Ultrasound measuring muscle thickness

Muscle Thickness Unveiled: How Ultrasound Measures Up

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Elliot Brynn in Health & Wellness November 2026 • 4 min read.

"Explore the reliability of ultrasound in measuring muscle thickness and its impact on fitness and rehabilitation programs."


In the ever-evolving world of fitness and rehabilitation, accurately assessing muscle size is crucial. While methods like MRI and CT scans exist, they can be costly and expose individuals to radiation. This is where ultrasound (US) emerges as a valuable tool, offering a non-invasive and cost-effective way to measure muscle dimensions.

Ultrasound's ability to monitor muscle changes makes it ideal for tracking the effectiveness of diet and exercise programs, as well as rehabilitation efforts. By providing detailed insights into muscle thickness, US helps professionals tailor interventions and optimize outcomes for individuals seeking to improve their physical well-being.

However, the accuracy of ultrasound measurements is paramount. Even small variations in muscle thickness can be significant, highlighting the need for a reliable and precise measurement technique. This article explores the reliability of ultrasound in measuring muscle thickness, examining its applications and limitations in fitness and rehabilitation settings.

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From Manual Scans to Automated Analysis

Ultrasound emerged as a non-invasive way to measure skeletal muscle thickness, while magnetic resonance imaging and computed tomography remained widely accepted reference methods for assessing muscle size. The 2017 MUSA study addressed a central limitation of earlier techniques: their dependence on manual operators and the narrow focus of semi-automatic computer-aided methods. Ultrasound also introduced a distinctive measurement challenge because pressure applied during imaging can influence the determination of muscle size.

How Reliable is Ultrasound for Measuring Muscle Thickness?

Ultrasound measuring muscle thickness

A study published in the Brazilian Journal of Sports Medicine investigated the reliability of ultrasound in measuring the thickness of elbow and knee flexor and extensor muscles. Researchers Paulo Sergio Chagas Gomes, Cláudia de Mello Meirelles, and their team sought to determine the consistency of ultrasound measurements and quantify the typical error associated with this method.

The study involved 15 healthy volunteers who underwent ultrasound measurements of their elbow and knee muscles on two separate days. The researchers used specific anatomical landmarks to ensure consistent measurement sites. Here’s a breakdown of the key methods:

  • Participants: 15 healthy volunteers (8 women) with an average age of 33.9 years.
  • Ultrasound Technique: Two-dimensional B-mode ultrasound with a 7.5 MHz transducer.
  • Muscles Measured: Elbow flexors (EF), elbow extensors (EE), knee flexors (KF), and knee extensors (KE).
  • Measurement Sites: Anatomical landmarks identified and recorded to ensure consistency between measurements.
  • Procedure: Muscle thickness measured between the interfaces of bone and adipose tissue.
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Useful Tool, Contested Measure

Recent research describes ultrasound as a real-time method for measuring muscle thickness and cross-sectional area that is simple to use and suitable for bedside applications. However, a publication discussed on July 7, 2026, examined whether ultrasound muscle thickness accurately reflects resistance-training-induced muscle growth and reported findings with important implications for researchers and practitioners. Together, these sources show that practical usability and measurement validity are related but separate questions.

Where Ultrasound Falls Short

The predictive value of measurements derived from A-mode ultrasound, particularly across distinct anatomical sites, remains insufficiently explored in oncology and other medical conditions. In neuromuscular disease, ultrasonography appears promising as a biomarker, but further studies are needed to establish whether it can serve as a treatment-outcome surrogate in amyotrophic lateral sclerosis. ICU muscle ultrasound is bedside-accessible for assessing muscle mass and quality, yet its status as a standard of care is still an open question.

More Than Thickness

Muscle ultrasonography is described as a non-invasive and direct method for evaluating skeletal muscle mass. Muscle thickness and echo intensity, an indicator of muscle quality, are related to muscle strength, physical function, and muscle mass measured using DXA or BIA. This makes ultrasound relevant not only for estimating size but also for examining relationships between muscle structure, quality, and function.

The researchers used statistical analyses, including ANOVA and intraclass correlation coefficients (ICC), to assess the reliability of the ultrasound measurements. They also calculated the coefficient of variation (CV) and typical error of measurement (TEM) to quantify the precision of the method.

