title: Changes in sarcomere lengths of the human vastus lateralis muscle with knee flexion measured using in vivo microendoscopy.
abstract: Sarcomeres are the basic contractile units of muscle, and their lengths influence muscle force-generating capacity. Despite their importance, in vivo sarcomere lengths remain unknown for many human muscles. Second harmonic generation (SHG) microendoscopy is a minimally invasive technique for imaging sarcomeres in vivo and measuring their lengths. In this study, we used SHG microendoscopy to visualize sarcomeres of the human vastus lateralis, a large knee extensor muscle important for mobility, to examine how sarcomere lengths change with knee flexion and thus affect the muscle׳s force-generating capacity. We acquired in vivo sarcomere images of several muscle fibers of the resting vastus lateralis in six healthy individuals. Mean sarcomere lengths increased (p=0.031) from 2.84±0.16μm at 50° of knee flexion to 3.17±0.13μm at 110° of knee flexion. The standard deviation of sarcomere lengths among different fibers within a muscle was 0.21±0.09μm. Our results suggest that the sarcomeres of the resting vastus lateralis at 50° of knee flexion are near optimal length. At a knee flexion angle of 110° the resting sarcomeres of vastus lateralis are longer than optimal length. These results show a smaller sarcomere length change and greater conservation of force-generating capacity with knee flexion than estimated in previous studies.
authors.affiliation: Department of Mechanical Engineering, Stanford University, United States.
authors.forename: Xuefeng
authors.initials: X
authors.lastname: Chen
authors.affiliation: Department of Mechanical Engineering, Stanford University, United States; Department of Bioengineering, Stanford University, United States.
authors.forename: Gabriel N
authors.initials: GN
authors.lastname: Sanchez
authors.affiliation: Department of Biology, Stanford University, United States; Department of Applied Physics, Stanford University, United States; Howard Hughes Medical Institute, United States.
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Schnitzer
authors.affiliation: Department of Mechanical Engineering, Stanford University, United States; Department of Bioengineering, Stanford University, United States. Electronic address: delp@stanford.edu.
authors.forename: Scott L
authors.initials: SL
authors.lastname: Delp
mesh_terms: D000328:Adult; D004724:Endoscopy; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology / Q000648:ultrastructure; D052097:Quadriceps Muscle / Q000000981:diagnostic imaging / Q000502:physiology* / Q000648:ultrastructure; D012518:Sarcomeres / Q000502:physiology* / Q000648:ultrastructure; D055815:Young Adult
keywords: Knee; Microendoscopy; Muscle; Sarcomere; Strength