Introduction
The lumbopelvic–hip complex is commonly referred to as the “core” region of the body. Muscles and associated connective tissues in this region are essential in prevention of buckling of the vertebral column and returning the trunk to postural equilibrium after perturbation (29). In the sporting environment, core muscles (CM) play a role in controlling the position and motion of the torso over the pelvis. However, during many everyday activities, especially those associated with sports, the entire torso is involved in optimizing the transfer of energy to the extremities (14). A substantial portion of the torso musculature is involved in breathing, and it is well-known that breathing and postural controls are coordinated (8). The dual role of the respiratory muscles, in particular the diaphragm, is implied by the fact that when rhythmic, posturally destabilizing limb movements are performed, phasic contractions of the diaphragm are superimposed on tonic respiratory-related diaphragm activation, with the same frequency as the limb movements (11). Furthermore, global inspiratory muscle (IM) fatigue is associated with impairment of postural stability in healthy people (13). Recently, we have shown that the heavy fatiguing respiratory work of intense running independently led to global CM fatigue in runners (27). The magnitude of the global IM fatigue correlated (r2 > 59%) with the magnitude of the global CM fatigue, suggesting that the IM and CM work synergically during heavy exercise. However, the specific contribution of IM function to function of the core stabilizing system is unclear.
During running activities, when the body in an upright position, CM are actively involved in providing torso and lumbopelvic stiffness that helps to optimize running form and the kinetic chains of upper and lower extremities (3,14). After high-intensity running, we have previously shown a decline in performance of a sport-specific endurance plank test (SEPT), which suggests the presence of CM fatigue (27). In the same study, the occurrence of CM fatigue was also shown to impair individual's endurance running performance. The apparently crucial role of the CM during exercise has led to the suggestion that enhancing CM function would improve sports performance. However, most previous studies have failed to support this suggestion (10,18,19,22). Given the closely integrated relationship between CM and IM contributions to sports performance, it is perhaps unsurprising that interventions focusing only on the CM should fail to identify an improvement in sports performance. Breathing pattern is not typically controlled during CM training, and it is frequently the case that people hold their breath or breathe with a low tidal volume to focus the activity of the torso muscles on the core stabilizing challenge. It has recently been suggested that adopting a “functional,” holistic approach to CM and IM training (IMT) might be beneficial (16). However, the effectiveness of a dual CM/IM training intervention has never been tested.
The study was designed to test whether a CM training regimen that included IMT improved performance in a 1-hour treadmill run test in recreational runners. Since IMT per se improves running performance (4), the baseline IM function of all participants was standardized by subjecting them to a 4-week phase of IMT (foundation IMT) before the intervention phase. The intervention phase consisted of a 6-week treadmill interval training program; under the intervention condition, IMT was undertaken during CM training (ICT), which took place immediately after the interval training. Under the control condition (CON), only interval training was undertaken. We hypothesized that the improvement in the 1-hour treadmill run test would be greater in the ICT group than the CON group.
Methods
Experimental Approach to the Problem
Participants with matched gender, physical characteristics, and training background were assigned randomly to ICT or CON groups (Table 1). To distinguish the effects of the CM/IM intervention from the known effects of IMT on running performance (4,12), all participants completed a 4-week period of IMT before the interval training intervention phase of the study. Subsequently, both groups undertook 6-week interval running training, but only the ICT group received the CM/IM intervention. This study occurred during the annual break and early preparation phase of the yearly training plan of the participants. During the 10-week period, participants received no other specific running training.
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Table 1.: Physical characteristics and training background of participants in ICT and CON groups.*†
Subjects
Sixteen recreational runners (4 females and 12 males), who had received training at a local sports club in Hong Kong, China, volunteered to participate. Participants had no familial history of cardiovascular disease and consumed no related medication. Moreover, none had previous experience of specific IM or CM training. After an explanation of the purpose and requirements of the study, participants gave written informed consent. Local Ethics Committee approval was obtained.
