The application of localized cold or cryotherapy is commonly used for the treatment of acute soft-tissue injuries (5). Such treatments are proposed to reduce the inflammatory response in injured tissue and reduce local edema and pain, thereby promoting recovery from injury (24). In addition to soft-tissue injuries, inflammation is also integral to the etiology of exercise-induced muscle damage (31). As such, whole limb cooling via cold-water immersion (CWI) is frequently applied by elite athletes during the early recovery period from strenuous exercise to alleviate some of the physiological and functional deficits associated with exercise-induced muscle damage (2,16,33). Although the precise mechanisms underpinning enhanced recovery with CWI are not currently known, they may be mediated in part, through marked reductions in skeletal muscle blood flow (12,23), which in turn reduces edema and the induction of inflammatory events (21,32).
Despite alleviating exercise-induced muscle damage and augmenting recovery of muscle function, CWI-induced reductions in postexercise muscle blood flow may have potential detrimental effects on other components that are integral to recovery. In this regard, the restoration of muscle glycogen represents a crucial component in the recovery of exercise capacity (4). Much of the literature to date has therefore focused on optimizing the timing (17), type (7), and amount (36) of exogenous CHO provision so as to maximize muscle glycogen resynthesis rates. However, despite ensuring adequate CHO availability, postexercise reductions in muscle blood perfusion may also limit muscle glucose uptake (27), and hence muscle glycogen resynthesis during short-term recovery (10).
Glucose uptake by skeletal muscle remains elevated after exercise, particularly in the initial stages (∼60 min) when the highest rates of muscle glycogen resynthesis are observed and glucose uptake is insulin independent (28). This coincides with a sustained elevation in skeletal muscle blood flow (13,29) that is proportional to the intensity and duration of the prior exercise (30,35). At rest, increases in muscle blood flow enhance glucose uptake independent of insulin effects (3,9). In contrast, few studies have investigated the influence of changes in postexercise muscle blood flow on muscle glucose uptake while in the fed state. Nevertheless, limited data suggest that postexercise (60 min cycling at 60% V˙O2max) reductions in skeletal muscle blood flow (achieved via local histamine H1- and H2-receptor blockade) reduces muscle interstitial glucose concentration during a 2-h postexercise recovery period despite the provision of approximately 100 g of exogenous CHO at 60 min postexercise (27). Such data have obvious implications for the restoration of muscle glycogen concentration, although these authors did not measure glycogen resynthesis. Furthermore, the associated problems with reduced postexercise muscle blood flow may become particularly pertinent when the exercise has induced considerable glycogen depletion and CHO availability is limited in the recovery period, conditions that are often more representative of the challenges facing the elite athlete (8).
With this in mind, the aim of the present study was to therefore test the hypothesis that postexercise CWI (considering the associated reductions in skeletal muscle blood flow) attenuates muscle glycogen resynthesis. To this end, we measured rates of muscle glycogen resynthesis in the vastus lateralis muscle during a 4-h recovery period from an exhaustive exercise bout that posed considerable glycogen depletion. Importantly, we used a postexercise CHO feeding protocol of only 0.6 g·kg−1 body mass per hour, rates of CHO provision, which are known not to induce maximal muscle glycogen resynthesis but may be more representative of actual practices undertaken by elite athletes (8). On the basis of previous data from our laboratory demonstrating marked reductions in postexercise muscle blood flow after CWI (23), we used an immersion protocol consisting of 10 min of whole lower-limb immersion at a water temperature of 8°C.
METHODS
Subjects.
Nine recreationally active subjects (age = 24.8 ± 4.5 yr, height = 176.2 ± 5.5 cm, body mass = 73.7 ± 9.4 kg, V˙O2peak = 55 ± 6 mL·kg−1·min−1, peak power output [PPO] = 280 ± 23 W; mean ± SD) volunteered to participate in this study. The experimental procedures and potential risks associated with the study were explained, and subjects gave written informed consent before participation. Subjects refrained from additional exercise outside of the study requirements as well as from alcohol and caffeine intake for at least 48 h before any of the testing sessions. None of the subjects had history of neurological disease or musculoskeletal abnormality, and none of the subjects were under pharmacological treatment during the study. The study was approved by the Ethics Committee of Liverpool John Moores University.
