Table 2.

Comparison between myotonometry and shear wave elastography for muscular stiffness assessment.

	Shear Wave Elastography	Myotonometry
Instrument characteristics	• Objective [4,18]      • Non-invasive [1,4,18]
	Real-time [1,3,64] Required technical expertise [18]	Less expensive [1,4] Handheld [1,3,4] Easy to use [1,3,4,66]
Structures assessed	Deep [1,4]	Superficial [1,4]
Type of stiffness measured	Passive [1,3]: resistance to elongation or shortening or, in physical terms, the change in tension per unit change in length [67]	Dynamic [1,25,68]: resistance to a force that deforms muscle initial shape [3,25,68]
Measurement mode	Elastic [4]/shear [3] modulus, that uses ultrasound radiation forces [4]	Damped oscillation method following a dynamic transformation of the muscle in response to a short-term external mechanical impulse [69]
Measurement process	Transducer parallel to the muscle fibers [1,4,18]—wave travel horizontal (along fibers [15]) to the point of application through tissue [4] Transducer held stationary for 10 s with minimal pressure applied on the skin [1,3,4]—acoustic radiation force to perturb muscle tissues [70] Measurement estimate based on the velocity of ultrasound propagation from an entire defined region of interest [3,4,18] and based on tissue density [15]—converted into Kpa values through Young’s modulus formula for every pixel [3,71]	Probe perpendicular to the skin surface [1,3] Constant pre-compression force (0.18 N) in the underlying tissues, followed by a short mechanical impulse (0.4/0.6 N for 15 ms) [1,4,18] Recording of muscle oscillation [4], reflecting viscoelastic properties of the tissue [3] Data by computational software, calculated from the acceleration of the testing probe during oscillations [3]
Measurement Interpretation	Velocity of shear waves (proportional to shear modulus [64]) rise with increase in passive muscle stiffness [1,64]	Higher values of dynamic stiffness imply more energy to modify the shape of the tissue [3]
Scapular muscles Assessed	In healthy subjects: UT [1,22,64,65,72] MT, LT and SA [64] Levator scapulae [22,64,72] In pain conditions: UT [1,22]; Levator scapulae [22]	In healthy subjects: UT [1,7,14,21,26,63] In pain conditions: UT [7,14,25,26,66] MT and LT [25]
Results SWE vs. Myotonometry	Myotonometry presented lower coefficient of variability [4] and similar values of reliability compared to SWE Myotonometry present high to very high reliability for upper limb, lower limb and spine muscles [3,4,25,56] in healthy subjects, and low to very high reliability for UT in both healthy [7,21,25,48,63] or with a musculoskeletal disorder subjects [7]; while SWE present moderate to very high reliability for upper limb, lower limb and spine muscles [3,4,56,64,65] in healthy subjects. Myotonometry present ability to discriminate between different muscle contraction intensities, but the same did not happen always for SWE [4]

Legend: dynamic stiffness (DS); healthy subjects (HS); maximum amplitude of the acceleration of oscillation (amax); maximum displacement of the tissue (Δl); middle trapezius (MT); lower trapezius (LT); pain conditions (PC); probe mass (mprobe); shear wave elastography (SWE); upper trapezius (UT).