Procedures
Figure 1 summarizes the phases of testing and training of the study. Before the 6-week intervention phase, 4-week phase of foundation IMT was undertaken, according to the established guidelines of functional IMT (16). Before IMT, baseline IM and CM functions were assessed, and exercise tests were performed. Identical tests were repeated after completion of the subsequent 6-week intervention. Both IM and CM functions were also assessed between the 2 phases of the study. All tests were performed in an air-conditioned laboratory. Before each test, the participants refrained from eating for at least 2 hours and from participation in strenuous physical activity for at least 1 day. All tests were scheduled to occur at the same time of the day and were separated by a minimum of 3 days.
Figure 1.: The timeline of testing and training.
Preliminary Tests and Familiarization Trials
Before the experimental trials, physical characteristics including lung function were assessed. After this, participants were familiarized with the assessment of CM and IM, as well as the 1-hour treadmill performance test. This familiarization period introduced the testing equipment and protocols, as well as providing the participants with the experience of exercising to the limit of tolerance.
Incremental Treadmill Test
The onset of blood lactate accumulation (OBLA), running economy at OBLA (REOBLA), and maximal oxygen uptake (V̇o2max) of the participants were determined by performing a standard maximal incremental treadmill test held on a separate day. The test protocol designed for the combined measurements has been described previously in detail (5). The OBLA was defined as the running speed at the stage during which blood lactate concentration was at or close to 4 mmol·L−1. The REOBLA in ml·kg−1·min−1 was calculated based on the average V̇o2 during the final minute of the 3-minute stage corresponding to OBLA. The V̇o2max was the highest 10-second mean value. After the incremental test, a running speed that elicited approximately 65% V̇o2max was identified from the linear relationship of steady-state V̇o2 vs. speed. The defined speed was used in the subsequent 1-hour treadmill run.
Inspiratory Muscle and Core Muscle Function Tests
Global IM function was measured by performing maximal inspiratory efforts at residual volume against an occluded rubber-scuba-type mouthpiece with a 1-mm orifice. The inspiratory mouth pressure at quasi-zero flow (P0 in cmH2O) provided a surrogate measure of IM strength. The maximum rate of pressure development (MRPD in cmH2O·ms−1) that occurred during the initial onset of the P0 effort was also recorded. The maximal inspiratory efforts were repeated at least 5 times until the results were stable (vary by <10% in consecutive 3 maneuvers), and the highest value was recorded for analysis (9).
The protocol of the SEPT, which has been shown to be valid and reliable for assessing athletes' global CM function, has been described in detail previously (26). Briefly, participants were required to maintain the prone bridge with good form throughout the following stages with no rest in between: (a) hold the basic plank position for 60 seconds; (b) lift the right arm off the ground and hold for 15 seconds; (c) return the right arm to the ground and lift the left arm for 15 seconds; (d) return the left arm to the ground and lift the right leg for 15 seconds; (e) return the right leg to the ground and lift the left leg for 15 seconds; (f) lift both the left leg and right arm from the ground and hold for 15 seconds; (g) return the left leg and right arm to the ground, and lift both the right leg and left arm off the ground for 15 seconds; (h) return to the basic plank position for 30 seconds; (i) repeat the steps from (a) to (i) until the maintenance of the prone bridge failed.
The conditions of the SEPT were standardized by using identical body posture. The distances between the left and right elbows (medial epicondyle), the left and right feet (first metatarsal), and the elbow and feet on the left and right sides of the body were measured during the familiarization trial while the participants were comfortably performing the prone bridge on a bench. Furthermore, 2 elastic strings of ∼80 cm length which were attached horizontally to a pair of vertical scales were placed beside the bench during the test. The 2 strings maintained at a distance of 10 cm were adjusted up and down until a height was reached that was at the same level as the participants' hip (the iliac crest was evenly in between the 2 strings). This setting acted as a reference for the objective monitoring of hip displacement during the test. The measured distances between elbows and feet, as well as the hip height, remained constant in subsequent experimental trials. During the assessment, the test administrator sat 1 meter away from the bench with the seat height adjusted to a level so that the hip displacement of the participants could be monitored horizontally. The participants were then asked to maintain the prone bridge throughout the test with maximum effort. For each time that the hip was beyond either of the reference lines, a warning was given. The test was terminated when the hip failed to be maintained at the required level after receiving 2 consecutive warnings. The measured time to the limit of tolerance was used as the index of global CM function.
