In vivo measurement of human tendon properties during activities of daily liv-ing using guided transmission mode ultrasonography

Measurement of tendon loading patterns during physical activity is important for understanding tendon injury. Alt-hough tendon forces have been commonly estimated from direct techniques, such as implantable transducers, these methods are highly invasive and interfere with normal gait patterns. Indirect methods such as estimates based on elec-tromyography, force plates and motion analysis are more widely used but are known to overestimate tendon loads by as much as 50% compared to direct measures. This paper explores the clinical measurement of tendon properties through the use of a novel, non-invasive guided ultrasound measurement technique. The technique takes advantage of the known relationship between transmission speed of ultrasound waves and the modulus and density of the material through which it propagates. In tendon, this relationship is governed by the classic Newtonian–Laplace equation with some adjustment for Poisson’s effects in elastic media. With application of physiological tensile load to tendon, the in-stantaneous elastic modulus of tendon increases along with the axial transmission velocity of ultrasound in the tendon. Hence, the change in transmission velocity of ultrasound is related to the magnitude of load applied to the tendon. This paper summarises the findings of a series of studies peformed within our laboratory, in which the properties and loading pattern of the Achilles or patellar tendon have been measured during common physical activities in healthy adults and in patients with tendon disorders. Ultrasound transmission velocity in healthy tendon usually ranges between 1700m/s and 2200m/s during activities, such as walking or squatting, and show a highly reproducible velocity-time pattern (typi-cal within-subject CV <5%). In addition to providing new insights into the differential loading patterns in injured ten-dons and their interplay with common rehabilitation approaches, the findings also highlight the potential of guided ul-trasound transmission techniques for functional assessment and monitoring of human tendon in vivo.