title: Estimation of cat medial gastrocnemius fascicle lengths during dynamic contractions.
abstract: In typical muscle models, it is often assumed that the contractile element (fascicle) length depends exclusively on the instantaneous muscle-tendon length and the instantaneous muscle force. In order to test whether the instantaneous fascicle length during dynamic contractions can be predicted from muscle-tendon length and force, fascicle lengths, muscle-tendon lengths, and muscle forces were directly measured in cat medial gastrocnemii during isometric and dynamic contractions. Two theoretical muscle models were developed: model A was based on force-time data obtained during the activation phase and model D on force-time data obtained during the deactivation phase of isometric contractions. To test the models, instantaneous fascicle lengths were predicted from muscle-tendon lengths and forces during dynamic contractions that simulated cat locomotion for speeds ranging from 0.4 to 1.6m/s. The theoretically predicted fascicle lengths were compared with the experimentally measured fascicle lengths. It was found that fascicle lengths were not uniquely associated with muscle-tendon lengths and forces; that is, for a given muscle-tendon length and force, fascicle lengths varied depending on the contractile history. Consequently, models A and D differed in fascicle length predictions; model D (maximum average error=8.5%) was considerably better than model A (maximum average error=22.3%). We conclude from this study that it is not possible to predict the exact fascicle lengths from muscle-tendon lengths and forces alone, however, adequate predictions seem possible based on such a model. The relationship between fascicle length and muscle force and muscle-tendon length is complex and highly non-linear, thus, it appears unlikely that accurate fascicle length predictions can be made without some reference contractions in which fascicle length, muscle-tendon length, and force are measured simultaneously.
authors.affiliation: Faculty of Kinesiology, Human Performance Laboratory, The University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada.
authors.forename: Motoshi
authors.initials: M
authors.lastname: Kaya
authors.forename: Wendy
authors.initials: W
authors.lastname: Carvalho
authors.forename: Tim
authors.initials: T
authors.lastname: Leonard
authors.forename: Walter
authors.initials: W
authors.lastname: Herzog
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D002415:Cats; D007537:Isometric Contraction / Q000502:physiology; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000502:physiology*; D013710:Tendons / Q000502:physiology