title: Muscle fatigue increases the amplitude of fluctuations of tangential forces during isometric contractions.
abstract: The orientation of the resultant force during a given contraction depends on the architecture of the muscles involved, and cooperation among muscles may induce force fluctuations in multiple directions. A novel setup for recording three-dimensional forces was developed to assess the effects of muscle fatigue on submaximal contractions. Fifteen subjects performed isometric dorsiflexions, elbow flexions, knee extensions, plantarflexions, and trunk extensions (12 s, 2.5-80% of maximal voluntary force) before and after a fatigue protocol. Electromyography (EMG) was recorded from relevant muscles. The standard deviation of force in the three directions and the agonist EMG amplitude increased with target force (p<.045), while the coefficient of variation of force and total excursions of the center of pressure showed maximal values at low target forces, reaching a plateau at moderate forces. Fatigue induced higher force fluctuations in task-related and tangential directions and increased agonist EMG amplitude for all muscle groups (p<.04). Fluctuations of tangential forces were greater during knee extensions compared with other muscle groups (p<.007). The present data demonstrate that fatigue increases the amplitude of force fluctuations in task-related and tangential forces. Moreover, alternation of activity between multiple synergist muscles is associated with increased fluctuations of tangential forces, especially during fatigue.
authors.affiliation: Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction, Department of Health Science and Technology, Aalborg University, Denmark. sauros@hst.aau.dk
authors.forename: Sauro E
authors.initials: SE
authors.lastname: Salomoni
authors.forename: Thomas
authors.initials: T
authors.lastname: Graven-Nielsen
mesh_terms: D000328:Adult; D001676:Biofeedback, Psychology / Q000502:physiology; D001696:Biomechanical Phenomena / Q000502:physiology*; D004576:Electromyography; D005260:Female; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011403:Pronation / Q000502:physiology; D012815:Signal Processing, Computer-Assisted; D013484:Supination / Q000502:physiology; D016474:Weight-Bearing / Q000502:physiology; D055815:Young Adult