Introduction
Athletes engage in resistance exercise across a season, to maintain and develop muscular hypertrophy, strength and power. Each individual resistance exercise session induces a systemic signalling response, which accrues over time following multiple repeated exposures. Athletes also often complete post-resistance exercise recovery strategies, e.g., water immersion, to promote recovery between training sessions. The hydrostatic pressure and temperature effects of water immersion alter the acute post-exercise physiological responses (Petersen and Fyfe
2021), by reducing acute muscle damage and swelling in athlete (Horgan et al.
2022a,
b,
c). Over time, these acute recovery responses may accumulate to accrue (promote) or mediate (diminish) the chronic systemic inflammatory signalling responses to resistance exercise, (Cornish et al.
2020).
Cold water immersion (CWI) involves the immersion of the body, i.e., single limb, partial- or whole-body (to neck, excluding head), in water temperatures ranging between 8 and 20 °C for a duration range of 5 to 30 min (Moore et al.
2022). The use of CWI throughout a taper period during competition preparation resulted in unclear (15 °C × 15 min, whole-body) (Halson et al.
2014) or positive (15 °C × 15 min, whole-body; 10 °C × 10 min, partial-body) (Halson et al.
2014; Tavares et al.
2020) performance responses, which may be associated with attenuated increases in post-exercise inflammation. If the practice of applying post-resistance exercise CWI (8 °C × 20 min, single-limb; 10 °C × 10 min, partial-body) occurs repeatedly over time, this may result in the accumulation of multiple acute reductions in myofibrillar protein synthesis (Fuchs et al.
2020a,
b) and the down-regulation of satellite cell activity and hypertrophy signalling pathways (Roberts et al.
2015). These findings have contributed to scepticism regarding repeated CWI application following resistance exercise in athletes.
Optimising anabolic hormonal status, i.e., testosterone, has also been shown to be favourable for increases in lean muscle mass and strength in healthy males (Bhasin et al.
2001). A single post-resistance exercise hot water immersion (HWI) session (39 °C × 15 min, whole-body) has been shown to result in small acute increases in testosterone concentration in athletes, as compared to a contrast water immersion strategy (Horgan et al.
2022b(in press)). Acute responses to post-resistance exercise HWI application also resulted in increased perceived sleep quality, while reducing next day perceptions of fatigue in athletes (Horgan et al.
2022b(in press)). As such, HWI may offer practitioners an alternative post-resistance exercise strategy to promote positive adaptive responses in athletes. Physiologically, HWI (~ 44 °C × 15–20 min, partial-body) increases peripheral muscular blood flow, as well as skin, muscle and core temperature (Bonde-Petersen et al.
1992). As highly trained athletes are reported to immerse themselves in hot water temperatures of ~ 38–42 °C (Rodrigues et al.
2020; Méline et al.
2021), it remains unclear if this strategy will stimulate positive adaptation (Fuchs et al.
2020a,
b; Stevens et al.
2020) when applied repeatedly over time. Hence, the cumulative effects of applying post-resistance exercise CWI versus HWI across multiple sessions on the medium- to long-term hormone responses in athletes warrants further investigation.
Compared to recreationally active subjects, highly trained athletes demonstrate different inflammatory responses to exercise (Damas et al.
2018). Increased training age and status is generally associated with lower exercise-induced muscle damage, e.g., creatine kinase (CK), myoglobin (Damas et al.
2018); adapted increases, e.g., interleukin (IL)-4, IL-6, IL-7, IL-8, IL-15, insulin-like growth factor (IGF)-1, heat shock protein (HSP)-72; or decreases, e.g., IL-1ra, IL-1b,(Dennis et al.
2004) tumour necrosis factor (TNF)-α in inflammatory cytokines and myokines (Cornish et al.
2020), growth factors (Kraemer et al.
2017), and temperature-dependent, muscle regulating HSP-72 (Fyfe et al.
2019). Research investigating repeated post-resistance exercise water immersion in athletes is ambiguous, with few studies highlighting the discrepancies between responses in trained versus untrained subjects (Fyfe et al.
2019). Many studies utilise ‘recreationally active’ subjects, from which results cannot be directly translated to athletes. In studies which utilised highly trained athletes, CWI application occurred following sport-specific training and not following resistance exercise (Ihsan et al.
2021).
To date researchers have not investigated the cumulative effects of repeated post-resistance exercise water immersion on the adaptive inflammatory responses in a highly trained athlete cohort (Roberts et al.
