Plantarflexor moment arms estimated from tendon excursion in vivo are not strongly correlated with geometric measurements

Josh R Baxter,Stephen J Piazza

doi:10.1016/j.jbiomech.2018.06.020

Josh R Baxter, Stephen J Piazza

Open Access

https://doi.org/10.1016/j.jbiomech.2018.06.020

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Abstract

Geometric and tendon excursion methods have both been used extensively for estimating plantarflexor muscle moment arm in vivo. Geometric measures often utilize magnetic resonance imaging, which can be costly and impractical for many investigations. Estimating moment arm from tendon excursion measured with ultrasonography may provide a cost-effective alternative to geometric measures of moment arm, but how well such measures represent geometry-based moment arms remains in question. The purpose of this study was to determine whether moment arms from tendon excursion can serve as a surrogate for moment arms measured geometrically. Magnetic resonance and ultrasound imaging were performed on 19 young male subjects to quantify plantarflexor moment arm based on geometric and tendon excursion paradigms, respectively. These measurements were weakly correlated that approached statistical significance (R2 = 0.21, p = 0.052), and moment arm from tendon excursion under-approximated geometric moment arm by nearly 40% (p < 0.001). This weak correlation between methods is at odds with a prior report (N = 9) of a strong correlation (R2 = 0.94) in a similar study. Therefore, we performed 92,378 regression analyses (19 choose 9) to determine if such a strong correlation existed in our study population. We found that certain sub-populations of the current study generated similarly strong coefficients of determination (R2 = 0.92), but 84% of all analyses revealed no correlation (p > 0.05). Our results suggest that the moment arms from musculoskeletal geometry cannot be otherwise obtained by simply scaling moment arms estimated from tendon excursion.

Highlights

Plantarflexor moment arm is an important musculoskeletal parameter that simultaneously determines muscle mechanical advantage and influences the amount of muscle fiber shortening or lengthening that occurs during a given joint rotation
In vivo measurements of plantarflexor moment arm are often made using one of two methodological paradigms: 1) geometric methods that quantify the distance between the tendon line of action and the joint axis of rotation [1]; and 2) tendon excursion methods based on the Principle of Virtual Work that consider plantarflexor moment arm to be equal to the amount of tendon travel occurring per unit of joint rotation [2]
The results of the present study do not support our hypothesis that plantarflexor moment arms measured in vivo using tendon excursion/US would be strongly correlated with those measured using the geometric/MR method

Summary

Introduction

Plantarflexor moment arm is an important musculoskeletal parameter that simultaneously determines muscle mechanical advantage and influences the amount of muscle fiber shortening or lengthening that occurs during a given joint rotation. Tendon excursion/US measures were found to underestimate by nearly 30% those measured with geometric/MR measures; which may be an artifact of tendon relaxation caused by variations in tendon slack length or compliance Regardless of these differences, the finding of such a strong correlation suggests that tendon excursion/US may be a viable surrogate for geometric/MR measures of plantarflexor moment arm. Before reaching this conclusion, it is important to see this finding replicated in a larger number of participants

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