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Relationship between isometric contraction intensity and muscle hardness assessed by ultrasound strain elastography

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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.


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.


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).


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.

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Analysis of variance


Coefficient of variation


Ethylene vinyl acetate


Maximum voluntary contraction


Region of interest


Standard deviation


Strain elastography


Strain ratio


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The authors would like to thank Mr. Naoyuki Murayama and Ms. Yoko Fujihara of Hitachi, Ltd. (Japan) for their technical assistance in the study.

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Correspondence to Kazunori Nosaka.

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Communicated by Nicolas Place.

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Inami, T., Tsujimura, T., Shimizu, T. et al. Relationship between isometric contraction intensity and muscle hardness assessed by ultrasound strain elastography. Eur J Appl Physiol 117, 843–852 (2017).

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