title: Human calf muscle responses during repeated isometric plantarflexions.
abstract: In the present study, we measured the contraction-induced shortening (dL) of individual synergistic human muscles in a repeated motor task to assess their contractile behaviour. Ultrasonography was used to obtain dL measurements in the gastrocnemius (GS) and soleus (SOL) muscles of six men performing 11 consecutive isometric plantarflexions. Contractions 1 and 11 were performed with maximal effort, and contractions 2-4, 5-7 and 8-10 were performed with efforts generating 50, 70 and 90%, respectively, of the plantarflexion moment produced in contraction 1. In contractions 5-10, the SOL muscle dL was similar (p > 0.05) to that produced in contraction 1 (approximately 6 mm), indicating that the SOL muscle became fully activated at 70% of the maximum plantarflexion moment. The GS muscle dL in contractions 10 and 11 exceeded by approximately 0.5 mm (p < 0.05) and 1.3 mm (p < 0.01), respectively, that generated in contraction 1 (approximately 10 mm), despite evidence obtained by superimposed stimulation that contraction 1 was produced with full motor unit activation. The consequent paradox that the GS muscle would produce in contractions 10 and 11 a greater activation and therefore more force than its actual potential is resolved when considering the interaction between the time-dependent tensile response of tendon and the performance of muscle as dictated by the sliding filament mechanism of contraction.
authors.affiliation: Institute for Biophysical and Clinical Research into Human Movement (IRM), Manchester Metropolitan University, Alsager ST7 2HL, UK. c.n.maganaris@mmu.ac.uk
authors.forename: Constantinos N
authors.initials: CN
authors.lastname: Maganaris
authors.forename: Vasilios
authors.initials: V
authors.lastname: Baltzopoulos
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Sargeant
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D003198:Computer Simulation; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology*; D010807:Physical Endurance / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D019415:Torque; D014463:Ultrasonography