title: Relationship between isometric contraction intensity and muscle hardness assessed by ultrasound strain elastography.
abstract: BACKGROUND
  Ultrasound elastography is used to assess muscle hardness or stiffness; however, no previous studies have validated muscle hardness measures using ultrasound strain elastography (SE). This study investigated the relationship between plantar flexor isometric contraction intensity and gastrocnemius hardness assessed by SE. We hypothesised that the muscle would become harder linearly with an increase in the contraction intensity of the plantar flexors.
  
  
  METHODS
  Fifteen young women (20.1 ± 0.8 years) performed isometric contractions of the ankle plantar flexors at four different intensities (25, 50, 75, 100% of maximal voluntary contraction force: MVC) at 0° plantar flexion. Using SE images, the strain ratio (SR) between the muscle and an acoustic coupler (elastic modulus 22.6 kPa) placed over the skin was calculated (muscle/coupler); pennation angle and muscle thickness were measured for the resting and contracting conditions.
  
  
  RESULTS
  SR decreased with increasing contraction intensity from rest (1.28 ± 0.20) to 25% (0.99 ± 0.21), 50% (0.61 ± 0.15), 75% (0.34 ± 0.1) and 100% MVC (0.20 ± 0.05). SR decreased linearly (P < 0.05) with increasing MVC from rest to 75% MVC, but levelled off from 75 and 100% MVC. SR was negatively correlated with pennation angle (r = -0.80, P < 0.01) and muscle thickness ( r= -0.78,  P< 0.01).
  
  
  CONCLUSIONS
  SR appears to represent muscle hardness changes in response to contraction intensity changes, in the assumption that the gastrocnemius muscle contraction intensity is proportional to the plantar flexion intensity. We concluded that gastrocnemius muscle hardness changes could be validly assessed by SR, and the force-hardness relationship was not linear.
authors.affiliation: Centre for Exercise and Sports Science Research, School of Medical and Health Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027, Australia.|Faculty of Sport Sciences, Waseda University, Tokorozawa, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Inami
authors.affiliation: Tsujimura Surgery Hospital, Kariya, Japan.
authors.forename: Toru
authors.initials: T
authors.lastname: Tsujimura
authors.affiliation: Graduate School of Health and Sport Sciences, Chukyo University, Nagoya, Japan.
authors.forename: Takuya
authors.initials: T
authors.lastname: Shimizu
authors.affiliation: Graduate School of Health and Sport Sciences, Chukyo University, Nagoya, Japan.
authors.forename: Takemasa
authors.initials: T
authors.lastname: Watanabe
authors.affiliation: Centre for Exercise and Sports Science Research, School of Medical and Health Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027, Australia.
authors.forename: Wing Yin
authors.initials: WY
authors.lastname: Lau
authors.affiliation: Centre for Exercise and Sports Science Research, School of Medical and Health Sciences, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027, Australia. k.nosaka@ecu.edu.au.
authors.forename: Kazunori
authors.initials: K
authors.lastname: Nosaka
mesh_terms: D054459:Elasticity Imaging Techniques; D005260:Female; D006801:Humans; D007537:Isometric Contraction; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging / Q000502:physiology*; D016059:Range of Motion, Articular; D055815:Young Adult
keywords: Gastrocnemius; Muscle force; Muscle thickness; Pennation angle; Strain ratio