It has been suggested that precise control of the trunk position and motion over the pelvis could optimize the energy transfer in the kinetic chains from torso to extremities for performing athletic activities composed of highly loaded movements (Kibler et al., 2006). Such specific bodily control in the exercising human depends on the outputs of the core muscles (CM) that mainly function at the lumbo-pelvic-hip region as well as the proximal lower limbs (e.g. external and internal obliques, rectus abdominis, erector spinae). The critical role of the CM in the kinetic chains had led to associations that CM function may limit the performance of athletes in sports activities, especially those performed with the body in an upright position, as observed in running. It had been further postulated that enhanced specific CM function could improve associated sports performance. However, such postulation has never been concluded well in previous studies (Stanton et al., 2004; Tse et al., 2005; Nesser et al., 2008; Sato and Mokha, 2009; Okada et al., 2011), partly due to unclearness of the roles that specific CM have in various sports performance (Hibbs et al. 2008). In fact, CM function has never been explored as a factor limiting performance capacity in runners. The fundamental questions (1) whether CM function would be changed with fatigue during intense running, and (2) whether the fatigue of the CM would impair the capacity of running exercise, have never been addressed. Such unclear scenarios have been ascribed to the lack of core test specifically designed for assessing sports-related core muscle function in athletes. Since core muscle load during various stability tests depends on the joint torques required to hold a specific posture, those core tests established in rehabilitation settings may not be accurate to reveal the functional capacity of the complex core anatomy that is specific to dynamic athletic performance (McGill et al., 2010). Recently, a sport-specific core muscle test developed by Mackenzie (2005) had been validated. The plank maneuver test interspersed with the alternate raising of the arms and legs challenges the trunk flexors and lumbar extensors in a manner that is similar to that occurring in performing sports movements. The test has been demonstrated to be a valid and reliable method, with adequate sensitivity to assess the change in global core muscle function with fatigue in athletes (Tong et al. 2014).
It has been reported that the tremendous ventilatory demand corresponding to >85% V[O.sub.2max] during continuous exercise would cause inspiratory muscle (IM) fatigue and associated exercise intolerance, while concomitant expiratory muscle fatigue is less likely to occur (Chevrolet et al., 1993; Dempsey et al., 2008; Ross et al., 2008; McConnell, 2011). From an anatomical view, both inspiratory and expiratory muscles in humans play a dual role in breathing and core stability during exercise. It was not known if the respiratory work during intense continuous running would contribute to the potential change in global CM function. The diaphragm, the major IM, has been shown to be activated during nonrespiratory activities including power lifting and bicep curls (Al-bilbeisi and McCool, 2000). However, the interaction of CM and IM functions in core stabilization during intense continuous running is not clear. The purpose of this study were to investigate (1) the occurrence of CM fatigue and its limitation to exercise performance during continuous high-intensity running to exhaustion; and (2) whether the respiratory muscle work performed during intense running would contribute to the potential occurrence of CM fatigue. In this study, the new validated sport-specific endurance plank test would be used for examining the changes in global core muscle function with fatigue.
Nine male recreational long-distance runners, who were asymptomatic for cardiorespiratory disease or lower back disorder, and engaged in regular long-distance running training with a lack of experience of regular CM / IM training, voluntarily participated in this study. Physical characteristics and athletic training background of the runners are shown in Table 1. After being fully informed of the experimental procedures and possible discomfort associated with the exercise test, subjects gave their written consent to participate. The local Ethical Committee for the Use of Human & Animal Subjects in Research provided ethical approval of the study.
Subjects were required to perform two trials of continuous runs on a treadmill at an intensity corresponding to 85% V[O.sub.2max] until volitional exhaustion. The first trial (CR trial) was to detect the occurrence of global CM and IM fatigue subsequent to intense running by comparing the results of post-exercise sport-specific endurance plank test (SEPT) and maximum inspiratory mouth pressure ([PI.sub.max]) measurement, respectively, with corresponding baseline values measured on separate days. The change in handgrip strength (HG), non-exercising muscle function, was also measured to evaluate if the potential decline in post-exercise SEPT and [PI.sub.max] was the result of reduced motivation or general whole-body fatigue (Ozkaplan et al., 2005). The measurements of hG, [PI.sub.max] and SEPT were performed in sequential order, and the sequence was identical in all trials. The second trial (CR_F trial) was to investigate if CM fatigue would limit running performance. The running time to exhaustion subsequent to a specific CM fatigued workout in the CR_F trial was compared with that without a preceded fatigued workout in the CR trial. The order of CR and CR_F trials were randomly assigned to subjects, with five performing the CR trial followed by the CR_F trial, with the rest of the subjects performing the trials in reverse order. To examine the independent contribution of respiratory muscle work, during intense running, to the occurrence of CM fatigue, a trial of voluntary isocapnic hyperpnea was performed. This was made possible by mimicking the ventilatory response recorded during the CR trial while subjects were free from whole-body exercise (Mimic trial). Post-hyperpnea SEPT performance was then compared with baseline values.
All trials were performed in an air-conditioned laboratory. Before each trial, the subjects 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 and were separated by a minimum of 3 days.
Preliminary testing and familiarization trials
Table 2 shows the timeline of preliminary testing, familiarization and experimental trials. Prior to the experimental trials, physical characteristics, including lung function were measured. Following this, subjects were familiarized with the measurements of CM function, IM function, HG strength, and the treadmill run. This familiarization period utilized, the testing equipment, protocols, and provided the subjects with the sensation of exercising to exhaustion.
The linear relationship between running speed and steady-state VO2 as well as aerobic capacity of the subjects were assessed by performing a standard graded treadmill testing protocol (Eston and Reilly, 2009) held on a separate day. Following the graded test, a running speed, which would elicit approximately 85% V[O.sub.2max], was selected from the linear relationship of steady-state V[O.sub.2] versus speed. The defined speed would be applied in the continuous intense running test during experimental trials.
Following the graded test, the measurements of...