One-Hour Treadmill Running Test
The 1-hour treadmill running test was performed on a separate day after the muscle function evaluations. The running protocol has been described previously in detail (1). Briefly, participants ran continuously on a treadmill (h/p/cosmos; Pulsar, Nussdorf, Germany) for 60 minutes with gradient of 1%. For the first 30-minute, participants ran at fixed speed equivalent to 65% V̇o2max. For the second 30-minute, participants maximized the running distance they achieved by manually adjusting, at their own will, the running speed. This took place each minute with a resolution of 1 km·h−1. During running, heart rate (Polar HR monitor, Polar, Kempele, Finland), and ratings of perceived exertion (Borg RPE scale 6–20) and of perceived breathlessness (Borg RPB scale 0–10), were collected every 3 minutes. Respiratory responses were monitored continuously, starting from the 27th minute, until the end of test (Vmax 229d; Sensormedics, Yorba Linda, CA, USA). A 25-μL fingertip blood sample was taken preexercise and postexercise for assessment of blood lactate concentration ([La−]b; YSI 1500 Sport Analyzer; YSI, Yellow Springs, OH, USA). Immediately after postexercise blood sampling, P0 and SEPT measurements were performed in sequential order. After the 6-week interval training phase, the 1-hour performance run was repeated twice, which permitted evaluation of the reliability of the maximum distance covered. Moreover, an additional running trial was completed, during which the preintervention trial running speed was replicated (ISO), which permitted direct comparison of the cardio-respiratory and perceptual responses during the entire 1-hour run, as well as the exercise-induced changes in IM and CM global functions. To ensure consistency across trials, participants were blinded to any feedback on the distance covered in any trial and were provided no verbal encouragement during the treadmill runs.
Inspiratory Muscle Training
The protocol for the 4-week IMT program has been described previously and shown to be effective (25). Briefly, participants in both groups performed 30 inspiratory efforts twice per day, 6 days per week, for 4 weeks. Each effort required the participant to inspire against a pressure-threshold load equivalent to 50% P0 by using an IM trainer (POWERbreathe Classic L3; POWERbreathe International, Southampton, United Kingdom). During the training, the participants were instructed to initiate every breath from the residual volume in a powerful manner. The inspiratory effort was continued until the inspiratory capacity for the preset loading limited further excursion of lung volume. For training progression, the inspiratory load was increased by 10–15 cmH2O, once the participant had adapted (i.e., they were able to complete 30 maneuvers without a break).
Interval Training
Table 2 shows the protocol of the 6-week high-intensity interval running training program recommended by Fox and Mathews (6) for enhancing, mainly, aerobic exercise capacity. The program consisted of 3–4 sessions per week. Each session comprised 1–3 sets with different repetitions of selected distances of 100, 200, 400, 600, 800, and 2,400 m in each set. The ratio of the work to recovery duration was 1:3 for distances ranging from 100 to 400 m, 1:2 for the 600 m distance, and 1:1 for the 800 m distance. The running training was performed on the high-speed treadmill (h/p/cosmos; Pulsar) with a gradient of 0%. The initial speed for each distance was set according to the participant's maximal speed during the graded treadmill test (100 and 200 m: 100%; 400 m: 90%; 600 and 800 m: 80%; and 2,400 m: 70%). After the initial trial of each distance in the training program, running speeds in each subsequent training distances were adjusted voluntarily on a trial-by-trial basis, such that the limit of tolerance (>90% HRmax) was attained at the end of the set. During recovery intervals, the participants walked briskly on the treadmill at 5 km·h−1.
