Alteration in the strain properties of the Achilles tendon may lead to adaptations such as pathological stiffening. Stiffer tendons have less adaptive ability, which may predispose to the development of tendinopathy. There is a current lack of methods to measure in-vivo tissue strain. A two-probe ultrasound procedure has been proposed to reduce measurement error associated with a one-probe procedure. However, reliability of the two-probe procedure has not been established. The study aimed to determine intra-rater, inter-session and inter-rater reliability of a two-probe ultrasound procedure to measure Achilles tendon strain.
Twentynine healthy participants (19 females, 10 males mean age 33.6 years) were included. Achilles tendon images were acquired with a two-probe ultrasound procedure as the ankle moved through a standardised range of motion (20° plantarflexion to 10° dorsiflexion). Both probes were positioned longitudinally, one probe over the musculotendinous junction and the second over the calcaneal insertion of the Achilles tendon. Repeat measurements were taken at a second session at the initial study visit and four weeks later. Strain measures were calculated from the pre-captured images using Motion Analysis 2014v1 software by two independent raters. Intra-rater, inter-session and inter-rater reliability were calculated using intraclass correlation coefficients (ICC). In addition, 95% confidence intervals for the ICC and the standard error of measurement (SEM) were also calculated.
Excellent intra-rater within-session reliability (ICC = 0.84, P < 0.001), poor inter-session reliability (ICC = 0.18, p = 0.397) and excellent inter-rater reliability (ICC = 0.88, p = 0.003) was demonstrated for the measurement of Achilles tendon strain using the two-probe procedure.
Discussion and Clinical Relevance
The two-probe procedure to measure Achilles tendon strain is reliable for repeated measurements within the same day. However, measurement error increased when strain was measured on different days, which may be due to a combination of examiner error and participant factors. Measurement of Achilles tendon strain offers an additional tool for the evaluation of the tendon’s mechanical characteristics. The ability to reliably quantify strain may allow clinicians to identify those at risk of Achilles tendinopathy and to formulate more effective management plans.