Assessment of Quadriceps Muscle Characteristics in Female Division I Athletes: A Validation Study of Wireless Probes Against Standard Ultrasound Units

Abstract

Objective:

Wireless ultrasound probes offer a quicker, more affordable option for muscle quality assessment compared with standard cart units, yet their effectiveness for evaluating larger muscles such as the rectus femoris in terms of cross-sectional area (CSA) and echo-intensity (EI) is unclear due to limited field of view. This study evaluates whether rectus femoris thickness and EI measured with a wireless probe correlate with CSA and EI obtained from a standard cart ultrasound.

Methods:

A cross-sectional, convenience sample of 29 division I college female athletes (age: 20.1 [1.1] y, height: 169.7 [7.4] cm, mass: 69.7 [10.0] kg) were recruited. Panoramic thigh ultrasound images were acquired with a standard ultrasound cart to assess the rectus femoris CSA and EI at 50% of the thigh length. A wireless ultrasound probe was used to acquire stationary images with the knee in the same position to assess rectus femoris thickness and EI. A Pearson product–moment correlation was used to determine the association between the muscle outcomes obtained with the standard cart ultrasound and wireless ultrasound probe.

Results:

Standard ultrasound CSA (10.1 [2.0] cm²) and wireless ultrasound thickness (2.0 [0.3] cm) were strongly associated (r = .71, P < .001). Standard ultrasound EI (56.2 [5.1] arbitrary units) and wireless ultrasound EI (62.0 [6.3] arbitrary units) were moderately associated (r = .49, P = .007).

Conclusion:

Wireless ultrasound offers a fast and accessible method for assessing muscle thickness in female division I athletes when compared with similar muscle size and quality metrics measured on panoramic images acquired with a standard ultrasound cart. The echogenicity indices from wireless and standard ultrasound are significantly associated between units; however, wireless ultrasound systematically overestimates echogenicity compared with the standard.

Ultrasound imaging is a minimal risk and accessible tool to assess muscle quality across populations.^1,2 Diagnostic ultrasound is valuable for tracking changes in muscle size and assessing quadriceps function by measuring cross-sectional area (CSA) or muscle thickness.^3–5 Additionally, echo-intensity (EI) provides information regarding the amount of noncontractile tissue within a muscle and is considered an assessment of the overall muscle quality.^1,6 The quadriceps play a crucial role in knee stabilization, and any alterations in their function can significantly increase the risk of injuries and the development of chronic conditions.^7–11 Quadriceps muscle CSA and EI have been extensively assessed with standard ultrasound units with a panoramic imaging function to assess the full cross-section of the muscles.^1,12 Advancements in ultrasound imaging have improved accessibility and reduced costs; however, the recent advancements in wireless ultrasound continues to break down the cost barrier. The portability and cost of wireless ultrasound allows for a wider range of individuals to utilize imaging techniques, while opening the door to more diverse locations such as on-field assessments. While it has been found that wireless ultrasound can improve the patient experience by reducing the overall time required for imaging assessments,¹³ previously published research has shown that wireless probes may introduce errors when measuring characteristics, such as fascicle length, pennation angle, and muscle thickness of the rectus femoris when compared with high-end or standard ultrasound units.¹⁴ Wireless ultrasound has been found to have both positives in use while still having potential negatives; however, these varying results show that more work is needed to validate the use of portable ultrasound on specific measurements of muscle size.

Wireless ultrasound probes lack panoramic imaging capabilities, limiting their ability to capture full CSA measurements particularly in larger muscles. Unlike standard ultrasound units that can generate panoramic images, wireless probes typically rely on single static images taken at a specific location, often the thickest portion of the muscle. This methodological difference may introduce bias when comparing muscle size assessments between wireless and standard ultrasound probes. However, prior research using standard ultrasound has shown a strong correlation between muscle thickness and CSA, suggesting that muscle thickness can serve as a reliable surrogate for overall muscle size.^15–17 While this relationship is well established with standard ultrasound, it remains unclear whether a single-image muscle thickness measurement from a wireless probe can accurately reflect whole-muscle CSA obtained through panoramic imaging. Given the increasing use of wireless ultrasound for clinical and field-based assessments, further research is needed to validate its ability to estimate muscle size when panoramic imaging is unavailable.

