title: The Etiology of Muscle Fatigue Differs between Two Electrical Stimulation Protocols.
abstract: This study aimed at investigating the mechanisms involved in the force reduction induced by two electrical stimulation (ES) protocols that were designed to activate motor units differently.
  
  
  
  The triceps surae of 11 healthy subjects (8 men; age, ~28 yr) was activated using ES applied over the tibial nerve. Two ES protocols (conventional [CONV]: 20 Hz, 0.05 ms vs wide-pulse high-frequency [WPHF]: 80 Hz, 1 ms) were performed and involved 40 trains (6 s on-6 s off) delivered at an intensity (IES) evoking 20% of maximal voluntary contraction. To analyze the mechanical properties of the motor units activated at IES, force-frequency relation was evoked before and after each protocol. H-reflex and M-wave responses evoked by the last stimulation pulse were also assessed during each ES protocol. Electromyographic responses (∑EMG) were recorded after each train to analyze the behavior of the motor units activated at IES. Metabolic variables, including relative concentrations of phosphocreatine and inorganic phosphate as well as intracellular pH, were assessed using P-MR spectroscopy during each protocol.
  
  
  
  Larger H-reflex amplitudes were observed during WPHF as compared with CONV, whereas opposite findings were observed for M-wave amplitudes. Despite this difference, both the force reduction (-26%) and metabolic changes were similar between the two protocols. The CONV protocol induced a rightward shift of the force-frequency relation, whereas a significant reduction of the ∑EMG evoked at IES was observed only for the WPHF.
  
  
  
  These results suggest that a decreased number of active motor units mainly contributed to WPHF-induced force decrease, whereas intracellular processes were most likely involved in the force reduction occurring during CONV stimulation.
authors.affiliation: 1Aix-Marseille University, CNRS, Center for Magnetic Resonance in Biology and Medicine (CRMBM) UMR 7339, Marseille, FRANCE; 2INSERM, Cognition, Action, and Sensorimotor Plasticity (CAPS) UMR 1093, University of Burgundy, Faculty of Sport Sciences, Dijon, FRANCE; 3APHM, La Timone Hospital, Imaging Center, CEMEREM, Marseille, FRANCE; and 4Deparment of Rheumatology, Sainte Marguerite Hospital, Marseille, FRANCE.
authors.forename: Alain
authors.initials: A
authors.lastname: Martin
authors.forename: Sidney
authors.initials: S
authors.lastname: Grosprêtre
authors.forename: Christophe
authors.initials: C
authors.lastname: Vilmen
authors.forename: Maxime
authors.initials: M
authors.lastname: Guye
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Mattei
authors.forename: Yann
authors.initials: Y
authors.lastname: LE Fur
authors.forename: David
authors.initials: D
authors.lastname: Bendahan
authors.forename: Julien
authors.initials: J
authors.lastname: Gondin
mesh_terms: D000328:Adult; D004558:Electric Stimulation / Q000379:methods*; D004576:Electromyography; D005260:Female; D006181:H-Reflex / Q000502:physiology; D064368:Healthy Volunteers; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011999:Recruitment, Neurophysiological / Q000502:physiology; D013979:Tibial Nerve / Q000502:physiology; D055815:Young Adult