title: Mechanisms of force depression caused by different types of physical exercise studied by direct electrical stimulation of human quadriceps muscle.
abstract: OBJECTIVE
  Force production frequently remains depressed for several hours or even days after various types of strenuous physical exercise. We hypothesized that the pattern of force changes during the first hour after exercise can be used to reveal muscular mechanisms likely to underlie the decline in muscle performance during exercise as well as factors involved in the triggering the prolonged force depression after exercise.
  
  
  METHODS
  Nine groups of recreationally active male volunteers performed one of the following types of exercise: single prolonged or repeated short maximum voluntary contractions (MVCs); single or repeated all-out cycling bouts; repeated drop jumps. The isometric force of the right quadriceps muscle was measured during stimulation with brief 20 and 100 Hz trains of electrical pulses given before and at regular intervals for 60 min after exercise.
  
  
  RESULTS
  All exercises resulted in a prolonged force depression, which was more marked at 20 Hz than at 100 Hz. Short-lasting (≤2 min) MVC and all-out cycling exercises showed an initial force recovery (peak after ~ 5 min) followed by a secondary force depression. The repeated drop jumps, which involve eccentric contractions, resulted in a stable force depression with the 20 Hz force being markedly more decreased after 100 than 10 jumps.
  
  
  CONCLUSIONS
  In accordance with our hypothesis, the results propose at least three different mechanisms that influence force production after exercise: (1) a transiently recovering process followed by (2) a prolonged force depression after metabolically demanding exercise, and (3) a stable force depression after mechanically demanding contractions.
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania.
authors.forename: Albertas
authors.initials: A
authors.lastname: Skurvydas
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania.
authors.forename: Gediminas
authors.initials: G
authors.lastname: Mamkus
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania.
authors.forename: Sigitas
authors.initials: S
authors.lastname: Kamandulis
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania.
authors.forename: Vilma
authors.initials: V
authors.lastname: Dudoniene
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania.
authors.forename: Dovile
authors.initials: D
authors.lastname: Valanciene
authors.affiliation: Institute of Sports Science and Innovation, Lithuanian Sports University, Lithuania, Sporto 6, 44221, Kaunas, Lithuania. hakan.westerblad@ki.se.|Department of Physiology and Pharmacology, Karolinska Institutet, 171 77, Stockholm, Sweden. hakan.westerblad@ki.se.
authors.forename: Håkan
authors.identifier: http://orcid.org/0000-0002-8180-3029
authors.initials: H
authors.lastname: Westerblad
mesh_terms: D004558:Electric Stimulation / Q000379:methods*; D015444:Exercise / Q000502:physiology*; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D053580:Muscle Strength / Q000502:physiology*; D064797:Physical Conditioning, Human / Q000379:methods / Q000502:physiology; D010807:Physical Endurance / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology*; D020127:Recovery of Function / Q000502:physiology; D055815:Young Adult
keywords: Eccentric contractions; Isometric contraction; Prolonged low-frequency force depression; Recovery; Wingate cycling