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. Author manuscript; available in PMC: 2025 Jan 31.
Published in final edited form as: J Biomech. 2024 Nov 12;177:112427. doi: 10.1016/j.jbiomech.2024.112427

Effect of joint angle positioning on shearwave speed and variability with ultrasound shearwave elastography in asymptomatic Achilles and patellar tendons

Rachana Vaidya a, Stephane Cui a, Bryson Houston b, Andrew North b, Menghan Chen a, Josh Baxter b, Jennifer A Zellers a,*
PMCID: PMC11784490  NIHMSID: NIHMS2047996  PMID: 39546816

Abstract

This study investigated the impact of joint positioning on ultrasound shear wave elastography measurements in the Achilles and patellar tendons. Twenty-eight healthy adults underwent SWE assessment of shear wave speed (SWS) and coefficient of variation in SWS (CV-SWS) at three ankle positions (neutral, 10° plantar flexion, and 20° dorsiflexion) and two knee positions (90° flexion and full extension), at two academic sites. Participant positioning for ankle testing differed between sites (prone vs long-sitting)—while knee testing used consistent positioning. At the ankle, both joint and participant positioning significantly affected SWS. In the prone position, SWS was lower in neutral compared to dorsiflexed position (3.07 ± 1.13 m/s vs. 3.95 ± 1.03 m/s, p = 0.013). In long-sitting, SWS was lower in neutral compared to plantarflexed position (2.85 ± 0.53 m/s vs. 4.86 ± 1.92 m/s, p = 0.016); and SWS was higher in the plantarflexed position when participants were in long-sitting compared to prone (4.86 ± 1.92 m/s vs. 3.25 ± 1.13 m/s, p = 0.016). Participant positioning affected CV-SWS, with higher variability observed in prone compared to long-sitting in plantarflexed (29.3 ± 15.5 % vs 12.4 ± 9.12 %, p = 0.005) and neutral ankle angles (p = 0.03).

At the knee, joint position significantly influenced SWS, with higher values in flexed versus extended positions (6.48 ± 3.1 m/s vs. 4.60 ± 2.3 m/s, p = 0.007). Extending the knee reduced CV-SWS compared to flexed position (14.5 ± 11.2 vs 19.2 ± 13.4, p = 0.044). In conclusion, joint position significantly affected SWS measurements in both the Achilles and patellar tendons, while participant positioning influenced measurement variability. Thus, standardizing joint and participant positioning is important to enhance the reliability of SWE assessments of tendon elasticity.

1. Introduction

The structural integrity and functional capacity of tendons, particularly the Achilles and patellar tendons, are essential for effective performance of the musculoskeletal, especially in locomotion activities (Magnusson et al., 2008; Wang, 2006). Elastography, an advanced imaging suite that assesses tissue shear elasticity has shown promise in evaluating tendon tensile behavior, an area increasingly relevant in musculoskeletal health assessments (Corrigan et al., 2019b; Laurent et al., 2020). Although elastography’s utility has been widely used in diagnosing conditions such as breast cancer, liver fibrosis, and thyroid nodules, its application in musculoskeletal health is a burgeoning field of interest (Koo and Hug, 2015).

Ultrasound shear-wave elastography (US-SWE), a non-invasive ultrasound modality within elastography, facilitates the measurement of shearwave speeds in targeted tissues. This is achieved through the generation of acoustic radiation force and subsequent measurement of shearwave speed with a transducer, which correlate to the shear elasticity of the tissue under examination. The results are displayed as a color map, providing a visual representation of tissue shear speed across a region of interest (Taljanovic et al., 2017) (Fig. 1). The efficacy of SWE in Achilles and patella tendon has been affirmed in previous studies (Corrigan et al., 2019a; Taş et al., 2017a) with its ability to target small, specific tissue regions allowing for precise evaluation of tendon pathologies, such as tendinopathy (Ito et al., 2023; Prado-Costa et al., 2018).

Fig. 1.

Fig. 1.

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Basic physical principle of shear-wave elastography (SWE). Application of high intensity ultrasound for a duration on the order of 100 μs generates shear waves in tissue. The induced micron-level shear wave displacements are detected by speckle tracking methods used in color flow imaging. With the B-Mode image for guidance, the user can adjust the size and position of the region of interest (ROI) to align to the anatomy of interest. Shearwave speed (m/s) is averaged across the ROI and relates to tissue elasticity (Healthcare, 2020).

