title: Initial conditions influence the characteristics of ballistic contractions in the ankle dorsiflexors.
abstract: This study investigated the influence of different initial conditions on a subsequent fast (ballistic) isometric contraction of the ankle dorsiflexor muscles. Surface electromyograms (EMGs) of dorsiflexor and plantarflexor muscles were recorded during ballistic contractions performed without any pre-activation (BAL) and in ballistic contractions preceded by a sustained submaximal contraction (20% MVC) that was followed either by a rapid voluntary relaxation of the agonist muscle (VRBAL) or by a rapid antagonist (reversal) contraction (ARBAL). In the latter condition, three different antagonist torque levels were compared (25, 50 and 75% MVC). The results showed that the mean average rate of torque development was significantly (P < 0.001) greater for the ARBAL condition (968.5 ± 183.9% MVC/s) compared with the VRBAL (509.3 ± 78.7% MVC/s) and BAL (461.8 ± 79.9% MVC/s) conditions. Furthermore, the mean value recorded for VRBAL was significantly (P < 0.05) greater than for BAL condition. The faster increases in torque during the VRBAL and ARBAL conditions were associated with a greater agonist EMG activity. Compared with VRBAL, performance during the ARBAL condition was improved by a greater level of antagonist coactivation and, in some trials, by the presence of a silent EMG period between the end of the antagonist activation and the onset of the agonist ballistic contraction. Together, these results indicate that the initial conditions can have a substantial influence on the rate of torque development during ballistic contractions performed in isometric conditions.
authors.affiliation: Laboratory of Applied Biology, Université Libre de Bruxelles, 808 route de Lennik, CP 640, 1070, Brussels, Belgium.
authors.forename: Chris
authors.initials: C
authors.lastname: Richartz
authors.forename: Morgan
authors.initials: M
authors.lastname: Lévénez
authors.forename: Julien
authors.initials: J
authors.lastname: Boucart
authors.forename: Jacques
authors.initials: J
authors.lastname: Duchateau
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D001696:Biomechanical Phenomena / Q000502:physiology; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D019415:Torque; D055815:Young Adult