Table 2.: Protocol of the 6-week high-intensity interval running program.
Functional Core Muscles/Inspiratory Muscle Training
For the ICT group, 4 inspiratory-loaded CM training exercises were performed immediately after all interval training sessions (16). The CM training program was endurance running-specific (7) and consisted of (a) Bridge—Lie on the back, prop on the hands with body weight on the heels, and maintain a straight body line. Brace the abdominal muscles and raise alternately the straightened left and right legs as high as possible; (b) Swiss ball squat thrust—Maintain a press-up position with ankles resting on the Swiss ball. Lift the pelvis up from the straight body line and bend the knees, and return to starting position; (c) Dynamic “bird dog”—Lift the left hand and right knee from a plank position and extend the arm and leg until both are horizontal. Return the arm and leg to original position and extend the other arm and leg; and (d) Raised alternating crunch—Maintain body in a “V” shape with flexed hip on the floor while hands are behind the head. Rotate the trunk alternatively with elbow toward the opposed bended knee. During each CM exercise, inspiratory load was imposed simultaneously using a POWERbreathe IM trainer at mouth. Participants inhaled forcefully through the device as they initiated the required body actions from the starting position and exhaled slowly when returning to the starting position. The load on the pressure-threshold device was set at 50% of the post-IMT P0 throughout the intervention. The 4 CM exercises were performed for 2 sets with 10 repetitions in each set in the first week. The repetitions were increased progressively to 15 in next 2 weeks. In the following 3 weeks, the number of sets increased to 3, and the repetitions in each set were increased progressively from 12, depended on participants' adaptation. For the incorporated inspiratory-loaded breathing activity, the increase in the number of inspiratory efforts paralleled the changes in the number of repetitions of each CM exercise.
Statistical Analyses
Kolmogorov-Smirnov test and Levene's test of equality of error variances was applied and revealed that the data were normally distributed in groups, and the error variances of dependent variables were equal across groups. Independent t-tests were applied to examine the difference in variables between groups (ICT vs. CON). A series of 2-factor analysis of variance was applied to analyze the between-group and within-group effects (pre-IMT and post-IMT, post-ICT, and ISO) on most of the dependent variables. Post hoc analyses using Newman-Keuls were performed when interaction effects were significant. Effect size of selected mean differences was described by calculating Cohen's d. Intraclass correlation coefficient (ICC) was used to reveal the reliability of the postintervention running performance. Relationships between variables were determined using Pearson's correlation test. All tests for statistical significance were standardized at an alpha level of p ≤ 0.05, and all results were expressed as the mean ± SD.
Results
In this study, all participants complied with the protocols of IMT, treadmill interval training, and functional CM/IM training.
Global Inspiratory Muscle and Core Muscle Functions
After the 4-week foundation phase of IMT, P0, MRPD, and SEPT increased significantly (p ≤ 0.05) in ICT and CON groups (Table 3); the increases were similar in both groups. When the variables were expressed as percentage of corresponding pre-IMT values, the increase in P0 was correlated with the increase in SEPT performance (r = 0.66, n = 16, p ≤ 0.05). After the 6-week interval training phase, SEPT performance increased further (Cohen's d = 0.78, p ≤ 0.05) from post-IMT level in ICT group (Table 3), but not in the CON group. Moreover, no significant change in P0 or MRPD was found in comparison with the post-IMT values in either group.
Table 3.: Changes in global inspiratory muscle and core muscle functions, and in variables during the maximum graded treadmill test between preintervention and postintervention.*
Onset of Blood Lactate Accumulation and REOBLA
Compared with preintervention, OBLA increased similarly at the 10-week time point in both groups during the maximal graded treadmill test (Table 3). However, REOBLA (measured at the speed corresponding to the preintervention OBLA) increased significantly (Cohen's d = 0.98, p ≤ 0.05) in the ICT group, but not the CON group. The change in REOBLA was correlated with that in SEPT (r = 0.69, n = 16, p ≤ 0.05; Figure 2A), when both variables were expressed as percentage of preintervention values.
