title: Passive muscle length changes affect twitch potentiation in power athletes.
abstract: BACKGROUND
  A conditioning maximal voluntary muscle action (MVC) has been shown to induce postactivation potentiation, that is, improved contractile muscle properties, when muscles are contracted isometrically. It is still uncertain how the contractile properties are affected during ongoing muscle length changes. The purpose of this study was to investigate the effects of a 6-s conditioning MVC on twitch properties of the plantarflexors during ongoing muscle length changes.
  
  
  METHODS
  Peak twitch, rate of torque development, and rate of torque relaxation, rising time, and half relaxation time were measured from supramaximal twitches evoked in the plantarflexors of 11 highly trained athletes. Twitches were evoked before a 6-s MVC and subsequently on eight different occasions during a 10-min recovery for five different modes: fast lengthening, slow lengthening, isometric, fast shortening, and slow shortening of the plantarflexors.
  
  
  RESULTS
  The magnitude and the duration of effects from the conditioning MVC were significantly different between modes. Peak twitch, rate of torque development, and rate of torque relaxation significantly increased for all modes but more so for twitches evoked during fast and slow shortening as compared with lengthening. Rising time was reduced in the lengthening modes but slightly prolonged in the shortening modes. Half relaxation time was significantly reduced for all modes, except fast lengthening.
  
  
  CONCLUSIONS
  The findings show that the effects of a conditioning MVC on twitch contractile properties are dependent on direction and velocity of ongoing muscle length changes. This may imply that functional enhancements from a conditioning MVC might be expected to be greatest for concentric muscle actions but are still present in isometric and eccentric parts of a movement.
authors.affiliation: 1Sports Sciences Department, University of Beira Interior, Covilhã, PORTUGAL; 2Research Center for Sport, Health and Human Development, PORTUGAL; 3Biomechanics and Motor Control laboratory, Swedish School of Sport and Health Sciences, Stockholm, SWEDEN; and 4Department of Neuroscience, Karolinska Institutet, Stockholm, SWEDEN.
authors.forename: Paulo
authors.initials: P
authors.lastname: Gago
authors.forename: Mário Cardoso
authors.initials: MC
authors.lastname: Marques
authors.forename: Daniel Almeida
authors.initials: DA
authors.lastname: Marinho
authors.forename: Maria M
authors.initials: MM
authors.lastname: Ekblom
mesh_terms: D000328:Adult; D000842:Ankle / Q000502:physiology; D004558:Electric Stimulation; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000502:physiology*; D013177:Sports / Q000502:physiology*; D019415:Torque; D055815:Young Adult