(51) INFLUENCE OF VIGOROUS INTENSITY HEART RATE-CLAMP EXERCISE ON NEUROMUSCULAR EFFICIENCY

Purpose: Exercise prescription to improve performance is often based on percentages of the maximum heart rate (HR_max) that reflect vigorous intensities (77-95%HR_max) to accumulate 20-60 min per day. Neuromuscular efficiency (NME: normalized maximal voluntary isometric contraction [MVIC] force divided by normalized electromyographic amplitude [EMG AMP]) provides a noninvasive assessment of neuromuscular fatigue. This study examined the changes in NME following HR-clamp trials within the vigorous intensity range. Methods: Ten men (mean±SD, 25.9±5.0yr, 178.7±6.1cm, 74.9±15.0kg, HR_max=178±12b×min^-1) performed a cycle ergometer graded exercise test, followed by three separate HR-clamp trials at 77%HR_max (HR_L, time to exhaustion [T_Lim]=58.5±4.6 min, range=45.3-60.0min), 86%HR_max (HR_M, T_Lim=48.4±16.5 min, range=20.33-60.0min), and 95%HR_max (HR_H, T_Lim=9.0±6.7 min, range=2.0-22.0min) to exhaustion or up to 60 min. EMG signals were measured from the vastus lateralis of the dominant leg during a 6-sec MVIC performed before (pre-test) and immediately after (post-test) the HR-clamp trials using a custom-built bike isometric torque collection hardware. The total work (TW) for each trial was determined by calculating the product of the average power output (PO) for the HR-clamp trial and the elapsed time in seconds. The pre- and post-test MVIC force and EMG AMP were normalized to the respective pre-test MVIC values and used to calculate NME. One-way repeated measures ANOVAs and post-hoc Bonferroni corrected t-tests were used to examine average PO and TW across HR-clamp trials. Separate, t-tests about a single mean (pre-test NME = 1.0) were used to examine NME for each HR-clamp trial. Results: There was significantly less TW performed for the HR_H (109±72kJ) compared to HR_L (475±81kJ) and HR_M (460±162kJ) trials (all p< 0.001), but there was no difference between HR_L and HR_M (p=1.000). There were significant differences among all HR-clamp trials for average PO (p< 0.001, HR_L:135.47±21.23W, HR_M:159.78±23.40W, HR_H:209.69±26.13W), but there were no significant differences in NME at post-test compared to pre-test (NME=1.0) for HR_L (p=0.309, NME=0.94±0.35) or HR_H (p=0.087, NME=1.23±0.49). NME was significantly reduced for the HR_M trial (p=0.009, NME=0.81±0.21). Conclusions: Only the HR_M trial demonstrated a reduction in NME, despite similar TW between the HR_L and HR_M trials. This may have been driven by the location of the trials relative to the moderate, heavy, and severe exercise intensity domains. Specifically, the greater average PO for the HR_M trial compared to HR_L may have placed subjects in the heavy domain for a longer duration of the trial, resulting in greater metabolic byproduct accumulation and driving peripheral and central factors of fatigue. In contrast, the greater PO but shorter T_Lim and less accumulated work at HR_H trial may have limited the influence of accumulation on peripheral and central factors of fatigue. Practical Applications: When utilizing HR-clamp exercise, coaches and practitioners may consider the middle of the recommended intensity range to elicit a neuromuscular stimulus. The lower end of the range will allow for similar TW compared to the middle HR range but may avoid the accumulation of neuromuscular fatigue. The higher end of the HR range may be used for intervals, but a single session does not meet the recommended accumulated time for exercise prescription to improve endurance performance. Acknowledgements: None