Experimental protocol.
In a repeated-measures and randomized crossover design (at least 7–10 d occurred between trials), participants reported to the laboratory on the evening before the main experimental trial to perform a glycogen depleting bout of intermittent exhaustive cycling. At the cessation of exercise, participants were provided with a low-CHO meal ( <50 g) to be consumed within a 30-min period. The following morning, participants arrived at the laboratory in a fasted state and performed a steady-state cycling protocol to volitional fatigue. Upon the completion of this ride, subjects either underwent 10 min of CWI or seated rest at room temperature (CONT). After the recovery intervention period, exogenous CHO was then provided at a rate of 0.6 g·kg−1 body mass per hour. Thermoregulatory measures (rectal, muscle, and skin) and venous blood samples were obtained preexercise, immediately postexercise, and at 1, 2, and 4 h postexercise. Muscle biopsies were obtained from the vastus lateralis immediately postexercise and at 1, 2, and 4 h after exercise.
Preliminary testing.
During their first visit to the laboratory, participants performed a maximal incremental cycling test to volitional fatigue on a cycling ergometer (Ergo-bike 8000 TRS; Daum electronic GmbH, Fürth, Germany) to determine V˙O2peak and PPO. The incremental test protocol commenced at a workload equivalent to 3 W·kg−1 body mass, after which work rate was increased by 50 W after the first 150 s and by 25 W every 150 s thereafter until exhaustion (14). Oxygen uptake was measured continuously during exercise using an online gas analysis system (Metamax, Cortex, Germany). The V˙O2peak was stated as being achieved by the following end point criteria: 1) HR within 10 beats·min−1 of age-predicted maximum, 2) respiratory exchange ratio >1.1, and 3) plateau of oxygen consumption despite increased workload. PPO was defined as the highest power output that was able to be maintained for 60 s. The ergometer saddle and the handle bar position were recorded for each participant during preliminary testing and replicated during the experimental trials.
Evening glycogen depletion protocol.
Participants arrived at the laboratory on the evening before each main experimental trial (∼1900 h) and performed an intermittent bout of cycling to volitional fatigue. After a 5-min warm-up at 100 W, participants cycled for 2 min at 90% PPO, followed immediately by a 2-min recovery period at 50% PPO. They repeated this work to rest ratio until they could no longer complete 2 min cycling at 90% PPO, determined as an inability to maintain a cadence of 60 rpm. At this point, exercise intensity was lowered to 80% PPO, and when participants could no longer maintain this intensity, it was lowered to 70% and finally 60% PPO with the same work to rest ratio. When the participants were unable to cycle for 2 min at 60% PPO, the exercise protocol was terminated. This intermittent pattern of exercise was used so as to induce glycogen depletion in both type I and type II fibers (20). HR was measured continuously throughout exercise (Polar S610i, Kempele, Finland) and ratings of perceived exertion (RPE) were recorded at 2-min intervals during exercise (6). Water intake was consumed ad libitum throughout exercise in the initial trial and was recorded and repeated for the subsequent trial. Throughout exercise, subjects received no information in relation to time elapsed, power outputs, or number of stages completed. After the completion of the evening glycogen depletion protocol, subjects were provided with a low-CHO diet (∼0.6 g·kg−1 body mass; 332.80 kcal, 48% CHO, 43% fat, 9% protein) for their evening meal. Subjects were also supplied with a food diary to record their daily intake for 48 h before the day of this initial depletion ride and were asked to replicate this pattern of food intake for their remaining trial. The purpose of this initial glycogen depletion protocol was to ensure subjects reported to the laboratory on the morning of the main experimental trial in a glycogen-depleted state.
Experimental trials.
