Effect Of Proximity To Failure During Resistance Training On Longitudinal Markers Of Muscle Damage

PURPOSE: To compare the longitudinal change in the acute response of indirect markers of muscle damage (i.e., Creatine Kinase [CK], Lactate Dehydrogenase [LDH]) between training programs with different proximities to failure using the repetitions in reserve (RIR)-based rating of perceived exertion (RPE) scale. METHODS: Nineteen males (height: 174.60 ± 6.23 cm, body mass: 80.43 ± 10.97 kg) with ≥2 yrs of training experience were assigned to one of two groups: 1) 4-6 RPE, n = 9 (4-6 RIR), or 2) 7-9 RPE, n = 10 (1-3 RIR) for 8wks. Both groups performed the squat, bench press, and other accessory exercises 3x/wk on non-consecutive days (e.g., Mon., Wed., Fri.). The only difference between groups was the proximity to failure of each set (4-6 RPE = 4-6 RIR, 7-9 RPE = 1-3 RIR). Serum was collected immediately before and after the first training session of weeks 1, 2, and 7, and prior to the second training session of the same weeks. Linear mixed effects models were fit for each biomarker separately with fixed effects, and interactions thereof, for group, week, and timepoint. Random effects were set so that the intercept could vary per participant to account for repeated measures. Model coefficients are presented with 95% confidence intervals. Additionally, pairwise comparisons were made between groups to generate standardized mean differences for the damage response observed for each week pooled between groups. RESULTS: The model coefficient for the interaction of week by time point demonstrate evidence of the repeated bout effect for the response of CK (b = -8.52 [-19.25-2.22]), but not LDH (b = 1.05 [-15.19-17.29]) when groups are pooled. For the interaction of group by week model coefficients suggest the 7-9 RPE group demonstrated a greater acute response of CK (b = 13.43 [4.88-21.97]) and LDH (b = 13.44 [0.52-26.37]). Moreover, between groups standardized mean differences were g = 0.61, 0.12, -0.26 for CK and g = 0.41, -0.12, -0.18 for LDH in weeks 1, 2, and 7, respectively. CONCLUSIONS: Our findings indicate that resistance training with a 7-9 RPE may elicit marginally greater acute responses of CK and LDH than 4-6 RPE over time.