The effect of exercise-induced pain on endurance performance, and strategies to mitigate its impact

Exercise-induced pain (EIP) is a natural consequence of exercising intensely, and results due to an accumulation of endogenous algesic substances, an increase in muscular pressure and muscular distortion or tissue damage. However, the presence of EIP may have negative consequences for exercise and endurance performance, brought about by the physiological and/or psychological effect of pain. EIP has not been widely addressed in sport and exercise science research, and much of the contemporary literature has ignored its potential role in endurance exercise performance, despite the wide acknowledgement it gains in interviews with athletes, coaches, exercise scientists and health and fitness practitioners. Therefore, more empirical research needs to be completed that explores the role of EIP in endurance performance, and the physiological and/or psychological contribution it may make to fatigue and work rate regulation. Therefore, the main purpose of this thesis was to examine the effect of EIP on endurance exercise performance, and identify strategies to mitigate its impact in various endurance exercise tasks. Consequently, this thesis consists of 5 experimental studies, as outlined below. The 1st experimental study (Chapter 3) assessed the relationship between traditional experimental measures of pain (the cold pressor test (CPT) and algometry), EIP tolerance and participants' performance of a 10 mile (16.1 km) cycling time trial. The primary finding was that no correlation was found between experimental pain measures and TT performance (mean pain in CPT; R = 0.222; time lasted in the CPT; R = -0.292; PPT; R = -0.016). However, there was a significant correlation between EIP tolerance and TT performance (R = -0.83, P < 0.01). Correlation analysis revealed significant (P < 0.01) relationships between TT completion time and VO2max (R = -0.816, P < 0.001), PPO (R = -0.864, P < 0.001), GET (R = -0.454, P = 0.009), and RPE tolerance (R = -0.736, P < 0.01). Hierarchical multiple regression for physiological parameters (VO2max, GET and PPO) revealed that a significant model emerged (F (1,30) = 88.586, P < 0.01) when only PPO was used to predict TT completion time. PPO explained 74.7% variance (R Square = 0.747, Adjusted R Square = 0.739, ?R Square = 0.747, F (1,30) = 88.586, P < 0.01, Beta = - 0.864). Stepwise regression for pain and RPE predictor variables (mean pain in CPT, time lasted in the CPT, PPT, EIP tolerance, and RPE tolerance) revealed that all variables with the exception of time lasted in CPT and RPE tolerance contributed to a predictive model. EIP tolerance predicted TT completion time and explained 69.4% variance (R Square = 0.694, Adjusted R Square = 0.684, ?R Square = 0.694, F (1, 30) = 68.075, P < 0.01, Beta = - 0.833), PPT explained additional 4% variance (R Square = 0.040, Adjusted R Square = 0.716, ?R Square = 0.040, ?F (1, 29) = 4.390, P = 0.045, Beta = - 0.886), and mean pain in CPT also explained additional 4.4% variance (Square = 0.044, Adjusted R Square = 0.754, ?R Square = 0.044, ?F(1, 28) = 5.543, P = 0.026, Beta = - 0.881). Therefore, EIP tolerance, PPT and mean pain in CPT explained 77.8% variance in TT completion time. Regression analysis for pain and physiological predictor variables (mean pain in CPT, PPT, EIP tolerance, VO2max, PPO, GET) revealed that a significant model (P < 0.01) emerged when only PPO (Adjusted R Square = 0.739) and EIP tolerance (?R Square = 0.075) were used to predict TT performance. Therefore, PPO and EIP tolerance explained an overall 82.2% variance in the model. This study demonstrated for the first time that tolerance of EIP provides a good predictor of endurance performance, whereas traditional measures of pain do not. It is suggested that participants who are able to tolerate a greater pain for longer time period, are able to maintain a higher work rate and therefore finish the endurance performance task faster. The results suggest that EIP plays a crucial role in endurance performance, and that a high tolerance for EIP provides an important role as a predictor of endurance athletic performance. Finally, this study demonstrates that psychological variables (in this case pain tolerance), should be considered alongside physiological (e.g. VO2max, lactate threshold, exercise economy) variables, in identifying the determinants of endurance performance. The 2nd experimental study (Chapter 4) examined the effect of mirror visual feedback on EIP during isometric performance. Specifically, mirror visual feedback was