On the following morning and after a 10- to 12-h overnight fast, subjects arrived at the laboratory at ∼0700 h. Subjects voided and nude body mass was measured, a rectal probe was inserted, skin thermistors were attached, and an HR monitor was positioned across the chest. A venous cannula (BD P100 kit; Becton, Dickinson, England) was inserted into an antecubital vein, and after obtaining a resting blood sample, the cannula was then flushed with ∼2 mL of sterile saline (Kays Medical Supplies, Liverpool, UK) to keep the cannula patent and sterile, a procedure that was completed after each subsequent blood draw. After assessment of baseline measurements, subjects commenced a steady-state cycling protocol at 70% V˙O2peak. This protocol was maintained until the subject could no longer maintain a cadence of 60 rpm for 15 s. At this point, the workload was lowered to 60% V˙O2peak and finally 50% V˙O2peak. Exercise was terminated when the subject could no longer maintain a cadence of 60 rpm for 15 s at 50% V˙O2peak. At the point of exhaustion, subjects were immediately laid in a supine position, and a first muscle biopsy was taken from the vastus lateralis muscle. At 15 min after the completion of exercise, subjects were transferred to a water immersion facility wearing only training shorts to undergo the CWI protocol (10-min immersion of the lower limbs in 8°C) or remained seated in a semireclined position for the CONT trial. In the CWI trial, subjects were raised in a semireclined position using an electronic hoist (Bianca; Arjo Ltd., Gloucester, UK) and lowered into the water tank (ECB, Gloucester, UK) until the thighs were fully submerged for a duration of 10 min. After immersion, subjects were immediately raised from the water using the electronic hoist and remained seated in a semireclined position for 4 h under normal laboratory temperatures. During the first hour of recovery, subjects wore a dry T-shirt; however, they remained dressed in their wet shorts with wet limbs. During the final 3 h of recovery, the limbs were towel dried and the subjects were moved to a dry area within the laboratory. Rectal temperature, thigh, and calf skin temperature and HR (Polar Electro) were continuously monitored during exercise and the subsequent 4-h recovery period. Muscle biopsies were also taken at 1, 2, and 4 h after the cessation of exercise while muscle temperature and blood samples were taken immediately postexercise, postimmersion, and at 1, 2, and 4 h after exercise. During the 4-h recovery period, subjects consumed CHO at a rate of 0.6 g·kg−1 body mass in the form of sports drinks and CHO-containing bars. CHO was consumed immediately after immersion (30 min postexercise) and again at 1, 2, and 3 h postexercise. All trials were conducted under normal laboratory ambient temperatures (∼20°C).
Assessment of thermoregulatory responses.
A rectal probe (Krondalve 9 DK-2610; ELLAB A/S, Roedovre, Denmark) was inserted 15 cm beyond the external anal sphincter for the assessment of rectal temperature. A skin thermistor was attached to the upper thigh for the assessment of skin temperature. Muscle temperature was assessed using a needle thermistor inserted into the vastus lateralis (13050; ELLAB). Thigh skinfold thickness was measured using Harpenden skinfold calipers (HSK BI, Batty International, Burgess Hill, England) and divided by 2 to determine the thickness of the thigh subcutaneous fat layer over each participants vastus lateralis (11). The needle thermistor was then placed at a depth of 3 cm plus one-half of the skinfold measurement for determination of deep muscle temperature (3 cm). The thermistor was then withdrawn at 1 cm increments for determination of muscle temperature at 2 and 1 cm below the subcutaneous fat layer. Rectal temperature, skin temperature, and muscle temperature were recorded using an electronic measuring system (CTF 9004; ELLAB).
Blood analysis.
Blood samples were collected into serum separation tubes or vacutainers containing ethylenediaminetetraacetic acid and stored on ice or room temperature (serum tubes for 1 h) until centrifugation at 1500g for 15 min at 4°C. After centrifugation, aliquots of serum and plasma were stored at −80°C for later analysis. Samples were analyzed for serum glucose, NEFA, and glycerol concentration using commercially available kits (Randox Laboratories, Antrim, UK). Serum samples were also analyzed for insulin using a solid phase enzyme-linked immunosorbent assay (ELISA; Demeditec Diagnostics GmbH, Germany). Plasma epinephrine and norepinephrine concentrations were measured using liquid chromatography–tandem mass spectrometry (26). All samples were analyzed in duplicate.
Muscle biopsies.
Muscle biopsies were obtained from separate incision sites (2–3 cm apart) from the lateral portion of the vastus lateralis muscle using a Bard Monopty Disposable Core Biopsy Instrument 12 gauge × 10 cm length (Bard Biopsy Systems, Tempe, AZ). Samples were obtained (approximately 20 mg) under local anesthesia (0.5% marcaine) and immediately frozen in liquid nitrogen and stored at –80°C for later analysis.
