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Luu MJ, Jones KE, Collins DF. Decreased excitability of motor axons contributes substantially to contraction fatigability during neuromuscular electrical stimulation. Appl Physiol Nutr Metab 2021; 46:346-355. [DOI: 10.1139/apnm-2020-0366] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
The present study was designed to (i) determine the time course of changes in motor axon excitability during and after neuromuscular electrical stimulation (NMES); and (ii) characterize the relationship between contraction fatigability, NMES frequency, and changes at the axon, neuromuscular junction, and muscle. Eight neurologically intact participants attended 3 sessions. NMES was delivered over the common peroneal nerve at 20, 40, or 60 Hz for 8 min (0.3 s “on”, 0.7 s “off”). Threshold tracking was used to measure changes in axonal excitability. Supramaximal stimuli were used to assess neuromuscular transmission and force-generating capacity of the tibialis anterior muscle. Torque decreased by 49% and 62% during 8 min of 40 and 60 Hz NMES, respectively. Maximal twitch torque decreased only during 60 Hz NMES. Motor axon excitability decreased by 14%, 27%, and 35% during 20, 40, and 60 Hz NMES, respectively. Excitability recovered to baseline immediately (20 Hz) and at 2 min (40 Hz) and 4 min (60 Hz) following NMES. Overall, decreases in axonal excitability best predicted how torque declined over 8 min of NMES. During NMES, motor axons become less excitable and motor units “drop out” of the contraction, contributing substantially to contraction fatigability and its dependence on NMES frequency. Novelty: The excitability of motor axons decreased during NMES in a frequency-dependent manner. As excitability decreased, axons failed to reach threshold and motor units dropped out of the contraction. Overall, decreased excitability best predicted how torque declined and thus is a key contributor to fatigability during NMES.
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Affiliation(s)
- M. John Luu
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
| | - Kelvin E. Jones
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
| | - David F. Collins
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
- Faculty of Kinesiology, Sport, and Recreation, Neuroscience and Mental Health Institute, Sensorimotor Rehabilitation Neuroscience Group, University of Alberta, Edmonton, AB T6G 2H9, Canada
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Wernbom M, Aagaard P. Muscle fibre activation and fatigue with low-load blood flow restricted resistance exercise-An integrative physiology review. Acta Physiol (Oxf) 2020; 228:e13302. [PMID: 31108025 DOI: 10.1111/apha.13302] [Citation(s) in RCA: 42] [Impact Index Per Article: 10.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2019] [Revised: 05/12/2019] [Accepted: 05/17/2019] [Indexed: 12/13/2022]
Abstract
Blood flow-restricted resistance exercise (BFRRE) has been shown to induce increases in muscle size and strength, and continues to generate interest from both clinical and basic research points of view. The low loads employed, typically 20%-50% of the one repetition maximum, make BFRRE an attractive training modality for individuals who may not tolerate high musculoskeletal forces (eg, selected clinical patient groups such as frail old adults and patients recovering from sports injury) and/or for highly trained athletes who have reached a plateau in muscle mass and strength. It has been proposed that achieving a high degree of muscle fibre recruitment is important for inducing muscle hypertrophy with BFRRE, and the available evidence suggest that fatiguing low-load exercise during ischemic conditions can recruit both slow (type I) and fast (type II) muscle fibres. Nevertheless, closer scrutiny reveals that type II fibre activation in BFRRE has to date largely been inferred using indirect methods such as electromyography and magnetic resonance spectroscopy, while only rarely addressed using more direct methods such as measurements of glycogen stores and phosphocreatine levels in muscle fibres. Hence, considerable uncertainity exists about the specific pattern of muscle fibre activation during BFRRE. Therefore, the purpose of this narrative review was (1) to summarize the evidence on muscle fibre recruitment during BFRRE as revealed by various methods employed for determining muscle fibre usage during exercise, and (2) to discuss reported findings in light of the specific advantages and limitations associated with these methods.
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Affiliation(s)
- Mathias Wernbom
- Center for Health and Performance, Department of Food and Nutrition and Sport Science University of Gothenburg Gothenburg Sweden
- Department of Health and Rehabilitation, Institute of Neuroscience and Physiology, Sahlgrenska Academy University of Gothenburg Gothenburg Sweden
| | - Per Aagaard
- Department of Sports Sciences and Clinical Biomechanics, SDU Muscle Research Cluster (SMRC) University of Southern Denmark Odense M Denmark
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Desaulniers P, Lavoie PA, Gardiner PF. Effect of rat soleus muscle overload on neuromuscular transmission efficacy during continuous and intermittent activation. Exp Physiol 2005; 90:333-40. [PMID: 15640276 DOI: 10.1113/expphysiol.2004.029132] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Increased neuromuscular activity is known to provoke morphological and functional adaptations at the neuromuscular synapse. Most of these changes have been documented following endurance exercise training programmes. In this study, the effect of rat soleus muscle overload produced by tenotomy plus voluntary wheel-cage activity on neuromuscular transmission efficacy was investigated. The overload protocol increased miniature endplate potential (MEPP) and endplate potential (EPP) amplitudes by 17 and 19%, respectively (both P < 0.01), and increased MEPP frequency by 86% (P < 0.01). EPP amplitude rundown during continuous trains of activation was attenuated by approximately 10% in the overloaded group (P < 0.01). Also, during intermittent activation, the overload protocol attenuated EPP amplitude rundown, mainly by enhancing EPP amplitude recovery by approximately 10% during the quiescent periods (P < 0.01). Although the present results show that both the degree and direction of adaptation are similar to what has been observed at rat soleus neuromuscular junctions following an endurance training protocol, there are important nuances between the results, suggesting different mechanisms through which these changes may occur.
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