This study has identified that completing a passive warming strategy with the use of heated garments between an active warm-up and competition can improve subsequent performance in the form of increased PPO (ES = 0.54 [95% CI 0.17 to 0.91]. From the studies included, improvements have been demonstrated in aerobic and anaerobic type sports such as rowing [14], cycling [9, 20] and swimming [13]. Improvements have also been shown in the activities of vertical jumping [5, 21] and repeated sprint performance [5, 10, 21]. The included literature suggests that maintaining an elevated muscle temperature is only beneficial in the early stages of exercise, however, it should be noted that having a much greater start in a time trial, does not necessarily mean subsequent exercise performance will be improved due to elevated afferent feedback [24]. However, despite a consensus on the effect of heated garments, the degree of improvement varied widely, from less than 1 to over an 11% improvement, however, the studies included have exhibited sufficient quality as evidenced through extensive critique. The use of multiple variations of active and passive warm-up protocols, the wide variation in results may be attributable to the specifics of the warm-up practices employed, post-warm-up, transition period strategies and the exercise itself. This emphasises the need for continued research to determine which methods of warm-up are best for a given sport or activity.
Passive warm-up strategies and exercise performance
Asmussen and Boje [25] and Bergh and Ekblom [26] reported that the velocity-dependent effect of muscle temperature on maximum external power to be ~ 4% higher in force and power per 1 °C rise in quadriceps muscle temperature. Furthermore, per 1 °C increase in muscle temperature, observed a 2–5% [27], and a 2–10% [28] increase in peak power during exercise performance.
A passive warming strategy, unlike a continuous active warm-up, during a transition period between an active warm-up and exercise performance allows a rise in muscle and/or core temperature without the reduction of energetic substrates [9, 20]. Initial studies emphasising passive warming strategies were solely laboratory based, with the method of increasing body temperature being accomplished by methods, such as hot baths/showers. Although impactful, these passive warm-up strategies are not often practical in a sporting scenario. Therefore, alternate methodologies of passive warming have been sought after, given that: (1) a lengthy period is expected (transition phase), between the end of the warm-up and the beginning of an event; (2) muscle temperature starts to fall immediately following exercise termination; and (3) substantial declines in body temperature occurs as early as ~ 15–25 min post-exercise [4, 5].
External heating garments and blizzard survival jackets and performance
All of the included studies in this review used either blizzard survival jackets or external heating garments, the majority of heated garments used across various sporting activities have heat filaments in the fabric. Faulkner et al. [9] reported an improvement in muscle temperature (1 °C rise in muscle temperature at a depth of 1 cm and a 0.4 °C rise at a 3 cm depth) and ~ 9% improvement in relative and PPO during a sprint cycling task when using an active warm-up and heated tracksuit pants in the 30-min transition period in comparison to standard tracksuit pants [9]. Additionally, further research by Faulkner and colleagues reported that muscle temperature remained increased when wearing heated tracksuit pants solely during the transition period (36.9 ± 0.3 °C) and when worn throughout the active warm-up and transition phase (37.0 ± 0.2 °C) in contrast to only an active warm-up (36.6 ± 0.3 °C) [20]. Although, an additional performance benefit was not found when wearing the heated tracksuit pants during an active warm-up as well as during the transition phase [20]. Supporting this, Cook et al. reported that wearing a blizzard survival jacket produces a rise in tympanic temperature and improved a 20 m sled sprint performance [10]. Additionally, Kilduff et al. reported an improved repeated sprint performance in elite rugby players when an active warm-up was followed by wearing a blizzard survival jacket throughout a 15-min transition period [21]. The decrease in core temperature during the transition phase was minimised when the blizzard survival jackets were worn (− 0.19 ± 0.08 °C) compared to the standardised tracksuit top (− 0.55 ± 0.10 °C). Therefore, participants began the tests with an elevated core temperature, suggesting that an increased core temperature prior to exercise can improve exercise performance [21].