Muscle glycogen.
Approximately 3–6 mg of freeze-dried sample was powdered, dissected free of all visible nonmuscle tissue, and subsequently hydrolyzed by incubation in 500 μL of 1 M HCl for 3–4 h at 100°C. After cooling to room temperature, samples were neutralized by the addition of 250 μL of 0.12 mol·L−1 Tris/2.1 mol·L−1 KOH saturated with KCl. After centrifugation, 150 μL of the supernatant was analyzed in duplicate for glucose concentration according to the hexokinase method using a commercially available kit (GLUC-HK; Randox Laboratories). Glycogen concentration is expressed as millimoles per kilogram of dry weight, and intra-assay coefficients of variation was <5%.
Statistical analysis.
Statistical analysis was conducted using the Statistical Package for the Social Sciences (version 15; SPSS Inc., Chicago, IL). A two-factor (condition × time) within-participants general linear model was undertaken to determine any treatment differences between the CONT and the CWI conditions. The assumption of sphericity (homogeneity of covariance) was assessed and corrected for using the Huynh–Feldt epsilon (1). Because there were only two levels in the main effect of condition, follow-up multiple comparisons were not necessary. A significant effect of time was followed up with planned multiple contrasts in line with the a priori hypotheses. Therefore, data at the specific time points were compared using the Newman–Keuls multiple contrasts. Where a significant interaction between condition and time was observed, differences between conditions were examined at each time point using Newman–Keuls multiple contrasts. Exercise time during the evening and morning exercises bouts and the mean HR and RPE response to the morning bout was compared using a paired samples t-test. All data in text, figures, and tables are presented as means ± SD, with P values ≤ 0.05 indicating statistical significance.
RESULTS
Exercise capacity, physiological responses, and muscle glycogen depletion in response to the glycogen depleting exercise protocols.
Exercise time to exhaustion values for the glycogen depletion protocol for both the evening (CONT = 82 ± 13 min, CWI = 83 ± 13 min, P = 0.870) and morning rides (CONT = 58 ± 10 min, CWI = 59 ± 7 min, P = 0.857) were similar between conditions. Mean HR (CONT = 160 ± 12 beats·min−1, CWI = 159 ± 10 beats·min−1, P = 0.819) and RPE (CONT = 18 ± 1, CWI = 18 ± 1 beats·min−1, P = 0.237) during the morning exercise bout were also similar between conditions. Furthermore, postexercise muscle glycogen concentration (i.e., before beginning the CONT or CWI recovery interventions) was also similar between trials (CONT = 76 ± 43 mmol·kg−1 dw, CWI = 78 ± 26 mmol·kg−1 dw). Taken together, these data demonstrate that the physiological and metabolic stress induced by the depletion protocols was comparable between trials.
Thermoregulatory and physiological responses to the immersion protocol and recovery period.
Rectal and thigh skin temperature after exercise and the immersion protocol are shown in Figure 1. Rectal temperature declined after exercise (P < 0.01) with the greatest decrements observed between the cessation of exercise and immediately postimmersion (see Fig. 1A). However, there was no difference between conditions at any time point throughout the recovery period (P = 0.19). In contrast, thigh skin temperature was lower (P < 0.01) in CWI compared with CONT (see Fig. 1B) immediately postimmersion. Differences in skin temperature between CWI and CONT (P < 0.01) remained at 1 and 2 h postexercise such that a significant interaction effect occurred (P < 0.01).
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FIGURE 1: Rectal temperature (A) and thigh skin temperature (B) immediately after exercise and during 4 h of recovery in CONT and CWI (n = 9, mean ± SD). A main effect for time was found for rectal temperature (P < 0.01). A main effect for condition and time was found along with a significant interaction between condition and time for skin temperature (P < 0.01). *Significant difference from postexercise (P < 0.01). +Significant difference between conditions (P < 0.01).