used to deceive participants about the difficulty of the exercise task they were engaging in. It was hypothesised that increasing perceived task difficulty would increase expectation of EIP and reduce time to exhaustion, whereas decreasing perceived would elicit the opposite effect. The results supported the study hypothesis, and showed that the deception of task difficulty in the Experimental group led participants to produce significantly longer times to exhaustion when they thought the task was easier than it was, and significantly shorter times to exhaustion when they thought it was harder than it was (F (1,40) = 4.293, P = 0.045). The ANOVA revealed a significant main effect of condition for EIP during the TTE test (F (1, 40) = 8.736, P = 0.005), and a significant interaction effect of EIP between groups for each time condition were observed (F (1,40) = 7.163, P = 0.011). The ANOVA revealed a significant main effect of condition for RPE during the TTE test (F (1, 40) = 33.403, P < 0.001), and a significant interaction effect of RPE between groups for each time condition (F (1,40) = 13.367, P < 0.001). This was accompanied by significantly higher EIP and RPE when they thought the task was harder than it was, and significantly lower EIP and RPE when they thought the task was easier than it was. This is the first experimental study using the mirror box technique as a strategy to moderate EIP during isometric contractions. The results suggest that perceptions about exercise have a consequence for the EIP arising from them, supporting the psychological and subjective dimensions of pain perception. Previous experiments investigating transcutaneous electrical nerve stimulation (TENS) and interferential current (IFC) have been shown to elicit analgesic effects in a variety of conditions. Considering the emerging experiments and application of these techniques on exercise and the potential benefits of these strategies to mitigate of EIP impact, the 3rd experimental study (Chapter 5) investigated the effect of TENS and IFC on EIP during single limb, submaximal isometric contraction in healthy volunteers. The primary finding was that the ANOVA revealed a significant difference in the time to exhaustion between conditions (F (2, 34) = 6.763, P = 0.003). Pairwise comparisons revealed a significantly different TTE time between TENS (10 min 49 s ± 6 min 16 s) and SHAM conditions (7 min 52 s ± 2 min 51 s) (P = 0. 031) and between IFC (11 min 17 s ± 6 min 23 s) and SHAM conditions (P = 0.02). No significant difference between TENS and IFC conditions was observed (P > 0.05). The ANOVA also revealed a significant main effect of condition for exercise-induced pain during the TTE test (P = 0.035). No significant changes in rating of perceived exertion (RPE) were found between the three conditions (P > 0.05). A 3 x 8 (condition x iso-time) ANOVA revealed a significant interaction effect for exercise-induced pain over time between conditions during the TTE test with lower pain intensity in the TENS and IFC conditions (F (3.4, 58.4) = 3.671, P = 0.013). No interaction and main effects for RPE were found between the three conditions (P > 0.05). For the MVC, paired-sample t-tests demonstrated that MVC was significantly reduced following the TTE in the Sham (t (17) = 9.069, P < 0.001), TENS (t (17) = 7.037, P < 0.001) and IFC conditions (t (17) = 8.558, P < 0.001). No significant differences between conditions were found for the pre-MVC (F (1.4, 23.4) = 1.758, P = 0.188) or the post-MVC (F (2, 34) = 1.499, P = 0.238). This is the first experiment investigating the analgesic effect of TENS during exercise that uses a randomised, crossover and placebo controlled design. This experiment demonstrated for the first time that eliciting a reduction in EIP through TENS resulted in an improvement in single limb exhaustive exercise. An additional novel finding from this study was that the reduced EIP and improved endurance performance occurred despite no effect on RPE. The results suggest that TENS and IFC can elicit an analgesic effect on EIP, and that this reduction in muscle pain can improve time to exhaustion performance in the absence of changes to perceived exertion. The results suggest that EIP is a limiter of endurance performance in single limb exhaustive exercise, and questions the notion that changes to RPE must always occur when endurance performance is affected.The 4th experiment (Chapter 6) sought to apply the results obtained in the 3rd experiment to the performance of a 10-mile cycling TT in trained cyclists. The novel finding was that the ANOVA revealed a significant