title: Knee extensor fatigue threshold is related to whole-body VO2max.
abstract: OBJECTIVE
  Above a given exercise intensity, rapid muscle fatigue will occur. We explored the possibility of assessing torque threshold for peripheral fatigue during single-legged repetitive isometric knee extensor exercise. We hypothesized this fatigue threshold to be related to the general aerobic fitness level and the so-called "critical torque" (CT) established with a recently validated 5-min all-out test.
  
  
  METHODS
  Seventeen healthy men (VO(2max) = 44.7-69.6 mL·kg(-1)·min(-1)) performed six submaximal (20%-55% maximal voluntary contraction [MVC]) 5-min bouts of 60 repetitive contractions (3-s on, 2-s off). Torque was changed between bouts in steps of 5% MVC to estimate the highest intensity (fatigue threshold) at which average changes in rsEMG, EMG median power frequency, and tissue deoxygenation (near-infrared spectroscopy) of the three superficial knee extensor muscles were still <5%, signifying steady-state exercise with minimal peripheral fatigue. On another occasion, one bout was performed in an all-out manner with end-test torque representing CT.
  
  
  RESULTS
  Fatigue threshold (40.0% ± 8.1% MVC) was related (r(2) = 0.57, P < 0.05) to CT (53.1% ± 10.0% MVC), but it was consistently lower (P < 0.05) and only fatigue threshold was significantly related to VO(2max) (r(2) = 0.68), and the first (r(2) = 0.45) and second (r(2) = 0.63) ventilatory threshold obtained during cycle ergometry.
  
  
  CONCLUSIONS
  Performing submaximal bouts of knee extensor contractions, while monitoring EMG and deoxygenation, seems a feasible manner to estimate an aerobic capacity-related exercise intensity of peripheral fatigue onset. This test may be used to evaluate changes in endurance capacity of single muscle groups, without the necessity for all-out testing, which could be problematic with frail subjects.
authors.affiliation: Research Institute MOVE, VU University Amsterdam, The Netherlands. c.j.de.ruiter@vu.nl
authors.forename: Cornelis J
authors.initials: CJ
authors.lastname: de Ruiter
authors.forename: Ellen A
authors.initials: EA
authors.lastname: Maas
authors.forename: Mariska G H
authors.initials: MG
authors.lastname: Wesseling
authors.forename: Arnold
authors.initials: A
authors.lastname: de Haan
mesh_terms: D000328:Adult; D004558:Electric Stimulation; D016552:Ergometry; D005080:Exercise Test / Q000295:instrumentation / Q000379:methods; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D007717:Knee / Q000502:physiology*; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*; D010809:Physical Fitness / Q000502:physiology; D019415:Torque; D055815:Young Adult