The Verdict: Ultrasound as a Reliable Muscle Measurement Tool

The study concluded that ultrasound measurements of elbow flexors, elbow extensors, and knee extensors are highly reliable, making it a valuable tool for monitoring changes in muscle thickness due to diet, exercise, or rehabilitation programs. However, the measurements of knee flexors showed lower reliability and higher error, suggesting that this method may not be suitable for assessing changes in these muscles.

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Reliability and Automation

A 2025 study found that ultrasound generally demonstrated moderate-to-excellent reliability for measuring muscle architecture parameters in post-stroke patients, including muscle thickness, pennation angle, cross-sectional area, muscle volume, and length. At the same time, the MUSA study identified manual operator dependence as a major limitation in muscle ultrasound analysis. MUSA was developed and validated as a fully automated method intended to address the limitations of semi-automatic or single-muscle computer-aided approaches.

Toward Smarter, More Usable Imaging

Quantified muscle ultrasound can produce sensitive results, but its wider use is hampered by the need for device-specific reference values. Future efforts are expected to include dedicated muscle-ultrasound systems and artificial intelligence to improve image interpretation and usability. Muscle ultrasound also offers a patient-friendly direction because it is described as painless and free of radiation compared with modalities such as computed tomography and magnetic resonance imaging.

The Human Work Behind the Measurement

A 2022 systematic review described muscle ultrasound as having high utility in clinical practice and research. It also identified the training and time required for manual analysis as major challenges to achieving objective quantification. The broader issue is therefore not only whether ultrasound can visualize muscle, but whether measurements can be produced consistently and objectively in routine use.

Ultrasound changes beside other muscle measures

  • 16.6%Resistance training was associated with an average 16.6% increase in ultrasound-measured quadriceps thickness in healthy adults; this is a group average, not a measure of how much each person changed.[1]
  • 0.966, 0.956; 0.945, 0.919 ICCFor quadriceps thickness, intrarater ICCs based on the average of two measurements were 0.966 for the quadriceps as a whole and 0.956 for the rectus femoris. Inter-rater ICCs were 0.945 and 0.919, respectively; these reliability figures concern repeatability, not agreement with MRI.[2]

What thickness readings can and cannot represent

A small measurement error matters when the aim is to track subtle change: high repeatability does not by itself establish that thickness change accurately represents changes in muscle area or volume.[2] [3]

When muscle contours were hard to distinguish near the proximal insertions, some area values were estimated from the other measurements. That means at least part of the volume estimate depended on interpolation rather than directly clear contours.[4]

Ultrasound readings can shift with probe angle and pressure, so repeatability depends partly on how the probe is positioned and applied. A separate reliability protocol used minimal hand force and positioned the probe perpendicular to muscle fibers to limit those effects.[2] [5]

Ultrasound compared with MRI and clinical monitoring

MRI is treated as a reference for assessing muscle mass and change, but it is less accessible and less affordable than some other measurement approaches. Ultrasound thickness is therefore useful to compare with MRI, while the two measures should not be assumed to capture change in the same way.[3] [4]

In critically ill patients, rehabilitation and no-rehabilitation groups differed in rectus femoris and total anterior thigh thickness. Those group differences show that ultrasound readings can distinguish observed clinical groups, but they do not alone establish that thickness is a direct measure of strength or function.[6] [7]

Other ICU observations report changing rectus femoris thickness over hospital days, alongside reports of quadriceps thinning and losses in cross-sectional area during hospitalization. These examples concern monitoring acute illness, not validation of ultrasound thickness as a stand-in for MRI-measured training-related growth.[8] [9]

A reported daily muscle-area loss rate in mechanically ventilated patients was higher than ranges reported in other studies; the source notes that differing patient characteristics may explain the contrast. That variation cautions against treating measurements from different critically ill populations as directly interchangeable.[10]

What ultrasound thickness still leaves unresolved

  • How much do probe handling and muscle-boundary visibility contribute to differences between repeated readings and derived estimates?[4] [5]
  • Do thickness readings alone capture clinically meaningful changes when muscle strength is not measured alongside them?[7]
  • How robust are outcome predictions from ultrasound when several underlying studies were underpowered and may overstate predictive ability?[11]