Therefore, the purpose of this study was to validate a novel, portable, wireless ultrasound probe with a standard ultrasound unit for assessing a measure of quadriceps, specifically the rectus femoris, muscle size, and quality. Because the standard ultrasound unit allows for the acquisition of a panoramic image to capture the entire quadriceps muscle area and this feature is not available for wireless ultrasound probes, we sought to determine if quadricep thickness and EI from a stationary B-mode acquired with a wireless ultrasound probe (ie, Clarius) is associated with quadricep CSA and EI from a panoramic image acquired with a standard ultrasound unit (ie, General Electric). To achieve this purpose, we utilized both a wireless probe and standard ultrasound unit to capture images of the quadriceps at the same location. Validating the wireless probe will provide the knowledge if these cost-effective and portable probes can be used to assess larger muscles, such as the quadriceps, in Division I female athletes.

Methods

Study Design

This cross-sectional study of Division I female athletes aimed to validate wireless ultrasound probe assessed muscle thickness and EI with panoramic images acquired with a standard ultrasound unit CSA and EI.

Participants

We utilized participants from an ongoing longitudinal study tracking health and performance of female Division I college athletes throughout multiple athletic seasons. This study was approved by the Institutional Review Board at Michigan State University and all participants completed written, informed consent. For participants to be included in this study, they had to be a female between the ages of 18 and 25 and a member of an National Collegiate Athletic Association Division I athletics team at our institution. Participants were excluded if they experienced any head, upper, or lower body injuries within 4 weeks prior to testing or if they had any contraindications to the use of ultrasound, such as open wounds at the imaging site. All participant recruitment for this study occurred from April 13th, 2023 to April 28th, 2023. Participants were not asked to refrain from exercising prior data collection.

Ultrasound Image Acquisition With Standard Ultrasound Unit

A single investigator (Grozier) used a standard ultrasound unit (General Electric LOGIQ P9 R3) to perform panoramic transverse scans, capturing entire cross-sectional images of each participant’s quadricep muscle. All images were obtained in B-mode at a depth of 4 cm, with a frequency of 12 MHz, and a gain setting of 50. All participants were positioned supine on the examination table with the knee in 20 to 30 degrees of flexion with their knee supported by a bolster. A mark was placed at the midpoint (50%) of the distance between the anterior superior iliac spine and the lateral knee joint line. Ultrasound images were collected at this location for both standard and wireless devices. For the standard ultrasound, panoramic images were acquired across the thigh to capture the entire CSA of the rectus femoris. In contrast, for the wireless ultrasound, muscle thickness was measured at the thickest portion of the rectus femoris at the same location (50% of thigh length). During image collection, participants were instructed to keep their quadriceps relaxed. The probe was kept perpendicular to the skin at all times while the probe was moved medially to laterally for the panoramic image acquisition to include the entire quadricep muscle. Ample ultrasound gel was applied to improve transmission of ultrasound waves with limited interference. Three panoramic images were collected unilaterally for each participant at a depth of 4 cm, frequency of 12 Hz, and gain set at 50.

Ultrasound Image Acquisition With Wireless Ultrasound Probe

Wireless ultrasound, despite its enhanced portability, is limited to single-image capture in the quadriceps, unlike standard ultrasound units that offer panoramic views. As a result, images were obtained at the thickest portion of the rectus femoris, positioned at 50% of the thigh length, to assess muscle thickness. The same investigator (Grozier) performed image acquisition using the Clarius L15 HD3 wireless probe (Clarius Inc). Three single-image scans were collected on the same limb as the standard ultrasound cart images, using a depth of 4 cm and a frequency of 12 MHz.

Ultrasound Image Segmentations

All ultrasound images from the standard and wireless ultrasound units were processed using the open-source ImageJ software (National Institutes of Health). When segmenting the panoramic images, the entire cross-section of the quadricep was segmented by tracing inside the muscle’s aponeuroses while excluding any hyperechoic tissue during segmentation using the polygon function (Figure 1A).¹ The average CSA and EI for the segmented quadricep muscle were calculated from 3 images acquired using a standard ultrasound unit (Figure 1A). A single ultrasound reader completed all segmentations, having previously established high interrater reliability for muscle CSA compared with a trained reader in our lab on a reliability set of 30 images intraclass correlation coefficient ([ICC_2,k] = .99). Due to the wireless ultrasound probe’s inability to perform panoramic imaging, we assessed muscle size using muscle thickness on a single image, measured as the distance between the superior and deep aponeurosis at the thickest portion of the rectus femoris.^18,19 To evaluate quadricep EI, we used an oval function on the single stationary image to encompass the maximum visible muscle belly (Figure 1B). While the lack of panoramic imaging does not allow for the entire muscle to be captured, this technique provides a rapid, partial quantification without requiring full-muscle segmentation. EI, representing the brightness of the image, was averaged within the selected area on a scale from 0 to 255 (0 = black; 255 = white). Lower EI values (closer to 0, more hypoechoic) indicate higher muscle quality, whereas higher values (closer to 255, more hyperechoic) suggest lower muscle quality.⁶ The average muscle thickness and EI of the 3 images were used for analysis.