SWE has been instrumental in detecting shearwave speed changes associated with pathological changes in patellar (Breda et al., 2020) and Achilles tendinopathies (Aubry et al., 2015; Corrigan et al., 2019a; Dirrichs et al., 2016; Dirrichs et al., 2018; Hsiao et al., 2015; Zellers et al., 2017), highlighting its potential in early detection of tendon issues, injury risk assessment, and rehabilitation evaluation (Prado-Costa et al., 2018). However, despite these advances, one of the primary challenges with ultrasound SWE is the reproducibility and variability in measurements, particularly in musculoskeletal applications (Baumer et al., 2017).

Various factors that influence tendon elasticity have been identified, including age (Slane et al., 2017), BMI (Capalbo et al., 2016; Siddiqui et al., 2023; Taş et al., 2017b), sex (Ashton et al., 2023), and muscle strength (Chino and Takahashi, 2015; Koo and Hug, 2015; Kuervers et al., 2021; Taş et al., 2017a; Taş et al., 2017b), with studies noting variations in tendon material properties across these demographics. While previous research using SWE has demonstrated that joint angle position significantly affects tendon elasticity in both the patellar and Achilles tendons (Berko et al., 2015; Coombes et al., 2018; Hug et al., 2013; Kuervers et al., 2021), how joint position influences variability in shearwave speeds remains unknown.

Ensuring reliable data acquisition with shearwave elastography requires a thorough understanding of the factors that contribute to variability. Once factors contributing to variability in shear wave speed (SWS) measurement have been identified, they can be controlled through the optimization of scanning protocols, study design, or statistical methodologies. The aim of this study was to investigate how joint and participant position influences the variability of SWS. We hypothesized that positions placing the tendon on stretch would reduce the variability in SWS when compared to positions placing the tendon on slack as the tendon would be assessed in the linear region (rather than the toe region) of the stress–strain curve. The findings from this study provide valuable insights for clinical and research applications, guiding decisions on optimal joint and participant positioning to minimize variability. Ultimately, our findings aim to establish standardized SWE protocols that will improve reproducibility in research and clinical contexts.

2. Methods

2.1. Participants

The study was comprised of data pooled from two academic centers using the same make and model of ultrasound scanner and standardized joint positioning. The study was approved by both institutional review boards and informed consent was obtained prior to testing. To balance practical feasibility with sufficient statistical power to detect meaningful differences and correlations, we determined our sample size based on previous studies and practical considerations—including recruitment feasibility, time constraints, and resource availability (Berko et al., 2015; Coombes et al., 2018). A total of 28 participants with asymptomatic tendons were recruited for this study (Table 1). Inclusion criteria were individuals aged 18–65 years with asymptomatic tendons. Exclusion criteria were diagnosis of collagen disorders, tendinopathy (tendon pain) or history of tendon rupture, and signs of tendinosis on ultrasound imaging (fusiform tendon thickening, focal hypoechoic areas, loss of usual echotexture). Demographic information including age, sex, height, and weight were collected at the start of the study.

Table 1.

Demographic characteristics of study participants.

Demographic characteristic Site 1 (n = 15) Site 2 (n = 13) p-value All (n = 28)
Age (years) (mean ± SD) 23.22 ± 1.48 29.70 ± 6.56 0.001* 26.53 ± 5.75
BMI (kg/m2) (mean ± SD) 22.22 ± 3.02 24.43 ± 5.93 0.36 23.53 ± 4.63
Sex [n (%)]
Female 6 (40 %) 11 (84.6 %) 17 (60.7 %)
Male 9 (60 %) 2 (15.38 %) 11 (39.3 %)
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*

Indicates statistical significance (p < 0.05).

Participants were screened for palpatory tenderness of the Achilles and patellar tendons prior to ultrasound imaging to ensure participants met eligibility criteria. Long-axis B mode, extended field of view images of the Achilles and patellar tendons were acquired to screen for asymptomatic tendinosis.