Sources

  1. Effect of resistance training on quadriceps femoris muscle thickness obtained by ultrasound: A systematic review with meta-analysis. (pubmed.ncbi.nlm.nih.gov)
  2. Reliability of point-of-care ultrasound for measuring quadriceps femoris muscle thickness (pmc.ncbi.nlm.nih.gov)
  3. Ultrasound Muscle Thickness Is a Poor Index of Criterion Magnetic Resonance Imaging Measures of Resistance Training-Induced Muscle Growth (pmc.ncbi.nlm.nih.gov)
  4. Muscle thickness correlates to muscle cross‐sectional area in the assessment of strength training‐induced hypertrophy (pmc.ncbi.nlm.nih.gov)
  5. Brazil - Validity, reliability and measurement error of quadriceps femoris muscle thickness obtained by ultrasound in healthy adults: a systematic review Validity, reliability and measurement error of (www.scielo.br)
  6. Evaluating Muscle Mass Changes in Critically Ill Patients: Rehabilitation Outcomes Measured by Ultrasound and Bioelectrical Impedance (pmc.ncbi.nlm.nih.gov)
  7. Clinical review: peripheral muscular ultrasound in the ICU (pmc.ncbi.nlm.nih.gov)
  8. Point-of-Care Ultrasound to Evaluate Thickness of Rectus Femoris, Vastus Intermedius Muscle, and Fat as an Indicator of Muscle and Fat Wasting in Critically Ill Patients in a Multidisciplinary Intensi (pmc.ncbi.nlm.nih.gov)
  9. Longitudinal Ultrasound Monitoring of Peripheral Muscle Loss in Neurocritical Patients (pmc.ncbi.nlm.nih.gov)
  10. Ultrasonographic assessment of the muscle mass of the rectus femoris in mechanically ventilated patients at intensive care unit discharge is associated with deterioration of functional status at hospi (pmc.ncbi.nlm.nih.gov)
  11. The current use of ultrasound to measure skeletal muscle and its ability to predict clinical outcomes: a systematic review (pmc.ncbi.nlm.nih.gov)

About this Article -

Written with AI assistance from published research, and reviewed by the Mystum team. See our About page for more information.

Everything You Need To Know

1

What exactly is ultrasound and why is it used to measure muscles?

Ultrasound (US) is a non-invasive imaging technique that uses sound waves to visualize structures within the body, including muscles. In the context of fitness and rehabilitation, it is used to measure muscle thickness. This is significant because it offers a cost-effective and radiation-free alternative to methods like MRI and CT scans for monitoring muscle size. However, the accuracy of ultrasound measurements is paramount, as even small variations in muscle thickness can be significant.

2

Why is it important to measure muscle thickness using ultrasound in fitness and rehabilitation?

Ultrasound is important because it provides a non-invasive and relatively inexpensive way to track changes in muscle thickness. This is particularly valuable for monitoring the effectiveness of diet and exercise programs, as well as rehabilitation efforts. By accurately measuring muscle dimensions, professionals can tailor interventions to optimize outcomes for individuals seeking to improve their physical well-being. It is important to note that ultrasound can also be used to assess soft tissues, tendons, and ligaments, providing a more comprehensive assessment.

3

How reliable is ultrasound for measuring different muscle groups?

The reliability of ultrasound measurements varies depending on the muscle group being assessed. Studies have shown that ultrasound measurements of elbow flexors, elbow extensors, and knee extensors are highly reliable. However, the measurements of knee flexors showed lower reliability and higher error, suggesting that this method may not be as suitable for assessing changes in these muscles. Therefore, practitioners should be aware of these limitations when using ultrasound to monitor muscle thickness.

4

What are Intraclass Correlation Coefficients (ICC), and how do they relate to ultrasound measurements of muscles?

Intraclass correlation coefficients (ICC) are statistical measures used to assess the reliability of measurements. In the context of ultrasound measurements of muscle thickness, ICC values indicate the degree of consistency between repeated measurements. A higher ICC value suggests greater reliability, meaning that the measurements are more consistent and reproducible. In research, ICC helps determine if ultrasound is a reliable tool for measuring muscle thickness across different muscle groups.

5

What do the Coefficient of Variation and Typical Error of Measurement tell us about precision when using ultrasound?

Coefficient of variation (CV) and typical error of measurement (TEM) are statistical measures used to quantify the precision of ultrasound measurements. The CV expresses the variability of measurements as a percentage of the mean, while the TEM indicates the expected error in a single measurement. Lower CV and TEM values indicate greater precision, meaning that the measurements are more consistent and less prone to error. These measures are important for determining the accuracy and reliability of ultrasound in assessing muscle thickness.

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