Figure 2.: The change in the performance of sport-specific endurance plank test (ΔSEPT) plotted against the change in (A) the running economy at OBLA (ΔREOBLA), (B) the 1-hour treadmill run performance (Ex), in ICT and CON participants (n = 16). Solid line is the line of regression.
One-Hour Treadmill Run
For the 1-hour treadmill run, the constant speed for the first 30-minute for the ICT and CON groups were 11.9 ± 1.4 km·h−1 and 12.4 ± 1.7 km·h−1 (p > 0.05), respectively. Pre-IMT, the total distance covered in the 1-hour time trial did not differ between groups (12.82 ± 1.47 km vs. 12.92 ± 1.69 km, respectively, p > 0.05). After the 10-week intervention, the distance covered in the two 1-hour time trials was highly repeatable in both groups (ICT—first run: 13.16 ± 1.49, second run: 13.21 ± 1.47 km, ICC = 0.998. CON—first run: 13.05 ± 1.49, second run: 13.10 ± 1.52 km, ICC = 0.991). Only the results of the second postintervention run were analyzed subsequently. Both groups increased the distance covered during the time trial significantly, but the increase for the ICT group was significantly larger (0.39 ± 0.11 km vs. 0.18 ± 0.21 km, respectively; Cohen's d = 1.34, p ≤ 0.05). These changes (expressed as percentage of baseline) were correlated with the percentage changes in the OBLA (r = 0.52, n = 16) and SEPT performance (r = 0.57, n = 16, p ≤ 0.05; Figure 2B).
Immediately after the 1-hour run, [La−]b was higher, and there was evidence of fatigue, as indicated by significantly lower P0 and SEPT values postrun (p ≤ 0.05). Responses did not differ significantly (p > 0.05) within or between the ICT and CON groups (Table 4). In contrast, the ISO trial elicited a significantly attenuated postexercise decrease in P0 and increase in [La−]b, compared with preintervention in both groups, but the response for SEPT performance remained unchanged.
Table 4.: Changes in global inspiratory muscle and core muscle functions, and in blood lactate accumulation during the 1-hour time trial treadmill run in pre-IMT, post-ICT, and ISO trials.*
During the final minute of the 30-minute time trial (60th minute of exercise), HR, RPE, and RPB approached maximum. The HR and RPB were not different between preintervention (ICT: 185.0 ± 8.8 b·min−1, 9.0 ± 0.76; CON: 188.3 ± 5.5 b·min−1, 8.83 ± 1.60) and postintervention trials (ICT: 185.3 ± 9.4 b·min−1, 9.13 ± 0.99; CON: 186.7 ± 3.5 b·min−1, 8.17 ± 1.47, p > 0.05). In contrast, the RPE was slightly, but significantly, lower postintervention (ICT: 19.3 ± 0.7 vs. 19.0 ± 1.1; CON: 18.8 ± 1.6 vs. 17.8 ± 1.7, p ≤ 0.05). In the ISO trial, all variables were significantly lower (ICT: 177.0 ± 10.2 b·min−1, 6.50 ± 1.85, 16.0 ± 1.9; CON: 182.8 ± 3.4 b·min−1, 7.0 ± 1.9, 16.3 ± 2.4, p ≤ 0.05). The changes did not differ significantly between groups (p > 0.05).
During the 30-minute ISO time trial, there was a significant increase in the mean tidal volume (ICT: 1.55 ± 0.29 vs. 1.66 ± 0.34 L; CON: 1.53 ± 0.19 vs. 1.60 ± 0.2 L, p ≤ 0.05) and a significant decrease in breathing frequency (ICT: 59.3 ± 16.7 breaths per minute vs. 54.4 ± 17.4 breaths per minute; CON: 56.1 ± 9.4 breaths per minute vs. 53.5 ± 5.4 breaths per minute, p ≤ 0.05) in comparison with the corresponding baseline values. The changes did not differ significantly between groups (p > 0.05).