2.2. Shear wave elastography

Ultrasound scans were completed unilaterally, with the leg randomly selected prior to testing, to ensure unbiased limb selection. Leg dominance was not explicitly considered, as randomization was used to mitigate any potential effects of leg tested. Each site had a single assessor performing the scans (one assessor per site). The assessors were research trainees who had received training in all procedures by the senior authors (clinician-scientist/researchers with over a decade of experience in Achilles tendon quantitative imaging). SWE scans were performed using a Logiq E10s ultrasound with a ML6–15 linear array transducer (GE Healthcare, Chicago, IL, USA). The transducer was held in long-axis along the midline of the patellar and Achilles tendon, and images were captured with an SWE color map superimposed on a B-mode grayscale base image. Scans were acquired at frequencies between 7–15 MHz. The dynamic range of the machine is 0–15 m/s, however we truncated the display range to 7 m/s to enhance visualization of the color map in the images. Transducer position was marked on the skin to assist with consistent placement of the transducer on the tendon. Standardization of probe placement was based on anatomical landmarks. At the Achilles, the calcaneus bone was used as a reference point. In case of the patellar tendon, the region between the patella and tibial tuberosity served as a landmark. All SWE images were captured at the midportion of the free tendon to minimize technical challenges posed by adjacent bony structures and the musculotendinous junction, which can obscure clear elastography images. The joint positions during the scans were selected to capture a range of joint angles emphasizing angles typically encountered with daily activities, such as walking and squatting. These angles enable evaluation of tendon properties in relatively slack and tensioned states.

For Achilles tendon measurements, participants were positioned in a joint dynamometer (System 4, Biodex, Shirley, NY, USA) at three different ankle joint angles, including 0 degrees of flexion (neutral), 10 degrees of plantarflexion (plantar flexion), and 20 degrees of dorsiflexion (dorsiflexion). Due to differences in dynamometer positioning options, participants at testing site 1 were positioned in prone for the Achilles tendon measurements and participants were positioned in reclined long sitting for the Achilles tendon measurements at testing site 2.

For patellar tendon measurements, the participants were positioned in reclined sitting at two different knee joint angles, including 0 degree (full extension) and 90 degrees (passive knee flexion) (Fig. 2).

Fig. 2.

Fig. 2.

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Participant positioning for data acquisition. (A) For patellar tendon measurements, participants were positioned in two knee-joint angles – 0° (full extension) and 90° passive flexion. (B) For Achilles tendon measurement, participants were positioned in 3 different ankle-joint angles – 0° (neutral), 10° plantar flexion and 20° dorsiflexion. (Pictured in reclined, long sitting per site 2 protocol.).

The ankle and knee joint angles were set and maintained passively using the ankle and knee attachments of the dynamometer equipment along with strapping, which provided stabilization and prevented any unwanted movement during the scanning process. The order of the joint angle positions was randomized to minimize the potential influence of any order effect on the measurements. Participants were instructed to remain still and relaxed during the scan to minimize muscle activity.

For both tendons, three circles were selected across the region of interest and averaged for each image measurement (Fig. 3). The average and coefficient of variation of measurements from three separate images at each joint angle position was then calculated for each participant.

Fig. 3.

Fig. 3.

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Shear-wave elastography results for a representative participant at (A) extended knee, highlighting patellar tendon in the image, and showing ROI selection for the image. (B) Shear-wave elastography results for a representative participant with ankle at 0 degrees neutral position.

Statistical analysis:

To assess how consistent the measurements are across joint and participant positioning which directly contributes to identifying the most stable and reliable positions for tendon assessment; coefficient of variation in SWS (CV-SWS) was calculated between serial SWE measurements. CV-SWS was defined as the coefficient of variation within participant/between trials and was calculated for each participant using the formula: CV = (standard deviation/mean) * 100.

To assess the effects of joint angle and participant positioning on shear wave speed (SWS) variability (coefficient of variation) and magnitude, separate mixed-effect models were made for the ankle and kn. Because participants were positioned similarly between sites for the patellar tendon measurements, the effect of participant position was not included in the model. Tukey post-hoc analyses were conducted to identify significant differences between specific joint and participant positions. For the ankle, the Tukey test was used, while for the knee, the Bonferroni test was applied. By calculating CV between serial SWE measurements, we can assess how consistent the measurements are across conditions (joint angle and participant positions), which directly contributes to identifying the most stable and reliable positions for tendon assessment.