Postexercise muscle temperature was similar between conditions at a probe depth of 3 cm (CONT = 37.8°C ± 0.5°C, CWI = 37.6°C ± 0.4°C, P = 0.34), 2 cm (CONT = 37.4°C ± 0.1°C, CWI = 37.1°C ± 0.5°C, P = 0.26) and 1 cm (CONT = 35.7°C ± 1.3°C, CWI = 35.6°C ± 1.0°C, P = 0.61). Muscle temperature was reduced over time (P < 0.01; Fig. 2), decreasing immediately after immersion (P < 0.01) with further decreases generally observed after +1 h (P < 0.01). This reduction was dependent on probe depth (P < 0.01). At the deeper depths, greater declines in muscle temperature were observed after + 1 h (Figs. 2A and 2B). At a probe depth of 1 cm, the greatest decline in muscle temperature was observed immediately after immersion (Fig. 2A).
FIGURE 2: Muscle temperature immediately after exercise and during 4 h of recovery in CONT and CWI at temperature probe depths of 3 cm (A), 2 cm (B), and 1 cm (C) (n = 9, mean ± SD). Main effects for condition (P < 0.01) and time (P < 0.01) were found along with a significant interaction between condition, time, and probe depth (P < 0.01). *Significant difference from postexercise (P < 0.05). +Significant difference between conditions (P < 0.05).
The reduction in muscle temperature over time was also dependent on condition (P < 0.01). Tm was generally lower in CWI compared with CONT with the greatest difference generally observed at after +1 h; Fig. 2). These differences were in turn dependent on probe depth (P < 0.01). At the deepest depths, differences between conditions were least; however, muscle temperature was lower in CWI at both 2-cm (Fig. 2B) and 3-cm (Fig. 2A) depths (P < 0.01). At a depth of 1 cm (Fig. 2C), the greatest difference in muscle temperature occurred immediately after immersion (P < 0.01).
Plasma/serum metabolite and hormonal responses to exercise and during the recovery period.
Plasma/serum metabolites and hormonal responses to exercise and recovery are shown in Table 1. Exercise increased serum lactate, glycerol, and NEFA (P < 0.01), although the magnitude of change was not different between CONT and CWI (P = 0.50, 0.75, and 0.51, respectively). There was no difference between conditions for lactate (P = 0.16), NEFA (P = 0.51), or glycerol (P = 0.60) at any time point during the recovery period. Exercise-induced decreases and CHO-induced increases in serum insulin concentration during the recovery period (P < 0.01) were also not different between CONT and CWI at any time point (P = 0.170). In contrast, changes in blood glucose concentrations during recovery (P < 0.01) were significantly different between trials (P = 0.04). Although there was no difference in glucose concentration at baseline, postexercise, or 1 h postexercise, glucose was significantly lower in CWI at 2 and 4 h postexercise compared with CONT (P < 0.01).
TABLE 1: Serum glucose, lactate, NEFA, glycerol and insulin, and plasma epinephrine and norepinephrine at baseline, immediately after exercise, and during 4 h of recovery in the CONT and CWI conditions (n = 9, mean ± SD).
Exercise increased plasma epinephrine (P < 0.01) to a similar extent between conditions, and there was no difference between CONT and CWI at any time point during the recovery period (P = 0.11). A comparable increase in plasma norepinephrine was observed under both conditions after exercise (P < 0.01). However, the change in plasma norepinephrine during the recovery period was significantly different between conditions (P = 0.03), with a greater increase observed in the CWI condition compared with CONT (P < 0.01; Table 1)
Muscle glycogen resynthesis.
Postexercise muscle glycogen concentrations were comparable between conditions (∼75 mmol·kg−1 dw). There was a progressive increase in glycogen concentration during the 4-h recovery period such that each time point was significantly higher (P < 0.01) than the preceding time point (see Fig. 3). However, rates of glycogen resynthesis did not differ between conditions during the recovery period (P = 0.719), such that there was no difference in absolute glycogen concentration between CONT and CWI at any time point (P = 0.724). Total glycogen synthesis during recovery equated to 83 ± 43 mmol·kg−1 dw in CONT and 79 ± 58 mmol·kg−1 dw in CWI, thereby resulting in comparable synthesis rates of ∼20 mmol·kg−1 dw per hour.
