title: Muscle-specific acute changes in passive stiffness of human triceps surae after stretching.
abstract: OBJECTIVE
  It remains unclear whether the acute effect of stretching on passive muscle stiffness differs among the synergists. We examined the muscle stiffness responses of the medial (MG) and lateral gastrocnemii (LG), and soleus (Sol) during passive dorsiflexion before and after a static stretching by using ultrasound shear wave elastography.
  
  
  METHODS
  Before and after a 5-min static stretching by passive dorsiflexion, shear modulus of the triceps surae and the Achilles tendon (AT) during passive dorsiflexion in the knee extended position were measured in 12 healthy subjects.
  
  
  RESULTS
  Before the static stretching, shear modulus was the greatest in MG and smallest in Sol. The stretching induced significant reductions in shear modulus of MG, but not in shear modulus of LG and Sol. The slack angle was observed at more plantar flexed position in the following order: AT, MG, LG, and Sol. After the stretching, the slack angles of each muscle and AT were significantly shifted to more dorsiflexed positions with a similar extent. When considering the shift in slack angle, the change in MG shear modulus became smaller.
  
  
  CONCLUSIONS
  The present study indicates that passive muscle stiffness differs among the triceps surae, and that the acute effect of a static stretching is observed only in the stiff muscle. However, a large part of the reduction of passive muscle stiffness at a given joint angle could be due to an increase in the slack length.
authors.affiliation: National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima, 891-2311, Japan.
authors.forename: Kosuke
authors.initials: K
authors.lastname: Hirata
authors.affiliation: National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima, 891-2311, Japan.|Japan Society for the Promotion of Science, Tokyo, Japan.
authors.forename: Eri
authors.initials: E
authors.lastname: Miyamoto-Mikami
authors.affiliation: National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima, 891-2311, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Kanehisa
authors.affiliation: National Institute of Fitness and Sports in Kanoya, 1 Shiromizu, Kanoya, Kagoshima, 891-2311, Japan. miyamoto@nifs-k.ac.jp.
authors.forename: Naokazu
authors.initials: N
authors.lastname: Miyamoto
mesh_terms: D000125:Achilles Tendon / Q000502:physiology; D000328:Adult; D004548:Elasticity / Q000502:physiology*; D054459:Elasticity Imaging Techniques / Q000379:methods; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000502:physiology; D008297:Male; D053580:Muscle Strength / Q000502:physiology*; D052580:Muscle Stretching Exercises / Q000379:methods; D018482:Muscle, Skeletal / Q000502:physiology*; D055815:Young Adult
keywords: Ankle joint; Gastrocnemius; Shear modulus; Soleus; Ultrasound shear wave elastography