Discussion
The main findings of this study were that the addition of a 6-week period of “functional” CM/IM training to an interval training program resulted in significantly greater improvements in 1-hour running performance, running economy at the speed of the OBLA, and in a SEPT.
Before the interval training, the participants in both groups undertook a 4-week IMT program. This specific training had 2 purposes, (a) to control for the established ergogenic effects of IMT in both groups (12); (b) to prepare a strong foundation within the inspiratory musculature for the subsequent challenge of the inspiratory-loaded CM training in the ICT group (16). As expected, after the IMT, the global IM function of both groups improved significantly. For example, P0 increased by an average of 23%, which is similar to changes observed using identical IMT in previous randomized placebo-controlled trials (21,24). Global CM function, as assessed by the SEPT, also improved in response to IMT, and the change was correlated with the improvement in P0. The related improvement in the global function of these 2 musculatures (r = 0.66) resulting from a specific IMT has, to the best of our knowledge, never been reported previously. The present findings are consistent with the notion of the dual role of IM in breathing and core stabilization that has been demonstrated during simultaneous ventilatory challenge and isometric torso task (17). Our findings are also consistent with our previous observation of a correlation (r = 0.77) between the severity of fatigue of the inspiratory and core musculatures when participants mimicked their ventilatory responses to a high-intensity running, while they were resting in a standing position (27). When the data in the present and previous studies were combined to analyze, the change in P0 in the participants explained approximately 80% of the variance in the change in SEPT performance. Collectively, these data provide evidence to support the existence of an essential role for the IM in global CM function during postural stabilizing tasks (13). The data also raise the possibility that enhancements of CM function and, in turn, core stabilization may be another contributory mechanism underlying the ergogenic effect of the specific IMT (12).
In agreement with our previous findings (27), there was evidence of running-induced fatigue of the IM and CM in both groups. The occurrence of CM fatigue suggests that the musculature had worked intensively during the run, providing core stiffness that presumably helped to optimize running form (3,14). We have previously shown that fatigue of the CMs is associated with impairment of high-intensity running performance (27). After the second, 6-week intervention with inspiratory-loaded CM training (CM/IM training) combined with the high-intensity interval program, global CM function enhanced further in ICT group, compared with that of their CON counterparts. In contrast, there was little further improvement IM function, which is consistent with the plateau of improvement in P0 that has been shown previously after 4- to 6-week of IMT (20,28). However, it may also be due to the fact that the volume of IMT was lower, compared with the preceding 4-week phase of specific IMT. During the CM/IM phase, the inspiratory load was kept constant, and the training frequency and repetitions were dictated by the related prescriptions of the interval and core training programs. At first sight, the absence of a further improvement in P0 during the CM/IM phase might seem to undermine the importance of the contribution of IMT to this intervention. However, most previous studies that have added CM training alone have failed to observe any improvements in performance (10,19) or of putative mechanistic factors, such as REOBLA. The important contribution of the IM to CM performance is supported by the significant improvement in SEPT after the IMT phase. During the second phase of the study, the ICT group experienced the additional challenge of inspiratory loading during tasks that challenged core stabilization, which led to a further improvement in SEPT performance and running performance. We believe that it is the unique combination of CM/IM training that explains why our study showed a beneficial contribution of CM training, whereas other studies have not (10,19).
Because the distance covered during the 1-hour time trial phase of the run increased in both ICT and CON groups postintervention, we can conclude that the interval training improved performance, which was expected. The enhanced performance could be partly attributed to the augmentation of aerobic energy utilization during exercise in adaptation to the 6-week high-intensity interval training that was revealed by the significant relationship between the improvements of OBLA and exercise performance. The consequently lower reliance on energy generated from anaerobic glycolysis is evidenced by the lower [La−]b in the postintervention ISO trial. In a previous study, we have demonstrated that the response to similar treadmill interval training is enhanced by a preceding period of IMT (25). Moreover, the improved breathing pattern, postexercise P0, and RPB in the postintervention ISO trial suggested that the enhanced IM function resulting from the IMT might have alleviated the IM fatigue and breathing effort during the running exercise, and contributed to the enhancement of the exercise performance (24). Given that our combination of interval training, preceded by IMT, has already been shown to maximize the outcome of interval training, it is all the more impressive that the addition of CM/IM improved performance further.