2015). Studies utilising athletes in high performance training settings which investigate the underlying inflammatory adaptive responses to repeated post-resistance exercise water immersion are essential to advance the understanding of the role of post-resistance exercise water immersion, and how it can be periodised across training and competition. This study aimed to determine the cumulative effects following a repeated post-resistance exercise (i.e., 4 weeks @ 2/week) CWI (15 °C × 15 min, whole-body) versus HWI (39 °C × 15 min, whole-body) strategy, compared to CON, on the systemic (blood) inflammatory and hormone responses following a 4-week intervention. We hypothesised that following repeated bouts of post-resistance exercise water immersion that: chronic hormone (testosterone, and testosterone to cortisol ratio) and growth factor (IGF-1, IGFBP-3 and PDGF-BB) responses would increase following all water immersion strategies (i.e., CWI or HWI); post-resistance exercise CWI would attenuate, and HWI would promote increases in blood markers of muscle damage (CK, myoglobin), pro-inflammatory cytokines (IL-1b, IL-6, IL-8, IL-17a and TNF-α) and heat-shock protein (HSP-72); and post-resistance exercise CWI would promote increases in anti-inflammatory cytokines (IL-1ra, IL-4 and IL-10), compared to HWI.
Discussion
During an in-season phase, this study investigated the effects of repeated post-resistance exercise CWI, HWI or CON on inflammatoryresponses in highly trained athletes. We found that repeated post-resistance exercise HWI resulted in moderate reductions in anti-inflammatory cytokine IL-1ra (p = 0.013), and muscle damage, i.e., CK (p < 0.001); while, HWI also resulted in trivial increases (p < 0.001–0.026) in growth factors PDGF-BB and IGF-1, as compared to CON and CWI. Conversely, we found that repeated post-resistance exercise CWI resulted in trivial reductions (p = 0.025) in muscle damage, i.e., CK, and small increases (p = 0.004) in heat shock protein HSP-72, as compared to HWI. This is one of the first studies to investigate the effects of repeated post-resistance exercise water immersion on an extensive array of inflammatory measures in highly trained athletes.
This study’s findings for muscle damage (i.e., CK) following repeated post-resistance exercise water immersion are consistent with previous research investigating acute responses following a single immersion (Horgan et al.
2022a,
b,
c), where irrespective of temperature (i.e., cold vs. hot) post-exercise water immersion resulted in trivial-to-moderate reductions in CK, compared to CON. Research to date has predominantly focused on acute effects of post-exercise water immersion. As such, this study is one of the first to investigate the effects of repeated post-resistance exercise water immersion on exercise-induced muscle damage using an all-athlete cohort (Tavares et al.
2018; Méline et al.
2021). The contribution from muscle damage to long-term hypertrophic adaptive responses to resistance exercise is believed to reduce in proportion with increased training status (Damas et al.
2018). Therefore, muscle damage in trained athletes can be considered as non-desirable symptomology associated with resistance exercise. These findings are favourable in terms of the proposed hydrostatic pressure and temperature effects of post-exercise water immersion, which reduces muscle damage biomarkers following resistance exercise. This study observed a large variation in CK, which may be partly explained by other non-modifiable factors, e.g., genetics, ethnicity, gender or age (Scalco et al.
2016). A limitation of the study is the single time-point for blood measures at the end of each 4-week block, given differences in plasma kinetics between biomarkers (Scalco et al.
2016). Future studies may consider examining the longer-term (i.e., > 4-week) pre- versus post-resistance exercise and intervention response of these markers.
Chronic, resting, serum IL-1ra anti-inflammatory cytokine levels were moderately lower (
p = 0.013) following post-exercise HWI, compared to CON. The chronic effects of resistance exercise on serum cytokines in highly trained athletes remains uncertain. Anti-inflammatory cytokines are thought to play a key role in the inflammatory cascade initiated by damaged muscle from resistance exercise (Methenitis et al.
2021). In our study, the CON group displayed significantly higher muscle damage (i.e., CK), which may be a reason for the elevated IL-1ra when blood testing occurred ~ 36-h after the last session. Similarly, elevated core temperature (~ 39.5 °C) in mice has previously been associated with increased circulating IL-1 levels, providing accelerated recovery and adaptive responses (Dinarello
2012). It should be noted that while there may be some similarities between mice and humans in terms of thermoregulation, there equally are as many challenges in directly comparing both due to the unique thermoregulatory systems of both species (Gordon
2017). For example, humans can take up to 115 min for the rectal temperature to reach 38.8 ± 0.14 °C while sitting passively in HWI at 42 °C (Rodrigues et al.
2020). Future research utilising athletes should investigate the systemic effects that post-exercise water immersion has on acute and chronic responses of genetic and intracellular RNA signalling using closed tube technology, which isolates intracellular RNA, as well as on immune cell subsets using mass cytometry.
Pro-inflammatory cytokines were either positively (i.e., IL-8, IL-17a) or negatively (i.e., TNF-α) associated with percent of body-fat, which was utilised as a covariate in the statistical analysis. This is consistent with findings by Aguiar et al. (
2020), where adiposity was positively correlated with pro-inflammatory cytokines in master athletes (Aguiar et al.
2020), with the unexpected exception of TNF-α. A limitation of this study was that we did not provide complete dietary standardisation across the study, with dietary supplementation shown to reduce eccentric muscle damage (Córdova-Martínez et al.
2021) or inflammation markers (e.g., TNF-α) in athletes (Willis et al.
