title: Effect of chronic unloading and rehabilitation on human Achilles tendon properties: a velocity-encoded phase-contrast MRI study.
abstract: The objective of this study was to measure and monitor changes in Achilles tendon mechanical properties and force production capability of triceps surae muscles after 4 wk of limb suspension and 6 wk of physical rehabilitation. Five healthy volunteers underwent unilateral lower limb suspension followed by weekly physiotherapy. A velocity-encoded, phase-contrast magnetic resonance imaging (VE-PC-MRI) technique was used to estimate the tendon strain as a function of force produced during the submaximal isometric contractions. After limb suspension, triceps surae muscle strength decreased to 53.2 +/- 15.6% (mean +/- SD) of the presuspension level (P < 0.05). Young's modulus, estimated from the slope of the tendon stress-strain relationship, decreased by 17.1% (from 140.50 +/- 29.33 to 119.95 +/- 36.07 MPa, P < 0.05), while the tendon transition point, reflecting the "toe region," increased by 55.7% (from 2.2 +/- 1.0% to 3.4 +/- 1.24%). Muscle strength, tendon stiffness, and transition point recovered to presuspension levels by the end of 6 wk of rehabilitation. Calcaneus movement was significant during the "isometric" contraction, accounting for 52.13 +/- 7.63% of the tendon displacement. Tendon cross-sectional area determined from anatomic magnetic resonance axial images remained unchanged, suggesting that the altered tendon elastic modulus and transition point were largely due to material deterioration. The increase in the transition point following chronic unloading as measured by the VE-PC-MRI technique has not been previously reported and offers new insights into the biomechanical changes that may occur in the tendon crimp structure.
authors.affiliation: Muscle Imaging & Modeling Laboratory, Department of Radiology, RIL, School of Medicine, University of California-San Diego, 3510 Dunhill Street, San Diego, CA 92121-0852, USA.
authors.forename: Dongsuk
authors.initials: D
authors.lastname: Shin
authors.forename: Taija
authors.initials: T
authors.lastname: Finni
authors.forename: Sinyeob
authors.initials: S
authors.lastname: Ahn
authors.forename: John A
authors.initials: JA
authors.lastname: Hodgson
authors.forename: Hae-Dong
authors.initials: HD
authors.lastname: Lee
authors.forename: V Reggie
authors.initials: VR
authors.lastname: Edgerton
authors.forename: Shantanu
authors.initials: S
authors.lastname: Sinha
mesh_terms: D000125:Achilles Tendon / Q000473:pathology* / Q000503:physiopathology; D000328:Adult; D004548:Elasticity; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D008279:Magnetic Resonance Imaging; D008297:Male; D053580:Muscle Strength; D053581:Muscle Strength Dynamometer; D018482:Muscle, Skeletal / Q000473:pathology* / Q000503:physiopathology; D009133:Muscular Atrophy / Q000209:etiology / Q000473:pathology / Q000503:physiopathology / Q000534:rehabilitation*; D026741:Physical Therapy Modalities; D020127:Recovery of Function; D013314:Stress, Mechanical; D013997:Time Factors; D016896:Treatment Outcome; D018474:Weightlessness Simulation / Q000009:adverse effects*