Several intermittent-type team sports (e.g. soccer, handball) are played for between 60-90 min duration, with a half-time break of 10-20 min at the mid-way point (Russell et al., 2015). Half-time strategies for optimizing second-half performances are essential as passive recovery has been associated with physiological changes, such as decrease in core and muscle temperature, and reduction in blood glucose level, that may impair both the physical and cognitive performance of players (Mohr et al., 2004, Greig et al., 2007, Lovell et al., 2013a). In fact, the passive half time interval leads to a decrement in the high-intensity running performance and increase in the incidence of muscular injury in the initial 5-15 min of the second half of competitive match-play have been reported frequently in professional soccer players (Lovell et al., 2013b). Currently, strategies associated with the maintenance of physical performance employed during a 15-min half-time break include injury treatment, hydro-nutritional practices and heat maintenance (Russell et al., 2015). For initiating active recovery in working muscles and attenuating body temperature loss due to transient drop-off in muscle activity and associated blood flow, actively engaging in specific physical activity, termed as "re-warm-up exercises", has been suggested (Mohr et al., 2004). In fact, several re-warm-up regimes including a moderate-intensity run, whole-body vibration, and lower-body resistance exercise have been demonstrated effectively to protect against the decrements in functional ability of the lower limbs and subsequent sprint ability observed under passive control conditions (Mohr et al., 2004, Lovell et al., 2013b, Zois et al., 2013). However, available time and space limit the implementation of such activities in real-game settings (Towlson et al., 2013).
In a sporting environment, core muscles (CM) are commonly referred to as all the muscles between the knee and sternum with a focus on the abdominal region, low back and hip (Fig, 2005).During running exercise when the body is upright, the CM are actively involved in providing torso and lumbopelvic stiffness that helps to optimize running form and support the kinetic chains of the upper and lower extremities (Kibler et al., 2006, Borghuis et al., 2008). Apart from supporting core stability, a substantial portion of the CM, located in the torso, are inspiratory muscles (IM) concomitantly responsible for the breathing movement of the chest and abdomen in meeting the strenuous ventilatory demands (McConnell, 2009). It has been demonstrated that exhaustive high-intensity running exercise reduces the IM and CM function with fatigue, impairing exercise performance (Tong et al., 2014a, 2016). However, whether the re-warm-ups during the half-time break in team sports, observed in a game setting could facilitate the functional recovery of the potential fatigued IM and CM and associated maintenance of the second-half exercise performance have not been investigated.
Recently, a 6-wk functional IM training period, which was composed of four inspiratory-loaded CM exercises, was found to augment the global IM and CM functions in endurance runners, and enhance their running performance (Tong et al., 2016). The performance of such CM exercises at mild intensity with simultaneous inspiratory load was likely to activate the two muscle groups for subsequent exercise readiness (Lin et al. 2007). Such activities might also facilitate the recovery of exercise-induced IM and CM fatigue due to the previous notion that active recovery performed using the same muscle groups which were active during the preceded fatiguing exercise, compared with that which remained unaffected by fatigue, was more effective in restoring muscle function (Mika et al., 2016). Accordingly, it was reasonable to postulate that a single set of inspiratory-loaded CM exercises, which could be accomplished within a few minutes in a confined space, could be a potential alternative to current re-warm-up strategies to attenuate the possible decrements of IM and CM functions that might occur following the first half of intermittent-type team sports. This strategy may also help to improve blood perfusion in active muscles including the legs, and promote the sprinting ability of the players during the second half (Mohr et al., 2004). The purpose of this study therefore, was to investigate the effect of a single set of inspiratory-loaded CM exercises as a potential re-warm-up strategy in team sports.
