title: Differential strain patterns of the human gastrocnemius aponeurosis and free tendon, in vivo.
abstract: OBJECTIVE
  The mechanical characteristics of the human free tendon and aponeurosis, in vivo, remains largely unknown. The present study evaluated the longitudinal displacement of the separate free Achilles tendon and distal (deep) aponeurosis of the medial gastrocnemius muscle during voluntary isometric contraction.
  
  
  METHODS
  Ultrasonography-obtained displacement of the free tendon and tendon-aponeurosis complex, electromyography of the gastrocnemius, soleus, and dorsiflexor muscles, and joint angular rotation were recorded during isometric plantarflexion (n = 5). Tendon cross-sectional area, moment arm and segment lengths (L(o)) were measured using magnetic resonance imaging. Tendon force was calculated from joint moments and tendon moment arm, and stress was obtained by dividing force by cross-sectional area. The difference between the free tendon and tendon-aponeurosis complex deformation yielded separate distal aponeurosis deformation. Longitudinal aponeurosis and tendon strain were obtained from the deformations normalized to segment lengths.
  
  
  RESULTS
  At a common tendon force of 2641 +/- 306 N, the respective deformation and Lo were 5.85 +/- 0.85 and 74 +/- 0.8 mm for the free tendon and 2.12 +/- 0.64 and 145 +/- 1.3 mm for the distal aponeurosis, P < 0.05. Longitudinal strain was 8.0 +/- 1.2% for the tendon and 1.4 +/- 0.4% for the aponeurosis, P < 0.01. Stiffness and stored energy was 759 +/- 132 N mm(-1) and 6.14 +/- 1.89 J, respectively, for the free tendon. Cross-sectional area of the Achilles tendon was 73 +/- 4 mm2, yielding a stress of 36.5 +/- 4.6 MPa and Young's modulus of 788 +/- 181 MPa.
  
  
  CONCLUSIONS
  The free Achilles tendon demonstrates greater strain compared with that of the distal (deep) aponeurosis during voluntary isometric contraction, which suggests that separate functional roles may exist during in vivo force transmission.
authors.affiliation: Team Denmark Test Center/Sports Medicine Research Unit, Bispebjerg Hospital, Copenhagen NV, Denmark.
authors.forename: S P
authors.initials: SP
authors.lastname: Magnusson
authors.forename: P
authors.initials: P
authors.lastname: Hansen
authors.forename: P
authors.initials: P
authors.lastname: Aagaard
authors.forename: J
authors.initials: J
authors.lastname: Brønd
authors.forename: P
authors.initials: P
authors.lastname: Dyhre-Poulsen
authors.forename: J
authors.initials: J
authors.lastname: Bojsen-Moller
authors.forename: M
authors.initials: M
authors.lastname: Kjaer
mesh_terms: D000125:Achilles Tendon / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000502:physiology*; D000328:Adult; D004576:Electromyography; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology*; D008279:Magnetic Resonance Imaging; D008297:Male; D009068:Movement / Q000502:physiology; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology*; D012399:Rotation; D013314:Stress, Mechanical; D014463:Ultrasonography