Abstract
Muscle hardness and its relationship with different muscle lengths/positions are important for understanding its underlying physiological status, and yet remained unclear. This study aimed to detect the local muscle hardness at different muscle lengths and identify the influence of muscle position on muscle hardness in healthy adults. A total of 26 healthy adults participated in this study. Shear wave elastography (SWE) was used to measure the muscle hardness of the Rectus Femoris (RF), Tibialis Anterior (TA) and Gastrocnemius Medialis (GM). Each muscle was tested at both resting (RST) and mid-range lengthened (MRL) positions. A novel ultrasound probe placing method was introduced, applied, and evaluated in this study. Moderate to excellent intra-/inter-rater reliability (Intraclass Correlation Coefficient, ICC ≥ 0.70) was found for muscle hardness measurements. The muscle hardness significantly increased from the RST to MRL position for all three muscles (p < 0.001). This study found that the muscle hardness increased at its mid-range lengthened position from the resting position. The mid-range lengthened muscle position of TA and GM could also be sensitive enough to reflect the age-related changes in local muscle hardness. This study also highlights the importance of placing the assessed extremities in an appropriate and consistent position when assessing muscle qualities by ultrasonics in clinical practice.
Highlights
Muscle hardness, defined as a mechanical property that represents the transverse muscle stiffness [1], exhibits an intrinsic viscoelastic tension [2]
We reported the muscle hardness in terms of shear modulus within the region of interest for Tibialis Anterior (TA), Rectus Femoris (RF) and Gastrocnemius Medialis (GM) under both RST and mid-range lengthened (MRL) conditions
We demonstrated that the muscle hardness detected by shear wave elastography (SWE) increased when TA, GM and RF muscles were positioned at their midrange lengthened position
Summary
Muscle hardness, defined as a mechanical property that represents the transverse muscle stiffness [1], exhibits an intrinsic viscoelastic tension [2]. The lengthening of the skeletal muscle, accompanied with an angular motion, exhibits a resistance caused by the passive stiffness when its motor neuron is quiescent and myofibrils are not actively contracting [7]. This resistance becomes measurable and increases with increasing tension when the muscle goes beyond its slack length, which is usually assumed to be the length measured with the joint in middle-range position and when the net joint torque is minimized [8]. Even though the muscle morphology remains similar for six decades, the aging factor could begin to affect the characteristics of skeletal muscle as early as at 30 years of age [9,10]
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