3. Results

3.1. Joint position influences shearwave speed but not variability in the Achilles tendon

Joint position significantly affected SWS (p = 0.0054). Participant positioning did not have a significant effect on the SWS (p = 0.98), however there was a significant interaction between joint position and participant position (p = 0.003). Post hoc pairwise comparisons revealed lower SWS in the neutral position compared to the dorsiflexed position when the participant was positioned in prone (p =0.013), while SWS was lower in neutral compared to plantarflexed position when the participant was positioned in long-sitting (p =0.016). Additionally, SWS was significantly higher in the plantarflexed position when participants were positioned in long-sitting compared to prone (p = 0.015).

Joint position did not significantly affect variability in shear wave speed (CV-SWS; p = 0.54, Fig. 4). However, CV-SWS was affected by participant positioning (p < 0.001), with no interaction between joint position and participant position (p = 0.69). Higher variability was observed with the participant positioned in prone compared to long-sitting position in the plantar flexed (p = 0.005) and neutral ankle angles (p = 0.03). Variability was consistent across joint positions within each participant position.

Fig. 4.

Fig. 4.

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Violin plots representing Achilles tendon (A) Shearwave speed and (B) coefficient of variation at different ankle joint angles and participants positions. Significant differences between joint angles and participant positions are denoted by p-values.

3.2. Different joint positions influence shearwave speed and variability in the patellar tendon

Because participant position was held constant at the knee for both testing sites, data were pooled across sites for analysis. Joint position significantly influenced shear wave speed (p = 0.010), with higher shear wave speeds observed with the knee in the flexed position compared to the extended position (p = 0.007). Positioning the knee in extension reduced variability in shear wave speed measurement (p = 0.044, Fig. 5).

Fig. 5.

Fig. 5.

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Violin plots representing Patellar tendon (A) shear wave speed and (B) coefficient of variation at different knee joint angles. Significant differences between positions and sites are denoted by p-values.

4. Discussion

Our study confirms that, while joint position affects SWS, it did not significantly impact measurement variability (CV-SWS). Instead, positioning of the participant played a larger role, highlighting the importance of standardizing participant positioning to ensure reliable data acquisition. These findings are particularly important for researchers and clinicians looking to use US-SWE as a diagnostic or monitoring tool for tendon health assessment.

Variability in SWS in the Achilles tendon within each participant was more influenced by participant positioning than by joint position, which may reflect physiological differences (e.g., tightening of the posterior fascial chain) or technical differences (e.g., improved consistency of transducer pressure with different positioning of the assessor). Positioning the participant in reclined, long-sitting reduced the variability of the shear wave measurement in both ankle plantar flexion and neutral positions, however, positioning in long-sitting did result in unanticipated increases in shear wave speeds in the plantar flexed ankle position. It may be that positioning in long-sitting improved precision but reduced accuracy of shear wave measurement in the plantar flexed position. The results of this study suggest that positioning in reclined long-sitting in a neutral or dorsiflexed position or positioning in prone with the ankle in dorsiflexion may yield the most consistent measurement.

The effect of joint position on SWS in the Achilles tendon suggest that ankle positioning is important in obtaining accurate measurement. Our findings align with previous research which shows that dorsiflexed positions result in higher SWS (Berko et al., 2015; Coombes et al., 2018) which could be due to increased tendon tension and collagen fiber alignment (Blank et al., 2022). In plantarflexion, the Achilles tendon and associated muscles, like the gastrocnemius and soleus, are in a shortened state, reducing tension in the muscle–tendon unit and typically leading to lower SWS (DeWall et al., 2014). However, when participants were positioned in reclined long-sitting, higher SWS was observed in plantarflexion, which may reflect tension in the posterior fascial chain increasing gastrocnemius tension and altering muscle–tendon mechanics (Blank et al., 2022; Liu et al., 2020). In summary, our data in the Achilles tendon suggest that dorsiflexion provides the most consistent and reliable SWS measurements across both participant positions. Dorsiflexion aligns the tendon under greater tension, which likely results in more stable and higher SWS values compared to other joint positions. However, it is critical to maintain consistent participant and ankle positioning to minimize variability.

