After receiving ethical approval from the Northumbria University Faculty of Health and Life Sciences Ethics committee in accordance with the ethical standards established in the Declaration of Helsinki, fifteen male semi-professional soccer players from Level eight of the English Football League, gave written informed consent to participate in the study. Throughout the data collection period, four players sustained injuries which prevented them from completing the study, leaving eleven participants in total (three defenders, five midfielders, three attackers; 22 ± 1 years; stature 1.80 ± 0.10 m; mass 78 ± 8 kg). Players trained three to four times a week, in addition to at least one competitive match. The participants competitive season ran from August to May, with testing taking place in the mid-season phase of the players training year. Participants were required to refrain from physical activity and alcohol consumption for the duration of the study and in the 48 h prior to data collection and abstain from caffeine consumption for the 12 h prior to each experimental visit.

The study employed a randomized cross-over design to assess the effectiveness of PCM on recovery in the days following competitive soccer match-play. Participants visited the laboratory prior to commencement of the data collection period for habituation to the measurement tools employed in the study. For the experimental trials, participants were required to visit the laboratory prior to and 24, 48, and 72 h following two competitive soccer matches. The pre-match visit took place 24 h before the fixtures. On one occasion, players wore shorts fitted with PCM (Glacier Tek; USDA BioPreferred PureTemp, Plymouth, MN) that was either cooled (PCM_cold) or left ambient (PCM_amb), which served as a sham control. The order of the conditions was randomized using an online randomizer¹. Phase change material was applied to the quadriceps and hamstring muscle groups, and was worn for 3 h post-match. To ensure compliance with the intervention, away fixtures in which the team were required to travel back for ≥3 h were selected. The two fixtures were separated by 4–8 weeks. During each experimental visit, participants completed assessments of neuromuscular, physical, and perceptual function to ascertain the effect of PCM on recovery.

Voluntary activation was assessed through the interpolated twitch technique and was quantified by comparing the amplitude of the superimposed twitch force (SIT) with the potentiated twitch force (100 Hz) delivered 2 s following the MVC at rest using the following equation: Motor nerve VA (%) = [1 - (SIT/Q_tw,pot) × 100]. VA_TMS was assessed during two sets of contractions at 100, 87.5, 75, 62.5, and 50% MVC according to Dekerle et al. (2018), and the regression between SIT amplitude and contraction intensity was extrapolated to the y intercept to obtain an estimated resting twitch (ERT, Todd et al., 2003). The regression analysis confirmed a linear relationship at each time-point (r² range = 0.89 ± 0.04–0.93 ± 0.06). The estimated resting twitch (ERT) was calculated as the y-intercept of the linear regression between the mean amplitude of the SIT force evoked by TMS at each contraction intensity. Subsequently, VA_TMS was quantified using the equation [1 - (SIT/ERT) × 100]. The peak-to-peak amplitude of evoked MEP and M_max were measured offline.

Typical error as a coefficient of variation (CV, %) and intraclass correlation coefficients (ICC_3,1) between the two baseline visits were calculated to quantify the reproducibility of neuromuscular and physical function measures. Reproducibility coefficients were as follows: MVC (ICC = 0.97, CV = 1.7%), VA with motor nerve stimulation (ICC = 0.85, CV = 2.8%), M_max (ICC = 0.91, CV = 4.3%), Q_tw,pot (ICC = 0.84, CV = 4.3%), VA_TMS (ICC = 0.81, CV = 4.1%),MRFD (ICC = 0.89, CV = 0.9%), MRR (ICC = 0.80, CV = 2.6%), CT (ICC = 0.60, CV = 8.1%), and RT_0.5 (ICC = 0.71, CV = 11.1%), CMJ (ICC = 0.97, CV = 6.6%), DJ-RSI (ICC = 0.87, CV = 7.7%).

Data are presented as mean ± SD. A two-way repeated measures ANOVA with 2 treatment levels (PCM_cold vs. PCM_amb) with 4 time points (Pre-, 24, 48, and 72 h post-match) was performed. Normality of the data was assessed using the Shapiro–Wilks test. Assumptions of sphericity were explored and controlled for all variables using the Greenhouse-Geisser adjustment, where necessary. In the event of a significant interaction effect (treatment × time), Bonferonni post hoc analysis was performed to locate where the differences lie. Paired sample t-tests were used to assess differences in match-running and heart rate variables between the two conditions. The belief questionnaire was analyzed using the Wilcoxon signed-rank test. To estimate the magnitude of the treatment effects, Cohen’s d effect sizes (ES) were calculated with the magnitude of effects considered either small (0.20–0.49), medium (0.50–0.79), or large (>0.80). All data were analyzed using Statistical Package for Social Sciences (SPSS version 22.0). Statistical significance was accepted at P < 0.05.

