title: Vastus lateralis maximum force-generating potential occurs at optimal fascicle length regardless of activation level.
abstract: OBJECTIVE
  Despite the fact that everyday movements are hardly ever performed with muscles contracting maximally, our understanding of the force-length relationship is mostly based on in vitro studies using maximal activation. In this study, the in vivo submaximal and maximal force-length relationships of vastus-lateralis were investigated. Force-length relationships were obtained based on maximal and submaximal levels of force and, also, on EMG activation.
  
  
  METHODS
  Nine subjects performed isometric knee extensor contractions at ten knee angles (80°-170°). Knee extensor torque, and vastus-lateralis EMG and fascicle lengths were acquired simultaneously. Fascicle lengths and knee angles at peak force occurrence were compared across maximal and submaximal conditions.
  
  
  RESULTS
  The submaximal force-fascicle length relationships depend crucially on the approach used: in the force-based approach, peak forces are constrained to occur at the same MTU length and, because of series elasticity, occur at longer fascicle lengths for decreasing force levels. In contrast, in the activation-based approach, peak force occurrence is not constrained to a given muscle length for submaximal contractions and occurs at similar fascicle lengths but shorter MTU lengths (more extended knee angles) as force decreases.
  
  
  CONCLUSIONS
  Our results support the hypothesis that vastus-lateralis fascicle length for maximal force production is about constant for maximal and submaximal levels of activation, presumably taking advantage of optimal myofilament overlap at that fascicle length. This result implies that optimal vastus-lateralis lengths occur at different knee angles for different levels of activation, which is in stark contrast to findings in the literature in which submaximal force-fascicle length relationships were based on force rather than activation.
authors.affiliation: Ministry of Education of Brazil, CAPES Foundation, Brasília, 70040-020, Brazil. heiliane.fontana@ufsc.br.|Morphology Department, Federal University of Santa Catarina, Eng. Agronômico Andrei Cristian Ferreira, Trindade, Florianópolis, Santa Catarina, CEP 88040900, Brazil. heiliane.fontana@ufsc.br.
authors.forename: Heiliane
authors.identifier: http://orcid.org/0000-0002-5797-1233
authors.initials: H
authors.lastname: de Brito Fontana
authors.affiliation: Human Performance Laboratory, University of Calgary, 2500 University Dr NW, Calgary, AB, T2N 1N4, Canada.
authors.forename: Walter
authors.initials: W
authors.lastname: Herzog
mesh_terms: D000328:Adult; D003198:Computer Simulation; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D053580:Muscle Strength / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D052097:Quadriceps Muscle / Q000033:anatomy & histology* / Q000502:physiology*; D013314:Stress, Mechanical
keywords: Calcium sensitivity; Electromyography; Fascicle; Force–length relationship; In vivo muscle function; Muscle adaptation; Muscle properties; Series elasticity; Ultrasound imaging