A Simplified Method for Considering Achilles Tendon Curvature in the Assessment of Tendon Elongation

Abstract

The consideration of the Achilles tendon (AT) curvature is crucial for the precise determination of AT length and strain. We previously established an ultrasound-kinematic-based method to quantify the curvature, using a line of reflective foil skin markers covering the AT from origin to insertion. The current study aimed to simplify the method by reducing the number of markers while maintaining high accuracy. Eleven participants walked (1.4 m/s) and ran (2.5, 3.5 m/s) on a treadmill, and the AT curvature was quantified using reflective foil markers aligned with the AT between the origin on the gastrocnemius myotendinous-junction (tracked by ultrasound) and a marker on the calcaneal insertion. Foil markers were then systematically removed, and the introduced error on the assessment of AT length and strain was calculated. We found a significant main effect of marker number on the measurement error of AT length and strain (p<0.001). Using more than 30% of the full marker-set for walking and 50% for running, the R2 of the AT length error saturated, corresponding to average errors of <0.1 mm and <0.15% strain. Therefore, a substantially reduced marker-set, associated with a marginal error, can be recommended for considering the AT curvature in the determination of AT length and strain.

Keywords: Achilles tendon; tendon elongation; tendon strain; ultrasound; walking and running.

Figure 1

(a) Experimental setup for determining the Achilles tendon (AT) length during walking and running. Reflective foil markers on the skin were used to reconstruct the curvature of the AT (black markers) from origin to insertion (white markers). The gastrocnemius medialis (GM) myotendinous junction (MTJ), as the AT origin, was projected to the skin surface, and the coordinates of the ultrasound images were transferred to the global coordinate system using a tripod that was mounted on the ultrasound probe. (b) The length of the AT with all foil makers during walking, slow running and fast running throughout the gait cycle. Vertical lines separate stance and swing phases. GL: gastrocnemius lateralis, US: Ultrasound.

Figure 2

The absolute error of the Achilles tendon (AT) length measurement as the difference between the different marker-sets (MS) in comparison to the curved AT length (reference value) throughout the gait cycle. MSS is the straight AT length, and MS1-8 indicates the marker-set with one to eight foil markers. The vertical dashed line separates the stance and swing phase.

Figure 3

Averaged Achilles tendon (AT) length measurement error for each marker-set with respect to the curved reference length of the AT during the (a) stance phase and (b) swing phase of walking, slow and fast running. The marker-set with i number of foil markers (MSi) is the AT length with i = 1 to 8 foil markers. P5 to P9 are the clustered participants with the same number of foil markers. The error bars express the standard deviation of the AT length error throughout the phase (stance, swing), showing the variance over each phase and then averaged among individuals within each cluster. * main effect of phase ($$p<0.001$$), † main effect of marker-set ($$p<0.001$$), # interaction effect of phase and marker-set ($$p<0.001$$). The post hoc analysis for the comparison of marker-set between swing and stance phases revealed significant lower errors for MS1-3 ($$p<0.001,p<0.0001$$ and $$p=0.012$$, respectively) during the stance phase and no significant phase differences for MS4-8 ($$p=0.126,0.443,0.665,0.777$$).

Figure 4

(a) Individual Achilles tendon (AT) length measurement error with respect to the reference length (i.e., the AT length with all foil makers) as a function of relative marker number. The 0% indicates the straight AT length, and 100% the curved reference AT length. Note the different number of markers between individuals. (b) The resultant $$R^2$$ of the linear regression on the data presented in panel (a) in % intervals from 0% to 100% of the remaining markers.

Figure 5

Maximum strain error of the Achilles tendon (AT) during walking, slow and fast running for marker-set 1-8 (markers-set (MS) with 1 to 8 foil markers) with respect to the marker-set with all possible foil reflective markers. Participants were clustered into five groups of P5-9 (participants with 5 foil makers to participants with 9 foil markers). † main effect of marker-set ($$p<0.001$$).