Figure 1 —

Wireless and standard ultrasound images. Images of the quadricep were collected at 50% of thigh length between the anterior-superior-iliac-spine and the lateral joint line. A: Cross-section area of the quadricep segmented within ImageJ allows only the region of interest within the muscle belly to be used to assess muscle characteristics. B: Ultrasound image captured with the wireless ultrasound unit. *A singular line was traced from the superior to deep aponeurosis to assess muscle thickness. ^†The majority of the muscle belly is captured using the oval function within ImageJ. This area is used to assess echo intensity.

Statistical Analysis

Pearson product–moment correlation analyses were used to determine the association between the 2 ultrasound units when assessing CSA, muscle thickness, and EI. Additionally, to assess the consistency of athlete rankings between the wireless and standard ultrasound units for CSA, muscle thickness, and EI measurements from both devices, a Spearman rank-order correlation was performed.²⁰ Association levels were set at: weak (.0–.29), moderate (.30–.50), or strong (>.50).²¹ We decided to use correlation analyses instead of agreement statistics because we are comparing 2 muscle size outcome measures that are on different scales (ie, muscle thickness vs CSA). All statistical analyses were performed using IBM SPSS Statistics software with an alpha level of P < .05.²²

Results

Table 1 includes the participant demographics of the 29 Division I college female athletes (10 volleyball, 5 women’s soccer, 14 field hockey) enrolled in this study. There was a strong positive association (r = .71, P < .001) between the CSA (10.1 [2.0] cm²) assessed with the standard ultrasound unit and the muscle thickness (2.0 [0.3] cm) assessed with the wireless ultrasound probe (Figure 2A). Additionally, there was a moderate association (r = .49, P = .007) between the EI of the standard unit (56.2 [5.1] arbitrary units) and wireless probe (62.0 [6.3] arbitrary units) (Figure 2B). A strong positive correlation was found between athlete rankings for muscle thickness measured by the wireless and standard ultrasound units and CSA measured by the standard ultrasound unit (ρ = .77, P < .01). A moderate positive correlation was observed between athlete rankings for EI measurements from both devices (ρ = .40, P = .007), suggesting that rankings were generally consistent across measures.

Table 1.

Demographics

N	29
Age, y	20.1 (1.1)
Height, cm	169.7 (7.4)
Weight, kg	69.7 (10.0)
Standard ultrasound
CSA, cm2	10.1 (2.0)
EI, au	56.2 (5.1)
Wireless ultrasound
Muscle thickness, cm	2.0 (0.3)
EI, au	62.0 (6.3)

Abbreviations: au, arbitrary units; CSA, cross-sectional area; EI, echo-intensity.

Figure 2 —

Scatter plots comparing ultrasound characteristics between a standard ultrasound unit and wireless ultrasound characteristics. A: Cross-sectional area (10.1 [2.0] cm²) assessed using a standard ultrasound unit had a high positive association (r = .71, P < .001) with muscle thickness assessed using a wireless ultrasound probe (2.0 [0.3] cm). B: Echo intensity collected from standard (56.2 [5.1] arbitrary units) and wireless ultrasound (62.0 [6.3] arbitrary units) had a low association (r = .49, P = .007) between units.

Discussion

Our study demonstrates that wireless ultrasound can provide meaningful assessments of the quadricep muscle characteristics despite its inability to capture full panoramic images. We observed a strong association between muscle thickness measured with the wireless ultrasound and CSA measured with the standard ultrasound unit, suggesting that muscle thickness serves as a reasonable surrogate for muscle size when panoramic imaging is not available. This finding aligns with previous research showing that muscle thickness is a valid and practical measure of overall muscle morphology.^23,24 Additionally, the moderate association between EI rankings from the 2 ultrasound units indicates some alignment, though differences in EI measurements may be influenced by the variable gain setting function on the wireless ultrasound unit. Future research should continue to refine these methods and explore ways to enhance the comparability of EI measurements across devices.