FIGURE 3: Skeletal muscle glycogen content immediately after exercise and during 4 h of recovery in CONT and CWI (n = 9, mean ± SD). A main effect for time was observed (P < 0.01). *Significant difference from postexercise (P < 0.05). (a) Significant difference versus 1 h (P < 0.05). (b) Significant difference versus 2 h (P < 0.05).
DISCUSSION
The aim of the present study was to test the hypothesis that postexercise CWI attenuates muscle glycogen resynthesis during short-term recovery as mediated via associated reductions in skeletal muscle blood flow. We provide novel data by demonstrating that postexercise CWI does not attenuate muscle glycogen resynthesis during a 4-h recovery period from exhaustive exercise, even when CHO is consumed at suboptimal rates of 0.6 g·kg−1 body mass per hour. As such, we consider that athletes who incorporate CWI as a recovery strategy to alleviate symptoms of exercise-induced muscle damage therefore should not be concerned with potential negative effects of the associated reductions in muscle blood flow on the restoration of muscle glycogen stores.
Our rationale for the present study was based on recent observations that reductions in skeletal muscle blood flow (achieved via local histamine H1- and H2-receptor blockade) reduce muscle interstitial glucose concentration during a 2-h postexercise recovery period despite the provision of approximately 100 g of exogenous CHO at 60 min postexercise (27). Accordingly, a reduction in muscle glucose uptake could lead to reductions in muscle glycogen resynthesis during the early recovery period when glycogen synthesis is insulin independent. To test this hypothesis during conditions that more closely resemble training and nutritional practices of elite endurance athletes, we used an experimental protocol that induced considerable glycogen depletion but also deliberately incorporated a CHO feeding strategy that is considered suboptimal to promote maximal rates of muscle glycogen resynthesis. Our CWI immersion protocol consisted of 10 min of whole lower-limb immersion at a water temperature of 8°C, a protocol that has previously been shown in our laboratory to induce ∼50% reductions in femoral artery blood flow up to 30 min postexercise (23). Nevertheless, despite creating conditions that could compromise glycogen resynthesis (i.e., reduced blood flow and low-CHO availability), we observed CWI to have no detrimental effect on glycogen resynthesis. Indeed, net glycogen synthesis during the 4-h recovery period was 83 ± 43 mmol·kg−1 dw in CONT and 79 ± 58 mmol·kg−1 dw in CWI, thereby equating to comparable resynthesis rates of ∼20 mmol·kg−1 dw per hour. Such rates of glycogen resynthesis are comparable with other studies in the literature in which similar feeding protocols to that used in the present study have been used (19).
A limitation of the present study surrounds the failure to directly measure postexercise limb blood flow. However, we recently reported that the CWI strategy used in the present investigation markedly reduces postexercise whole limb blood flow (∼50%) compared with rest (23). Furthermore, this reduction in limb blood flow remains for up to 30 min postexercise. Consequently, because both the level of thermal strain (rectal, skin, and muscle temperature) attained after exercise and the magnitude of change in the thermoregulatory responses during the early stages of recovery after CWI are similar to our recent work, it is likely that similar reductions in limb blood flow will have occurred in the present investigation. At the skin temperatures presently observed, nonnoxious thermoreceptors will be activated provoking a profound reflex increase in sympathetic nerve activity (15). Indeed, plasma norepinephrine concentrations were significantly elevated during the recovery period in the CWI condition compared with CONT. The magnitude of this increase relates closely to previous observations reported in both the arms and the legs in response to the cold pressor test (18,22). Although alterations in sympathetic nerve activity associated with cooling may not necessarily translate into limb blood flow changes, this increased activity combined with our previous observations on blood flow changes suggests that the application of CWI in the present study is likely to have mediated marked reductions in limb blood flow.