The ergogenic effect of the IMT almost certainly contributed to the enhancement of the exercise performance in both ICT and CON groups, through the direct ergogenic effect of IMT (15), as well as through the potentiation of the interval training stimulus (25). However, the relatively greater enhancement in the ICT group (3.04% vs. 1.57%, Cohen's effect size = 1.34) may be, at least partly, the result of the additional integrated CM/IM training. The interrelationships among the improvements of SEPT, REOBLA, and exercise performance suggest that the augmented global CM function, resulting from the CM/IM training in the ICT group, was responsible for the increased distance covered during the running time trial and that this was underpinned by optimizing running economy.
Running involves continuous alternate unilateral hip flexion and extension that creates corresponding trunk rotation in the runners in reaction to their leg movement (23). During running, the CM is responsible for stabilizing the trunk by absorption of the disruptive torques, thus minimizing the diversion of leg force exertion (2). The greater CM activation (assessed using electromyography normalized to maximal voluntary contraction) of endurance trained runners during running exercise, relative to that of untrained individuals, has been suggested to underlie their stable and efficient running form, and resultant superior running capacity (2). In this study, it is reasonable to postulate that the further increase in CM function of the ICT participants might have improved their running economy by creating a solid base in the lumbopelvic–hip region, such that lower limb movements during the run were performed in a more linear manner, improving running performance. In light of the current findings and of previous evidence that prior CM fatigue impairs performance during high-intensity treadmill running (27), it is reasonable to suggest that CM function is a factor limiting the capacity for high-intensity endurance running.
In this study, the CM/IM intervention was designed to address real-world situation during exercise, where there is competition between the respiratory and nonrespiratory functions of the IM. Although we did not compare with the outcomes of CM training alone, without loaded-respiratory activity incorporated, it is logical to presume that the current CM/IM conditioning maneuver applied in the ICT group could result in greater adaptations in the global CM function, and the adaptations would be more functionally relevant to endurance running. Nonetheless, the magnitude of CM fatigue (expressed as percentage of pre-exercise SEPT performance) remained unchanged during the ISO run trial. This response differs from that of P0, which showed a significantly attenuated fatigue in both groups postintervention. It is unlikely that the lack of change in CM fatigue is due to inadequate sensitivity of the SEPT, as the test has been shown to be capable of detecting changes of as little as 3% (26). Although we do not have a direct measure of CM activation during the running, it is reasonable to assume that the CM output was augmented posttraining, providing greater core stabilization, which was manifest as a reduction in REOLBA, but resulting in the same magnitude of CM fatigue. Such findings in addition of the marked enhancements in the running economy and the maximum performance of the running time trial inferred that there may be ample space in endurance runners to improve their running performance through the specific CM conditioning. This may seem to be contrary to the uncertainty of the CM training effects on athletic performance reported previously (10), but the current findings suggest that the “functional” element of the training may underpin its success in transferring the effect of CM conditioning to athletic performance.
Practical Applications
This study demonstrates that the application of a 4-week IMT enhances global IM and CM function simultaneously. Furthermore, integration of CM/IM training into a high-intensity interval training program for 6 weeks, enhances CM function further, and augments the summative effects of the IMT and interval training on 1-hour running time-trial performance, possibly by optimizing running economy. Based on these findings, it is recommended that IMT and running-specific CM/IM training are included within high-intensity interval programs for endurance runners. Whether CM/IM training improves performance in shorter running events and team sports remains to be explored.
Acknowledgments
A. K. McConnell declares a beneficial interest in the POWERbreathe brand of inspiratory muscle training products.
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