Table 2.

Ultrasound metrics: assessment of muscle and fat tissue

Scale	Population	Description	Anatomical structures	Normal range	Variations from normal
Heckmatt Scale [60]	Children	Assesses muscle echogenicity from normal to increased fibrosis and fat infiltration	Quadriceps, especially the rectus femoris, due to its accessibility and size Medial gastrocnemius, for its propensity to show changes in echogenicity due to its composition and function	Grade 0 (normal) to Grade 1 (slight increase)	Higher grades indicate increased fat and connective tissue, suggesting muscle pathology
Muscle Quality Index (MQI) [79]	Adults	Combines measurements of muscle size with the degree of intramuscular fat for an overall assessment of muscle quality	Biceps brachii and triceps brachii in the upper extremities Rectus femoris and biceps femoris in the lower extremities, due to their size and role in locomotion	MQI < 25 (lower intramuscular fat)	Higher MQI scores may indicate sarcopenia or increased fat infiltration
Elastic modulus [77]	Adults	Quantifies muscle stiffness, often used in the assessment of fibrosis	Accessible and sizeable muscles such as the rectus femoris or biceps brachii These measurements are also useful in deeper muscles, but accessibility and the depth at which reliable measurements can be obtained may limit their use	< 8 kPa (indicative of normal tissue stiffness)	Increased values suggest greater tissue stiffness and potential fibrosis
Shear wave speed [77]	Adults	Measures the speed of shear waves through muscle, related to stiffness and used to assess fibrosis	Accessible and sizeable muscles such as the rectus femoris or biceps brachii These measurements are also useful in deeper muscles, but accessibility and the depth at which reliable measurements can be obtained may limit their use	1–3 m/s (indicative of normal tissue elasticity)	Higher values suggest increased muscle stiffness and potential pathology
Peak systolic velocity (PSV) [80]	All ages	Assesses the maximum blood flow velocity in muscle during the contraction phase of the heartbeat	Measurements are often taken in the major arteries supplying the muscle groups, such as the femoral artery for the lower limb muscles The brachial artery can be used for upper limb muscle assessments	Generally ranges between 30 to 100 cm/s, but this can vary widely based on the artery's size and the patient's hemodynamic status	Lower values may indicate reduced muscle perfusion and potential pathology
End-diastolic velocity (EDV) [80]	All ages	Measures the blood flow velocity in muscle during the relaxation phase of the heartbeat	Measurements are often taken in the major arteries supplying the muscle groups, such as the femoral artery for the lower limb muscles The brachial artery can be used for upper limb muscle assessments	Often ranges from 5 to 20 cm/s, again depending on the specific vessel and physiological conditions at the time of measurement	Lower values may indicate reduced muscle perfusion during relaxation
Resistive Index (RI) [62]	All Ages	Calculates the ratio of peak systolic velocity minus end-diastolic velocity to peak systolic velocity, indicating muscle vascular resistance	Similarly to PSV and EDV, RI measurements are typically taken in arteries within or supplying the muscle group of interest, such as the femoral artery for the lower limbs Small peripheral arteries within the muscle can also be assessed if the resolution of the ultrasound machine is sufficient	0.5–0.7 (indicative of normal vascular resistance)	Values outside the normal range suggest altered vascular resistance, which may reflect pathology