Introduction: Exercise-induced muscle damage (EIMD), caused by unaccustomed eccentric muscle contractions and subsequent mechanical damage, inflammation, and oxidative responses, is associated with soreness and impaired muscle function. Understandably, athletes often seek to mitigate these consequences by consuming anti-inflammatory (e.g. ibuprofen) and antioxidative (e.g. vitamin C) supplements so as not to interrupt training. However, it remains unclear whether such interventions impair adaptive responses, including the repeated bout effect (RBE), which attenuates subsequent muscle damage. Previous investigations have employed inadequate protocols to assess this.

Objectives: Assess whether anti-inflammatory (e.g. ibuprofen) and antioxidative (e.g. vitamin C) supplements attenuate the development of the RBE.

Methods: 23 adults (mean(SD) age: 22(1), BMI: 23.3(2.3) kg·m^-2, ♀:♂: 12:11) participated in a randomised, double blind, placebo controlled parallel group study. Participants completed two 20-minute bouts of stepping exercise to a bench 110% of tibia length with the dominant leg always acting eccentrically, separated by 3-weeks. In visit 1, one group consumed vitamin C (1000 mg) 2-h pre-exercise and ibuprofen (400 mg every 8-h) for 48-h post-exercise (SUPP), while the other consumed matched placebos (PLA). In visit 2, the acute effects of supplementation were removed, with all participants receiving placebo; the only difference between groups were the residual effects of treatment from visit 1. To ensure internal validity, the study was described such that it could be interpreted as being a randomised crossover design. This concealed its true objectives in researching the RBE, meaning volitional measures were not influence by the expectation of reduced damage. EIMD was quantified through jump height and force-velocity profiling using a leg press dynamometer at baseline and immediately-, 2-h-, 24-h- and 7-d-post-exercise. Membrane permeability (creatine kinase (CK)) was assessed at baseline and immediately, 1-, 2-, 24-, and 48-h-post-exercise. All research was approved by a University of Bath Research Ethics Committee.

Results: Following the bout of eccentric exercise in visit 1, indices of EIMD were evident in both groups, with reductions in jump height and interpolated maximum power (Pmax) at 2- and 24-h (p<0.05), but this did not differ between groups (p>0.59). Peak post-exercise CK was increased for both groups (p<0.05), with this increase being numerically greater in PLA (SUPP: 32.8(56.2) vs PLA: 185.0(286.7) U·L^-1; p=0.08; d=0.74). After exercise in visit 2, no between group differences in functional outcomes were observed; however, CK responses suggested an altered repeated bout effect. Compared to visit 1, CK responses in visit 2 were numerically, but not statically, attenuated in PLA (185.0(286.7) vs 121.5(134.0) U·L^-1; p=0.21; d=0.28) whilst accentuated in SUPP (32.8(56.2) vs 130.1(214.2) U·L^-1; p=0.27; d=0.62), and while time*group interaction was not statistically significant, the effect size indicated a potentially meaningful between-group difference throughout visit 2 (p=0.23; partial η²=0.14; d=0.78).

Conclusion: This interim analysis suggests mitigating EIMD indices through supplementation may potentially alter the RBE response of muscle membrane proteins to subsequent eccentric exercise without influencing functional markers. We intend to fulfil the target sample size of 52, and broaden the analysis of physiological markers to include more comprehensive assessment of muscle damage, oxidative stress, and inflammation.