The use of passive warming strategies are not common practice, however, the application of maintaining body and muscle temperature during a transition period is gaining recognition. Passive heat maintenance through the wearing of blizzard survival jackets and athletic heating garments appears to be an optimal technique in offsetting the reduction in core and/or muscle temperature and therefore improving exercise performance. However, athletic heating garments can have their limitations. Wired heated garments, do not provide uniform heat across the heating elements. Furthermore, for optimal heat transfer and increase in muscle temperature, the heating elements should be in direct contact with the skin, which involves the garment being tight to the skin [29]. Additionally, Faulkner et al. reported a significant decrease in muscle temperature during the inactivity period when the heated garments were worn, however, the decline in muscle temperature was significantly less compared to the application of standard tracksuit bottoms [9]. Furthermore, Raccuglia and colleagues demonstrated by using water-perfused heated trousers, heated to a higher temperature, 43 °C, successfully maintained and even increased muscle temperature in the passive recovery period following an active warm-up [30]. However, water perfused trousers are not very practical for use in a competition setting as they need to be connected to a heating system, consisting of a temperature-controlled water bath and powered water pump. Alternatively, numerous studies have reported a significant improvement in exercise performance following the application of battery-powered athletic heating garments which use integrated flexible heating elements [9, 13, 14, 20]. This allows consumers to use the garments portably, however without multiple batteries, for a limited time period.
All the reviewed literature found a significant improvement in exercise performance (P < 0.05); particularly time trial performances, which have shown to display a wide variety of percentage increases. When examining these in more detail, the type of exercises might influence the overall percentage improvement. For example, Cowper et al. exhibited a performance increase of 1.1%, this slight increase in performance, might be because of the competition duration being long (> 5 min) in comparison to the other studies which are all predominantly anaerobic and short-lasting in nature [14]. Limited studies have determined the physiological outcomes of passive warming for long-duration performances. This might be because a well-known limiting physiological factor for long-duration performances is excessive bodily heat [31, 32].
A rise in core temperature before exercise might be detrimental to long duration performance due to impaired thermoregulatory mechanisms [33, 34] and/or a decrease in heat storage capacity [35]. This increase in body temperature, may create a greater dependence on evaporative heat loss, thus an increase of sweating. Therefore, an individual may incur sub-optimal hydration over a prolonged period and subsequently hinder exercise performance [36]. Furthermore, González-Alonso et al. [33] found a detriment in time till exhaustion performance in hot environmental conditions (40 °C) when comparing 36 (CON), and 40 °C (HEAT) pre-exercise water immersions for 30 min. Observing that a reduction in performance was attributed to increases in heart rate and reductions in stroke volume paralleled the rise in core temperature [33].
When a heated jacket is utilised following an active warm-up in cool environments, body temperature would be comparatively lower than if the same protocol was applied in standard ambient temperatures (18–20 °C) [37]. During colder environments, a delayed duration that the body takes to reach critical core temperature would occur and performance might improve. Alternatively, in standard ambient conditions, the use of a heated jacket may elevate the core temperature to critical levels and possibly decrease the capacity for exercise performance [14]. Due to the paucity of work in this area, the papers included in this systematic review have differing ambient temperatures. It is known that a change in ambient temperature can significantly alter the thermoregulatory profile at rest [38] and affect the ability to exercise [39]. As such, the ambient temperature is an important factor to consider when using interventions during the passive warming period. Therefore, further research is needed to determine the effect on performance following the use of passive heating garments in below ambient temperatures.
Limitations
Several methodological problems in the studies reviewed could have impacted the outcomes reported. Some of the studies did not measure temperature change post active warm-up, therefore it might be difficult to distinguish whether the change in bodily temperature was due to the active warm-up or the passive heating garment. Furthermore, chosen studies in this systematic review and meta-analysis displayed small sample sizes, this led to many of the confidence intervals crossing the ‘‘zero point’’. While the conclusions from this review are based on mean data, it is important to state that even though none of the included studies displayed an absolute improvement in performance for passive heating when examining both the means and confidence intervals, no studies showed a mean or absolute decrease in performance. This demonstrates that passive heating during the period between active warm-up and performance has no effect on performance. There are certain weaknesses in this review, relatively small number of participants in some of the studies which increases the potential effects of chance. Furthermore, the limited number of studies included in the systematic review and meta-analysis and the studies that were included, some authors were unable to provide raw data to fully complete analyses.
The findings of this systematic review and meta-analysis offer a partial but best available evaluation of the influence of passive heating techniques prior to sport and exercise performance. This review aimed to eliminate possible sources of bias by employing a systematic review method, however, this does not ensure the absence of bias. Furthermore, a modified version of the PEDro scale was applied to distinguish between the quality of different studies. The modified PEDro scale has not likely to have biased our decisions since points are only granted to studies when the criteria are clearly fulfilled. Furthermore, after a precise reading of the included research articles, if it was not evident that the criterion was reported, a point was not presented for that specific criterion.