Match activity variables are displayed in Figure 1. No differences in playing time, match activity, or heart rate variables (mean HR 167 ± 9 and 165 ± 5, peak HR 192 ± 7 and 195 ± 9 for PCM_cold and PCM_amb, respectively) were found between the two conditions (P ≥ 0.10). Players were required to play at least 70 min in order to be included in the intervention; no players were excluded on this criterion. In terms of treatment order, six players wore PCM_amb first and five players wore PCM_cold.

Perceptual responses from the Elite Performance Readiness Questionnaire can be viewed in Table 1. Soccer match-play elicited fatigue [F_{(3, 30)} = 18.62, P < 0.001] and soreness [F_(3,30) = 17.99, P < 0.001] which persisted up to 72 h relative to baseline (all P ≤ 0.03). No effects of PCM were observed for any of the perceptual responses [F_{(3, 30)} = ≤ 0.65, P ≥ 0.59]. Analysis of the belief questionnaire revealed no differences in the perceived effectiveness of the two treatments either pre- or post-intervention (P = 0.56; Table 2).

Neuromuscular function variables are depicted in Figure 2. Soccer match-play elicited declines in MVC force [F_{(3, 30)} = 6.26, P < 0.01], VA measured with motor nerve stimulation [F_(3,30) = 5.05, P < 0.01], and Q_tw,pot [F_{(3, 30)} = 3.09; P = 0.03], with impairments in MVC and Q_tw,pot persisting for up to 72 h post-match (all P ≤ 0.04), and reductions in VA persisting for up to 48 h post-match (P = 0.03). VA_TMS did not change at any time-point [F_{(3, 30)} = 2.662, P = 0.129]. Similarly, no time effect was found for M_max [range 4.4–4.8 mV, F_{(3, 30)} = 0.808, P = 0.524], or any measure of muscle contractility [F_{(3, 30)} ≤ 0.768, P ≥ 0.547; Figure 3]. No treatment × time interactions were observed for any of the neuromuscular variables (F ≤ 2.73, P ≥ 0.18). However, a main effect of treatment was found for both MVC [F_{(1, 10)} = 6.254, P = 0.03] and VA with motor nerve stimulation [F_{(1, 10)} = 5.47, P = 0.04]. Post hoc analyses revealed that for both MVC and VA, there was no difference between PCM_cold and PCM_amb at 24 h (MVC: P = 0.28; motor nerve VA: P = 0.61), while at 48 h, MVC (P = 0.03; d = 0.26) and VA (P = 0.01; d = 0.66) were higher in the PCM_cold compared with the PCM_amb condition.

Physical function variables are displayed in Figure 4. Although a main effect for time on CMJ height was observed [F_{(3, 30)} = 5.01, P = 0.03], post hoc analysis revealed no significant differences relative to baseline (Figure 4A). Soccer match-play results in reductions in RSI [F_{(3, 30)} = 7.45, P = 0.02] which persisted for up to 48 h (P = 0.02; Figure 4B). There was no effect of PCM on any of the physical function variables [treatment × time F_(3,30) ≥ 1.05, P ≥ 0.20].

The magnitude of impairments in the maximal force generating capacity of the muscle and the time-course of recovery in the present study was similar to that observed following competitive match-play in a study conducted by Rampinini et al. (2011), but less than was observed by Brownstein et al. (2017), in which MVC remained 11% below baseline at 24 h post. Specifically, MVC was reduced at 24 (PCM_cold 5.2%, PCM_amb 7.5%) and 48 h (PCM_amb 4.3%), before recovering by 72 h post-match. Similarly, Q_tw,pot was reduced at 24 (PCM_cold 8.0%, PCM_amb 6.7%), and 48 h (PCM_cold 4.2%, PCM_amb 3.4%), before recovering by 72 h post-match. Voluntary activation measured with motor nerve stimulation was reduced at 24 h (PCM_cold 1%, PCM_amb 6%) before recovering by 48 h post-match. In addition, physical function measured through the DJ-RSI was impaired for up to 72 h post-match, while analysis of perceptual responses indicate that fatigue and muscle soreness persisted for up to 72 h post-match. Furthermore, the reduction in MVC, one of the most widely used indicators of EIMD (Goodall et al., 2017), along with the increase in muscle soreness for up to 72 h post-match, indicates that the competitive soccer matches involved in the study elicited muscle damage. The occurrence of muscle damage was likely a consequence of the high volume of decelerations recorded throughout the matches along with the numerous other eccentric actions associated with soccer match-play. Given that recovery of contractile and CNS function has been shown to occur rapidly following exercise that is metabolically, but not mechanically demanding (Skurvydas et al., 2016), it is likely that the prolonged impairments in Q_tw,pot and VA in the present study were a consequence of the muscle damage incurred during match-play along with the inflammatory response which ensues thereafter.

