Blood Flow Restricted Exercise Alters Motor Unit Recruitment And Firing Rate

PURPOSE: Despite suggestions that low-intensity (LI) blood flow restricted exercise (BFRE) may be as effective as high intensity (HI) resistance exercise for the purpose of enhancing the recruitment of type II motor units (MU); individual MU acute response to LI BFRE has never been observed and/or reported. Recently, the decomposition of the surface electromyographic (EMG) signals introduced new analysis methods to assess single MU properties. Through these methods we aimed to observe LI BFRE effects on the behavior of the individual MUs. METHODS: Eight men (26.0 ± 3.8 yrs) performed 5 sets of 15 reps of knee extensions at 20% 1RM (with and without BFR). BFR condition was set at 60% of the individual absolute arterial occlusion value. Torque was determined during preand post-exercise maximal voluntary contractions (MVC). Surface EMG activity was recorded from the vastus lateralis (VL) at preand post-exercise time point measurements. In each of these time points, isometric voluntary contractions were performed, matching trapezoidal target-force trajectories at 40% pre-MVC. Resulting surface EMG signals were decomposed and MU recruitment threshold, firing rates and MU action potential (MUAP) amplitudes were further analyzed. RESULTS: Torque only decreased after the LI BFRE condition (-20.5%; p<0.05). In the regression lines between MU recruitment threshold and firing rate, LI BFRE induced higher decrements in the linear slope coefficient (-165.1% ± 120.4, p<0.05) when compared to LI condition (-44.4 ± 33.1, p<0.05). Also, the MU firing rate vs. MUAP amplitude relationship had a notable shift to higher firing rate and MUAP amplitude values after the BFRE condition, reinforcing the evidence that new MUs with higher MUAP amplitude are recruited and MUs with similar MUAP amplitudes are activated at higher firing rates. CONCLUSION: LI BFRE induced a significant change in the MU recruitment pattern, with higher-threshold and lower-firing rate MUs being recruited earlier to compensate muscle failure. Moreover, there were strong evidences about an elevated firing rate in similar MUAP amplitude after LI BFRE condition. Acknowledgements: The authors would like to express their gratitude to Dr. Paola Contessa (Delsys Inc., Natick, USA), for all the invaluable assistance, comments and support.