The ability of wireless ultrasound to provide comparable measurements to standard cart-based ultrasound has important clinical and applied implications. Wireless ultrasound offers a cost-effective, portable, and time-efficient alternative for assessing muscle characteristics, making it particularly valuable for field-based settings or routine athlete monitoring. Its ease of use allows for rapid image acquisition and segmentation, enabling clinicians to track muscular adaptations throughout an athletic season without the logistical challenges associated with traditional ultrasound units. While panoramic imaging remains the gold standard for capturing full-muscle CSA, our findings suggest that single-image assessments with wireless ultrasound can still yield clinically relevant information.

While our findings support the utility of wireless ultrasound for assessing quadriceps muscle size, previous research has reported varying levels of agreement between wireless and standard ultrasound units across different muscles. For example, Ritsche et al observed inconsistent reliability depending on the muscle and characteristic being assessed. Their study found the highest agreement in the gastrocnemius medialis for fascicle length (ICC: .457–.899, SEM %: 6%–10.7%) and pennation angle (ICC: .423–.865, SEM%: 8.4%–12.6%), whereas the rectus femoris exhibited greater errors, particularly for fascicle length and pennation angle.¹⁴ These findings suggest that the accuracy of wireless ultrasound may be muscle- and metric-dependent, emphasizing the need for validation studies specific to each application. Despite these inconsistencies, wireless ultrasound technology continues to evolve, with improvements in imaging resolution and software algorithms likely to enhance its reliability for assessing muscle morphology in the future.

While our results suggest that muscle thickness measured with a wireless ultrasound probe provides similar results to CSA measured with a standard ultrasound unit, this study has several limitations. First, our sample consisted exclusively of Division I female athletes, limiting the generalizability of our findings to individuals with different body compositions, levels of fatty infiltration, or muscle groups beyond the quadriceps. Furthermore, participants were not asked to refrain from exercise prior to data collection, which could affect muscle characteristics. However, since the purpose of this study was to compare the measures across 2 scanners, any effects of prior activity would be assessed with both scanners and not affect the results of the current study. Clarius ultrasound devices are equipped with an auto-gain function that automatically optimizes imaging by adjusting gain to enhance image quality based on the tissue being scanned. This difference in gain could have resulted in the weaker association between scanners. Additionally, we choose to include the auto-gain function, as it helps maximize image quality based on the tissue being scanned. A recent study comparing knee cartilage characteristics between a standard and wireless ultrasound unit demonstrated poor agreement and reliability between EI measurements between the 2 units, even after multiple attempts to standardize the gain across images.²⁵ This suggests that gain standardization alone may not fully resolve discrepancies in EI measurements. Additionally, the greater sensitivity of EI to acquisition parameters, including scanner-specific auto-gain features, may have impacted the association between scanners. The use of the oval function to assess EI in the wireless ultrasound images may have contributed to differences between units, as this approach does not capture the full muscle cross-section. Another potential limitation is that the wireless ultrasound probe lacks panoramic imaging capabilities, preventing full-muscle CSA assessment. Instead, muscle thickness was used as a surrogate measure of muscle size. While this reduces direct comparability, our findings demonstrate a strong association between muscle thickness and CSA, supporting its validity as an alternative measure. Future studies should explore the impact of consistent gain settings and alternative EI segmentation methods to improve agreement between wireless and standard ultrasound assessments. Further research is also needed to determine whether these findings extend to other populations and muscle groups. This study compared muscle characteristics between wireless and standard ultrasound but did not include functional measures like muscle strength. Future research should incorporate functional outcomes to assess the clinical relevance of wireless ultrasound-derived measurements.

This study shows that wireless ultrasound can effectively assess muscle characteristics, even without panoramic imaging. Muscle thickness measured with wireless ultrasound strongly correlates with CSA from standard ultrasound, making it a useful alternative for muscle size evaluation. While EI measurements showed moderate agreement, differences may be due to variable gain settings, emphasizing the need for standardization. Wireless ultrasound is affordable, portable, and easy to use making it a practical tool for tracking muscle characteristics over time.

Key Points.

• This study validated wireless ultrasound muscle assessments, muscle thickness, and echo intensity, to standard ultrasound outcomes, cross-sectional area, and echo intensity, within the quadriceps.

• Wireless ultrasound offers a clinically assessable tool to assess quadricep muscle characteristics compared to standard ultrasound, despite its lack of panoramic imaging.