The decline in the maximum force generating capacity of the quadriceps was attenuated following PCM_cold, as indicated by the higher MVC at 48 h post compared with PCM_amb. Furthermore, the between-treatment differences in the magnitude of reduction in MVC between baseline and 24 (2.3%) and 48 h (4.3%) was greater than the measurement error obtained from the two baseline visits in the present study (1.7%). The attenuated decline in MVC following PCM_cold in the present study is in line with recent studies conducted by Clifford et al. (2018) and Kwiecien et al. (2018), which displayed a substantially accelerated recovery of MVC strength in the days following soccer match-play and an eccentric based exercise protocol, respectively. The magnitude of improvement in the present study, however, was lower than that of Clifford et al. (2018), who found a treatment × time interaction and large effect size between the PCM_cold and PCM_amb condition at 36 h post-match. This could have been due to the substantially lower decline in MVC in the present study (4 ± 5% reduction at 48 h compared with ∼15% at 36 h post-match in the study by Clifford et al. (2018)), possibly owing to participants continuing to train in the days post-match and potentially compounding reductions in MVC in the study by Clifford et al. (2018), while players in the present study refrained from physical activity in the 72 h post-match. Taking this into consideration, it could be suggested that PCM could be a useful tool during periods of heavy training and/or competition, during which impairments in muscle function could be compounded by limited recovery periods.

The ability of the muscle to generate maximum force is influenced by the capacity of the CNS to activate the muscle, and the efficacy of excitation-contraction processes occurring at or distal to the neuromuscular junction to produce force in response to neural input. In the present study, motor nerve VA was greater following PCM_cold compared with PCM_amb at 48 h, while, similar to MVC, the between-treatment difference in the reduction in VA between baseline and 24 (5.0%) and 48 h (3.8%) was greater than the VA measurement error (2.8%). Considering the higher VA following PCM_cold coupled with the lack of effect on contractile function (Q_tw,pot), this suggests that the beneficial effect of PCM_cold on MVC was primarily due to improvements in VA. It is difficult to deduce the precise mechanisms by which PCM_cold improved VA in the present study. However, while the mechanisms underpinning the prolonged reduction in VA post-exercise are largely unknown (Carroll et al., 2017), it is well established that muscle damage elicits impairments in VA which can take several days to resolve (Goodall et al., 2017). Factors associated with inflammation are thought to interfere with CNS function, through an increase in firing rate of group III and IV muscles afferents sensitive to various markers of muscle injury (e.g., bradykinin, histamines, and prostaglandins; Pitman and Semmler, 2012). Alternatively, or additionally, increases in the concentration of brain cytokines following eccentric exercise are potent modulators of brain function (Dantzer, 2001), and might also influence recovery of CNS impairment (Carmichael et al., 2006). Given that one of the proposed benefits of cryotherapy is to attenuate inflammation, it is possible that wearing PCM_cold post-match could have reduced the inflammatory response and thereby attenuated inflammation-induced perturbations in CNS function. However, given that markers of inflammation were not measured in the present study, this suggestion remains speculative. Further research examining the effects of PCM_cold on inflammation concurrent with measures of VA following damaging exercise is thus warranted.