In the patellar tendon, positioning the knee joint in extension reduced measurement variability, suggesting that knee extension may yield more reproducible measurement. Joint positioning also significantly influenced patellar SWS with greater knee flexion yielding higher shear wave speeds consistent with prior studies (Berko et al., 2015). In these flexed positions, tendons experience a shift towards the linear portion of the stress–strain curve. This shift is characterized by the alignment and stretching of collagen fibers within the tendons, leading to an enhanced resistance to deformation, and consequently, higher shear wave speeds. Conversely, in a more relaxed tendon state, the collagen fibers experience reduced tension, likely positioning the tendon in the toe region of the stress–strain curve and resulting in lower shear wave speeds.

Our results have important implications for clinical and research applications of shearwave elastography (SWE). Clinicians and researchers should maintain consistency in joint positioning throughout assessments to ensure reliable data. For the Achilles tendon, positioning the ankle in dorsiflexion may offer some benefit – particularly if needing to image the tendon with the participant positioned in prone. For the patellar tendon, positioning the knee in full extension yields improved measurement repeatability. Standardized protocols for participant positioning, repeated measurements, proper limb support for stabilized joint position, and consistency in assessor positioning are essential, especially in multisite studies, to pool data between sites and reduce measurement variability. ‘

Our previous work demonstrated that ultrasound scanning parameters, such as frequency settings, had minimal impact on SWS variability (less than 3 %), emphasizing the importance of participant-related factors in driving variability. While our frequency range (7–15 MHz) optimized image quality, this had no significant impact on SWS in our current study, supporting the reliability of our results across different trials. Nevertheless, variability in image acquisition parameters can introduce minor inconsistencies, which should be accounted for in clinical practice (Zellers et al., 2024). Important to the interpretation of the current study findings, this prior study found very minimal effect of site on shear wave speed measurement when participant positioning was consistent in both testing locations (Zellers et al., 2024).

US-SWE is a promising non-invasive technique for assessing tendon elasticity, however it faces several limitations in assessing tendon health reliably, including high sensitivity to participant positioning and probe placement, which introduces variability in results (Romano et al., 2021). Assessor technique and site-specific factors may affect measurement consistency (Pelea et al., 2023) while the lack of standardized protocols exacerbates these issues (Payne et al., 2018). Spatial and regional variations along the length of the tendon as well as medial lateral differences further complicates measurement procedures (DeWall et al., 2014; Slane et al., 2017). Additionally, its technical sensitivity to muscle activation complicates interpretation, leading to variability across different operators and clinics (Kot et al., 2012). Addressing these limitations is critical for SWE’s broader clinical application.

A limitation of our study is the lack of inter-assessor reliability due to the use of different participant pools at each site, though we previously found the effect of site (which includes variability introduced by rater) to have very minimal effect on tendon shear wave speed. Additionally, our relatively homogeneous sample in individuals without tendon injury limits the generalizability of our findings to broader populations. The variability introduced by positional differences across sites underscores the need for strict standardization in future studies to ensure consistent results across multiple locations.

5. Conclusion

Our study highlights the significant impact of joint and participant positioning on shear wave speed (SWS) and variability in both the Achilles and patellar tendons. The findings underscore the importance of maintaining consistent participant and joint positioning, to ensure reliable and reproducible data. Overall, these findings provide guidance for standardized protocols in tendon health assessment to reduce measurement variability and ensure consistency in clinical and research applications using shear wave elastography.

Acknowledgements

This study was supported by the National Institutes of Health/National Institute of Arthritis and Musculoskeletal and Skin Diseases (Grant #AR078898, AR080581). The authors have no other conflicts of interest relevant to this study.

Footnotes

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Rachana Vaidya:. Stephane Cui: Methodology. Bryson Houston: Methodology, Data curation. Andrew North: Methodology, Data curation. Menghan Chen: Validation, Methodology, Data curation. Josh Baxter: Writing – review & editing, Supervision, Resources, Investigation, Funding acquisition, Conceptualization. Jennifer A. Zellers: Writing – review & editing, Supervision, Resources, Investigation, Conceptualization.

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