The observation of similar muscle glycogen resynthesis rates in CWI and CONT conditions suggests that CWI induced reductions in blood flow do not limit the availability of blood glucose to skeletal muscle. Alternatively, there may have been an increase in glucose extraction in the CWI trial, so as to compensate for reductions in muscle blood flow and related glucose availability with postexercise CWI. Possible supporting evidence for this hypothesis is provided from the examination of our plasma glucose data throughout the recovery period. Indeed, although there were no differences in plasma glucose concentration at baseline, postexercise and 1 h postexercise, glucose values were significantly lower at 2 and 4 h postexercise in CWI compared with CONT. Although we did not directly measure rates of glucose delivery, uptake, or any signaling parameter regulating glucose uptake and glycogen storage, there are numerous data sets that support a cold-induced potentiation of glucose uptake in skeletal muscle. For example, Oliveira et al. (25) observed in rodent muscle that cold exposure (4 d exposure to ambient temperatures of 4°C) increased PGC-1α content, GLUT4 membrane localization, and muscle glucose uptake through an insulin independent signaling pathway. Similarly, Wende et al. (37) also observed that overexpression of PGC-1α in mouse skeletal muscle increases GLUT4 content, muscle glucose uptake, and postexercise glycogen resynthesis. In this regard, it is noteworthy that we have recently observed in a separate study from our laboratory that postexercise CWI significantly increases PGC-1α messenger RNA levels at 3 h postexercise when compared with exercise per se (Hwa-Joo et al., unpublished observations). When taken together, such observations suggest that any potential limitations of reduced blood flow associated with CWI may be offset by increased glucose uptake that is regulated through a PGC-1α-dependent pathway. Further studies are now required to directly test this hypothesis in human skeletal muscle.
In addition to any effect of cooling on skeletal muscle glucose extraction, it has been frequently shown that cold induced elevations in metabolism enhances plasma glucose uptake and oxidation (34). However, estimates of metabolic heat production were not derived in the present investigation; consequently, the degree to which difference in metabolic rate may have influenced glucose update is difficult to elucidate. Despite reduced skin temperature after CWI, rectal temperature remained similar between conditions and overt shivering was not evident during the recovery phase. This suggests the degree of cooling associated with CWI may not have been sufficient to play a major role in mediating the differences in glucose concentrations presently observed.
In addition to altered blood flow, the physiological effects of localized cryotherapy are thought to be partially mediated through reductions in tissue temperature per se (21,32). Consequently, whole limb CWI may be an effective mode of cryotherapy by virtue of its effect on muscle temperature. The current findings demonstrate that whole limb CWI decreases superficial and deep muscle temperatures confirming previous observations at rest (12). Furthermore, greater reductions in superficial temperature were initially observed after immersion with deep muscle temperature declining to a greater extent 1 h after immersion. This transition from superficial to deep tissue cooling over time confirms reports after localized (11) and whole limb immersion (12) at rest. The increase in skin and superficial (1 cm depth) muscle temperature during the initial 60 min after water immersion partly reflects removal of the cooling source per se and subsequent exposure of the limbs to room air temperature. In addition, hemodynamic exchange between the cooler surface and the warmer deeper tissue is likely to have promoted a net flow of heat leading to a gradual increase in superficial tissue temperature and a corresponding decrease in deep tissue temperature over time (11).
In summary, the present data demonstrate that postexercise CWI does not attenuate muscle glycogen resynthesis rates during short-term recovery. Interestingly, such similar net glycogen synthesis rates may be due, in part, to a compensatory increase in muscle glucose uptake given that plasma glucose values were significantly lower in CWI at 2 and 4 h postexercise compared with CONT. Further studies incorporating direct assessments of glucose delivery and uptake as well as measurement of signaling proteins regulating glucose uptake and glycogen storage are now required to fully test this hypothesis. We consider the present data to have practical implications and suggest that athletes who incorporate CWI as a recovery strategy to alleviate symptoms of exercise-induced muscle damage should therefore not be concerned with potential negative effects of the associated reductions in muscle blood flow on the restoration of muscle glycogen stores.
Warren Gregson has received funding from ECB Cold Spas for the CWI facility.
The authors acknowledge the efforts of all subjects who took part in this study. They also express their thanks to ECB Cold Spas for supplying the CWI facility and to Dean Morrey for assistance with the metabolite analysis. The authors also thank Prof. Louise Burke for her insightful comments during the development of the study experimental design.
Robert Allan, Susan Holden, Padraic Phibbs, Dominic Doran, Iain Campbell, Sarah Waldron, Chang Hwa Joo, and James Morton have no conflicts of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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