(6) INTENSITY SPECIFIC PHYSIOLOGICAL RESPONSES DURING SINGLE-LIMB LEG EXTENSION REPETITIONS TO FAILURE

Purpose: Critical power (CP) is an individualized fatigue threshold that separates the heavy from severe (SEV) exercise intensity domains for whole-body exercise and is derived as the slope of the relationship between total work and time to task failure (Tlim) at intensities performed in the SEV domain (Tlim = 2-15 min). The critical load (CL) has been established for resistance-based exercises but is derived from loads that result in Tlim in < 2 min. It is likely the CL estimate uses loads in the recently established “extreme” domain (EXT), and may not be analogous to the CP, as previously suggested. The purpose of this study is to determine if similar estimates of the CL are derived when using loads associated with the SEV (Tlim 2-15 min) and EXT domains (Tlim < 2 min) and evaluate physiological responses across domains. Methods: 13 trained participants (age: 22±1 yrs) completed a 1-repetition maximum (1RM) for the single-limb leg extension (dominant limb) and repetitions to failure at three loads in the SEV (Tlim in 2-15 minutes; S1-S3) and three loads in the EXT (70%, 80%, 90% 1RM), in a random order. Electromyography (EMG) was measured from the vastus lateralis and performance fatigability (PF) was assessed from pre- and post-exercise maximal voluntary isometric contractions (MVIC; %D). To determine the SEV (S1, S2, S3) and EXT (70%, 80%, 90% 1RM), the load lifted (kg) x the number of repetitions completed versus the number of repetitions completed was plotted for each domain. Paired samples t-tests were used to compare the slopes between the 2 relationships. Separate one-way ANOVAs and pairwise comparisons were used to examine %D EMG AMP from the first to last 10% of repetitions completed and PF. Results: The slope derived for SEV (2.6±2.0% 1RM, r=0.672±0.32) was less than the EXT (49.0±16.2% 1RM, r=0.99±0.01; p< 0.001). The %D EMG AMP was greater for SEV (49.3±20.5%∆) compared to EXT intensities (11.3±16.4%∆; p£0.09 for all SEV vs. EXT comparisons). There were greater changes in PF for all the SEV (-61.2±11.3%∆) compared to EXT intensities (-16.0±10.8%∆; p£0.001 for SEV vs. EXT comparisons). %∆ EMG AMP and PF were not different between intensities within the SEV domain (p >0.05) or intensities within the EXT domain (p=1.00). Conclusions: The slopes of the relationship between total work and repetitions completed when using intensities within the SEV (S1, S2, S3) vs. EXT (70%, 80%, 90% 1RM) domains provided different CL estimates. This suggests the existence of two separate fatigue thresholds, and that CL is not analogous to the CP, as previously suggested. Further, both %D EMG AMP and PF decreased from the SEV to EXT. This indicates the rate of increase of muscle excitation was lower at higher intensities, likely due to greater EMG AMP required at the onset of exercise at EXT intensities. The greater change in PF during SEV domain exercise suggests longer duration exercise may induce more fatigue, likely due to increased metabolite accumulation. Practical Applications: Most resistance exercise performed in athletic settings is completed at intensities within the EXT domain. Understanding the unique fatigue profile of loads associated with this domain provides insight into training adaptations and limitations during exercise performed at those intensities. Identifying these acute fatigue responses will allow coaches and practitioners to design training programs that target the appropriate neuromuscular stimulus desired for each individual. Acknowledgements: None