Nine male college athletes (Table 1), who had received training in different intermittent-type team sports (soccer and handball) for 2-3 hrs x [day.sup.-1], 3-4 days x [wk.sup.-1], for at least two years, volunteered to participate in the study. The sample size was estimated based on the assumption that the minimum practical important difference of the repeated-sprint performance was 1.2 [+ or -] 1.1% (Buchheit et al., 2009), and the expected typical error (within-subject SD) was 0.8% (Impellizzeri et al., 2008). The use of the acceptable precision a priori in sample size estimation was the approach developed for magnitude-based inferences (Hopkins, 2006). A sample size of >7 participants in the present study would provide maximal changes of 0.5% and 20% of type I and type II errors, respectively. After being fully informed of the experimental procedures and possible discomfort associated with the exercise test, participants gave their written informed consent. Ethical approval for this study was obtained from the Committee on the Use of Human and Animal Subjects in Teaching and Research of Hong Kong Baptist University. The study was conducted in accordance with the Declaration of Helsinki.
Participants performed a simulated team-sports intermittent exercise protocol (IEP) on a non-motorized treadmill in two phases [phase one (P1), phase two (P2)], interspersed by a 15-min half-time break, in separate experimental trials. Details of the IEP can be found below in the 'Intermittent exercise protocol' section. During the 15-min half-time break, the participants performed either an entire 15-min passive recovery (CON) or 11-min passive recovery plus 4-min specific re-warm-up exercises (RWU). The IM and CM functions, leg perfusion, and sprint performance at the onset of the P2-IEP following the RWU were compared correspondingly to those in the CON trial.
In the present study, the CM function of participants was assessed using a sport-specific endurance plank test developed previously in our laboratory (Tong et al. 2014b). Since the plank test would lead to severe local muscle fatigue in CM that has been shown to impair subsequent running performance (Tong et al. 2014a), the assessments of Pre-P1, Post-P1 and Pre-P2 IM and CM functions in CON and RWU trials were arranged on separate days. For monitoring the changes in leg perfusion, near-infrared spectros-copy was adopted. Since cutaneous reflex vasoconstriction and a resultant fall in skin temperature that could potentially occur in the transition from rest to exercise might confound the accuracy of near-infrared readings on the target muscles (Torii et al., 1992, Buono et al., 2005), skin temperature at selected body sites were measured subsequent to every near-infrared measurement for reference purposes.
Figure 1 shows the timeline of Pre-P1, Post-P1 and Pre-P2 measurements in each trial. The order in which the RWU and CON trials were performed was counterbalanced and the assignment of participants to testing protocols was done in a random fashion. In all trials, standardized whole-body warm-up exercises, which comprised of a 5-min motorized-treadmill run, a 10-min period of stretching, and five 6-s non-motorized treadmill runs with velocities ranging from moderate to maximum, were performed prior to the exercise tests. All trials were completed under controlled laboratory condition. The mean air temperature and relative humidity (20.0 [+ or -]1.6 [degrees]C, 73.5 [+ or -]3.1%) in the laboratory in the CON trials did not differ from those, respectively, recorded in RWU trials (19.5 [+ or -]1.5 [degrees]C, 72.8 [+ or -]1.6%, p > 0.05). Before each trial, the participants refrained from eating for at least two hours, and from participation in strenuous physical activity for at least one day.
All trials were scheduled to occur at the same time of day to control for diurnal variation effects and were separated by a minimum of 3 days.
Preliminary tests and familiarization trials: Prior to the experimental trials, physical characteristics, including lung function and aerobic capacity were measured. The details of the measurements of the lung function and the aerobic capacity test have been reported previously (Tong et al. 2001). Following preliminary testing, participants were familiarized with the measurements of IM and CM functions, as well as the sprint test and the IEP. This familiarization period introduced, the testing equipment and protocols, as well as providing the participants with the experience of exercising to the limits of tolerance.
Intermittent exercise protocol: The specific IEP performed on a non-motorized treadmill (Force 3, Wood-way, USA) was modified from a protocol used for prolonged intermittent-type sport simulation adopted in previous studies (Sirotic and Coutts, 2008). Briefly, the modified protocol was interspersed with six activities including standing still, walking, jogging, running, dashing and sprinting on the treadmill with the brake force set at 2 kp. The velocities for each activity were 0%, 20%, 35%, 50%, 70% and 100% of individual maximal sprint velocity, respectively. The maximal sprint velocity of participants was measured in a preliminary testing trial. The participants performed three maximal 4-s sprints on the treadmill, each separated by 14 s of passive recovery (Zois et al., 2013). The highest...