difference in completion time between conditions (F (2, 42) = 6.597, P = 0.003). Pairwise comparisons revealed that participants performed a significantly faster TT (P = 0.001) in the TENS condition (29 min 6 s ± 3 min 20 s) compared to the SHAM (29 min 39 s ± 3 min 34 s) condition. There were no significant differences (P = 0.872) between the IFC condition (29 min 28 s ± 3 min 34 s) and the SHAM, or the TENS and IFC conditions (P = 0.116). The ANOVA also revealed a significant main effect of condition for power output (F (2, 38) = 3.48, P = 0.041), mean HR (F (1.38, 29.06) = 4.016, P = 0.042) and mean B[La] (F (1.49, 31.37) = 7.54, P = 0.004). There was a significant difference in the mean EIP between conditions during the TT (F (2, 44) = 4.210, P = 0.022). Paired t-tests revealed that participants perceived significantly less pain during the TENS condition (3.5 ± 1.8) than in the sham condition (4.0 ± 2.0) (t (21) = 3.037, P = 0.006). No differences were observed between the TENS and the IFC condition (3.8 ± 1.9) or the IFC and Sham condition (P > 0.05). No significant differences in mean RPE were found between conditions during the TT (P > 0.05). Interestingly, this study also showed that TENS elicits an analgesic effect on EIP and improves the TT performance, whereas IFC technique does not elicit any reduction of EIP and consequently has no effect on whole-body endurance performance. This experiment demonstrated the first time that TENS intervention significantly improved completion time of the cycling TT, and that this was attained by the cyclists sustaining a greater power output (PO), heart rate (HR) and blood lactate (B[La]). Regardless of the increased physiological stress and metabolic rate induced by the higher PO, participants perceived EIP in the TENS strategy alongside in the absence of a difference in RPE between conditions. The improvement in dynamic endurance was probably the result of reduction in EIP for a given load. This is the first experiment showing that a TENS intervention can be used to elicit this analgesia to EIP, and suggests that there may be scope for TENS to be used during exercise in those where EIP negatively effects their engagement in physical activity. The final experiment in this thesis (Chapter 7) examined the effect of mood and emotional state on EIP and endurance performance. The use of painful images prior to endurance cycling performance was used to negatively affect mood, which was hypothesised to increase EIP. The primary finding was that the ANOVA revealed a significant difference in completion time between conditions (F (2, 40) = 8.480, P = 0.001). Pairwise comparisons revealed that participants performed a significantly faster TT (P = 0.003) in the pleasant condition (29 min 38 s ± 4 min 35 s) and the neutral condition (29 min 39 s ± 3 min 34 s) compared to the painful condition (30 min 19 s ± 5 min 7 s). There were no significant differences between the neutral condition and the pleasant (P = 1.000). The ANOVA also revealed a significant difference in PO (F (2, 40) = 6.318, P = 0.004), mean HR ((F (2, 40) = 4.502, P = 0.017) and mean B[La] (F (2, 40) = 5.724, P = 0.007) between conditions during ?the TT cycling performance, but no significant effect of condition for mean RPE or EIP (P > 0.05). In the FP, a ?significant main effect of condition for EIP (F (2, 40) = 4.363, P = 0.019), but no difference for RPE, HR or B[La]. This experiment demonstrated the first time that painful images negatively affect mood and elicit a compassionate hyperalgesia response to exercise. The results demonstrate that an increased pain sensation during exercise (induced via compassional hyperalgesia) can decrease TT performance, and highlights there is an emotional element to the processing of EIP that can be influenced by compassional hyperalgesia. This is probably the consequence of 'top-down' processing increasing the pain sensation elicited by a given 'bottom-up' stimulus. These results highlight the importance of maintaining a positive mood and emotional state prior to and during exercise. The experimental studies performed as part of this thesis provides unique empirical evidence to advance scientific knowledge and understanding of the phenomenon of EIP. This thesis provides further new insights into how different interventions both alleviate and exacerbate EIP, which subsequently influences endurance exercise performance. Furthermore, considering the lack of knowledge regarding the testing and role of EIP in exercise, this thesis contributes to and enhances scientific understanding for how to test for and control these variables.