2012). However, our randomised cross-over study design may reduce the effect of this limitation. Another consideration related to nutrition as a covariate for adaptive responses is whether the temperature effects of post-exercise water immersion may influence appetite. Future research should investigate the effects the temperature of post-resistance exercise water immersion may have on markers of appetite regulation.
Post-resistance exercise HWI resulted in trivial increases in IGF-1, compared to CWI or CON strategies, while post-resistance exercise HWI resulted in trivial increases in PDGF-BB, compared to CON. The authors are not aware of any research investigating post-resistance exercise CWI versus HWI in athletes on chronic systemic growth factor responses. Synergistic effects of IGF-1 and PDGF-BB have been reported to provide therapeutic anti-inflammatory osteoarthritic benefits (Montaseri et al.
2011). Collectively, the above results suggest CWI may lower, albeit with trivial effect (
g = 0.06–0.11), systemic circulating anabolic growth factors, i.e., IGF-1. Our finding is consistent with observations that post-resistance exercise CWI may attenuate the anabolic response in resistance trained males (Earp et al.
2019). In summary, synergistic effects in the systemic circulation of anabolic growth factors (i.e., IGF-1, PDGF-BB) accrue following repeated post-resistance exercise HWI application in athletes, and may contribute towards promoting an anabolic response in athletes, similar to previous studies utilising various forms of heating in combination with exercise stress to enhance adaptation and attenuate atrophy in human and rodent subjects (McGorm et al.
2018; McGorm
2019).
The immersion protocols in the current study reduced (~ 47%) or increased (~ 57%) thermal sensation following CWI or HWI, respectively. Our novel study utilising an all athlete participant cohort is the first to demonstrate small increases occurred in thermo-sensitive serum HSP-72 following repeated CWI, compared to HWI. Our findings are in contrast to evidence demonstrating no change in intramuscular HSP-72 following four weeks of repeated post-endurance exercise CWI (7-week @ 3-day/week, 15 min
\(\times\) 10 °C) following high-intensity interval cycling exercise in physically active participants (Aguiar et al.
2016), as well as evidence demonstrating that repeated post-resistance exercise CWI (4-week @ 3-day/week, 15 min
\(\times\) 10 °C) attenuated total intramuscular HSP-72 in recreationally active participants (Fyfe et al.
2019). This unexpected chronic increase in serum HSP-72 following CWI may be explained via a thermal shock-like response due to relative or absolute temperature changes following cooling and upon the homeostatic return to normothermia, i.e., re-warming following a cold shock (Neutelings et al.
2013). Similarly, HSP function and roles differ depending on the tissue of origin (i.e., intracellular versus extracellular), with environmental factors, and exercise mode, duration and intensity all influencing availability (Yamada et al.
2008). It is plausible that our findings are specific to extracellular serum HSP availability following post-resistance exercise CWI in athletes. Given the importance of HSPs in regulating exercise-induced muscular adaptation, this outcome may also partially explain our finding that post-resistance exercise CWI did not attenuate lean muscle mass in athletes. Similarly, reduced resting levels of serum HSP-72 have also been suggested as evidence of an improved inflammatory profile (Hoekstra et al.
2018), as well as evidence of improved heat adaptation via reduced HSP production in response to thermal stressors, e.g., HWI (Gibson et al.
2017).
Practical applications
Post-resistance exercise water immersion strategies, such as CWI, HWI, etc., may be used to provide trivial-to-moderate reductions in muscle damage (i.e., CK) following a 4-week in-season schedule in highly trained athletes. Water immersion strategies as utilised in this study, may be prescribed for highly trained athletes during an in-season competition phase, posing little risk of attenuating inflammatory adaptative responses. These results support the notion that water immersion may be periodised, i.e., the use of hot increases systemic growth factor circulation (i.e., PDGF-BB, IGF-1), while the use of cold increases thermo-sensitive systemic heat shock protein (i.e., HSP-72) circulation. Practitioners working with athletes should consider individual responses, e.g., as evident by mean ± 95% confidence interval ranges, to post-resistance exercise recovery strategies to identify each athlete’s ‘adaptive sweet spot’. Future study designs should consider the differences in inflammatory responses to exercise and hydrotherapy strategies for trained athletes versus recreationally active, non-athletes.
Acknowledgements
The authors would like to acknowledge that this research and work was carried out on the Traditional Lands of the Ngunnawal people, and pay our respect to Elders both past, present, and future. We would like to thank the subjects, as well as their strength and conditioning coach, Benjamin Norcott, for their time, dedication, and efforts during the study. The authors wish to thank the Systems Biology and Data Science Group at Griffith University for their assistance with analysing the immunology biomarkers. The assistance and support during data collection from Felicity Bright, Dr. James Broatch, Steven Hughes, Dr. Jessica Stephens, and Alice Wallett is also greatly acknowledged. The authors would also like to thank Dr. Marijke Welvaert and Dr. James Gilmore for their statistical advice. Finally, we would like to acknowledge the University of Canberra researcher development team for their assistance and support throughout this study.
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