A number of previous studies have shown that muscle damage leads to prolonged impairments in contractile function, as evidenced through protracted reductions in Q_tw,pot (Endoh et al., 2005; Goodall et al., 2017). It is likely that the prolonged reductions in Q_tw,pot following eccentric based exercise are a consequence of direct myofibrillar damage, disorganization of sarcomeres and interference with cellular Ca²⁺ handling which inhibit the excitation-contraction coupling process (Skurvydas et al., 2016). The lack of change in M_max, a measure of neuromuscular transmission, suggests that the prolonged reduction in Q_tw,pot in the present study was due to processes beyond the sarcolemma. In addition to measuring M_max and Q_tw,pot, we included measures of muscle contractility (namely, MRFD, CT, MRR, and RT_0.5) to attempt to provide further insight into contractile function following soccer match-play. Similar to previous studies (Rampinini et al., 2011; Brownstein et al., 2017), reductions in Q_tw,pot persisted despite a lack of change in measures of contractility, suggesting that impairments within the excitation-contraction coupling process which were not detectable through our measures of muscle contractility were responsible for the decline in Q_tw,pot. While we cannot discern the precise mechanism for the protracted impairment in contractile function, events that occur secondary to the initiation of muscle damage have also been implicated in impairments in excitation-contraction coupling. Specifically, the accumulation of reactive oxygen/nitrogen species has been shown to interfere with SR Ca²⁺ release, which has been attributed to redox modification of ryanodine receptors (Cheng et al., 2016), while reactive oxygen species are also thought to diminish the calcium sensitivity of myofilaments (Moopanar and Allen, 2005; Reid, 2008). In this regard, it was thought that the application of cryotherapy, which has been suggested to inhibit the inflammatory response and limit the generation of reactive oxygen/nitrogen species (White and Wells, 2013), could ameliorate the impairments in contractile function in the days following soccer match-play. However, the application of cold PCM had no effect on recovery of either Q_tw,pot. The lack of effect of PCM_cold on neuromuscular function could have been due to a number of factors. Firstly, whether or not cryotherapy actually reduces inflammation remains equivocal, despite its widespread application (Broatch et al., 2014; Peake et al., 2017). Veritably, studies have neither been consistent nor produced compelling evidence to support the role of cryotherapy in reducing inflammation and improving aspects of recovery (Leeder et al., 2012), and it has been suggested that many of the previously reported benefits of cryotherapy could simply be due to a placebo effect, rather than any physiological effect (Broatch et al., 2014). Despite the promising findings from recent studies using cold PCM as a recovery aid (Clifford et al., 2018; McHugh et al., 2018), and that applying these garments has been shown to reduce muscle temperature (Kwiecien et al., 2018), there is no evidence to suggest that cold PCM reduces inflammation. As such, it is possible that PCM_cold had no effect on the inflammatory processes suggested to interfere with contractile and CNS function. Secondly, as alluded to previously, the magnitude of the impairments in Q_tw,pot was relatively small, potentially limiting the ability to detect subtle differences between groups. Indeed, it would be reasonable to assume that the benefits of cryotherapy on recovery would only be evident were the impairments in neuromuscular function more substantial than those seen in the present study. Further research to examine the effects of wearing cold PCM on recovery of neuromuscular function following exercise which elicits substantially more damage is probably warranted.

Despite the improvement in MVC and motor nerve VA with PCM_cold, there was no improvement in measures of jump performance (CMJ or DJ-RSI). The lack of effect of PCM_cold on physical performance measures might have been due to the relatively modest improvement in MVC and VA in the present study. Thus, the functional relevance and meaningfulness of the differences between PCM_cold and PCM_amb for MVC and VA are unclear, and could be questioned. Although it is plausible that a reduced capacity of the CNS to activate muscles would impede the ability to perform tasks requiring maximal force production, given that current methods of determining VA are restricted to isometric or isokinetic contractions, it is not possible to accurately qualify the functional consequences of reduced VA on the performance of unconstrained physical tasks relevant to football performance.

This study used a competitive soccer match in order to study the effects of the application of cold PCM on recovery in the days post-match. While this approach provides the most ecologically valid means of investigating the effects of a recovery intervention following soccer match-play, one limitation of this method compared with a laboratory simulation is the lack of experimental control over the activity profiles of the players and the high inter-subject variability in match demands. Consequently, it is possible that differences between match-demands could have influenced the magnitude of fatigue and time-course of recovery following the two treatments. However, differences between the time-motion and heart rate variables between the matches were negligible. Furthermore, although simulated match protocols are designed to replicate the physiological demands of competitive matches, many of the neuromuscular, skill and cognitive demands associated with competitive match-play cannot be replicated through match simulations, and the validity of using these protocols when assessing the efficacy of a recovery intervention could thus be questioned. In addition, although no differences were found in the results from the belief questionnaires, on average, participants reported that they believed both PCM_cold and PCM_amb were “moderately effective” in improving recovery both before and following the intervention. As such, it is possible that a placebo effect could have influenced recovery under both conditions. However, the magnitude of fatigue and the time-course of recovery was similar to that observed following competitive match-play in professional soccer players (Rampinini et al., 2011), suggesting that any placebo effect on the results was negligible. Furthermore, that the participants believed both interventions to be moderately effective could be considered an important finding given that a growing body of evidence indicates that recovery is related to individual preference and perceptions of the intervention (Halson, 2014). Moreover, because local tissue temperature was not measured in the present study, it is unknown whether or not PCM_cold had the desired effect in regards to cooling the muscle. Nevertheless, previous work has displayed that PCM_cold reduced skin temperature to 22°C for 3 h following eccentric based exercise (Kwiecien et al., 2018). Thus, it is likely that the skin temperature was, similarly, decreased in the present study. Finally, another limitation of the present study was the 4–8 week gap between matches for each condition. Consequently, it is possible that players were in a different phase of the training cycle between the two matches, potentially influencing the magnitude of fatigue and time-course of recovery in response to competitive match-play. However, the majority of fixtures were separated by 6 weeks or less, with only two matches separated by 8 weeks. As such, it is likely that the influence of the duration between conditions had a negligible effect on the results of the study.