Tcore and PPO during exercise, no relationships were observed between thermoregulatory parameters at exhaustion and TTE. These results would suggest that Tcore is important for development of power-related variables during exercise but not for duration of exercise in athletes with tetraplegia. However, as noted previously, both Tcore and MST values were similar at exhaustion between trials. Furthermore, for the DUR trial where PPO of the initial sprints was lower, the total duration of exercise and total work done were greater. Consequently, the reduction in peak power from the first to the last sprint for CON and PRE was ∼13 to 15%, respectively, whereas for DUR, it was ∼3%. It is possible that the ability to exercise longer was, therefore, related to the lower power outputs achieved in the initial sprints. The decreased PPO for the initial sprints for DUR will not have stressed the metabolic processes within the muscle to as great an extent as for CON and PRE enabling better recovery between sprints.20 Less microdamage and/or cellular disturbance in the muscle tissue is, therefore, also likely to have occurred and, consequently, less muscle fatigue, contributing to a greater duration of exercise being completed. This may be more evident for athletes with tetraplegia because of the reduced active muscle mass when compared with that of the able-bodied. Therefore, a cooling technique such as DUR may be more appropriate when the initial power output is not crucial to the overall performance (eg, endurance events) when compared with those events where the duration is not prolonged but maintenance of peak power produce may be (eg, team sports). In the latter instance, DUR may be a more appropriate strategy.
Where the exercise duration is concerned, the DUR protocol, although not reducing body temperature to the level achieved during precooling, did offset the rate of gain in body temperature. A reduced rate of body temperature increase may have contributed to the lower perceptions of effort and thermal strain observed, resulting in an improved time to exhaustion. In addition, the athletes all commented upon the coldness of the ice vest when it was first worn. This may also have affected their initial sprint performance from a psychological or perceived comfort perspective and thus not eliciting as maximal an effort as for PRE and CON, consequently resulting in less metabolic disturbance as outlined above. The factors relating to initial power production may, therefore, be thermal in nature, but not those at the end of exercise. In support of this, the correlation between PPO and core temperature for DUR, although significant, was not as great as for CON and PRE and may reflect the more “acute” thermal comfort effects of donning the ice vest before exercise rather than removing it.
What is already known on this topic
Precooling has been shown to enhance performance in repeated sprint exercise in able-bodied subjects in a hot environment.
No study has examined the performance gains of either precooling or cooling in quadriplegic athletes.
What this study adds
Exercise durations were improved by both cooling strategies, but the peak physiological responses (PPO) are the same, which implies that the rate of heat gain is a predominant factor in determining exercise duration.
Tetraplegic athletes performing intermittent sprint exercise in hot conditions should use PRE or, if practical, DUR to improve performance.
There may be individual variability in the thermoregulatory responses according to the completeness and level of the spinal lesion. It is relevant to note that of the two subjects completing the ISP during CON, one had an incomplete lesion; of the four subjects completing PRE, two had incomplete lesions; and the only subject completing DUR had an incomplete lesion. Physiologically incomplete lesions could result in subjects having a greater amount of sensory information regarding thermal state and a greater surface area available for sweating, if present. However, the subjects with incomplete spinal lesion in the present study did not show any greater sweat losses (based on changes in body mass and generally within the error of the measurement) than those with complete spinal lesions, nor were their peak aerobic or anaerobic power outputs greater than the remainder of the group, suggesting no differences in heat production during exercise. It is, therefore, difficult to determine whether an incomplete lesion is beneficial to exercise capacity under the conditions studied.
An important point to note is that two subjects were withdrawn from the study because of observed increases in core temperature reaching the safety cut-off point. As these athletes could presumably have continued exercising voluntarily, for at least a short time, the importance of thermal perception should be considered. If subjects cannot perceive the increase in body temperature because of a large surface area of insensate skin, the chance that they may continue exercising when already hyperthermic, and increase the risk of heat injury, may exist. How this could be overcome practically in the absence of whole-body thermal perception is an interesting research question.
In conclusion, this study has shown that PRE and DUR strategies can improve the duration of repeated sprinting capacity in athletes with tetraplegia but for different reasons. It is recommended that athletes with tetraplegia performing intermittent sprint exercise in hot conditions should use PRE or DUR to improve performance, but the specific technique should match the power output characteristics of the event.
Competing interests None.