Validity of procedures involved in ultrasound-based measurement of human plantarflexor tendon elongation on contraction

Ultrasonography is becoming increasingly popular for studying the tensile behaviour of in vivo human tendons through elongation measurements during isometric contraction. For the plantarflexor tendons specifically, which receive much attention due to their functional role in locomotion, elongations are conventionally measured by fixing the scanning probe on the calf. Elongation corrections are also made to account for artifactual ankle rotations during the isometric test, by assuming that these occur round the tibio-talar joint axis, as is the case for passive ankle plantarflexion-dorsiflexion rotations. The present work was set out to examine the validity of these procedures. The displacement of the calcaneum, a skin marker on the calf, and the gastrocnemius myotendinous junction, were measured from rest to maximum isometric voluntary plantarflexion contraction (MVC) in six men by using ultrasound probes mounted on externally fixed points (active test). A passive ankle plantarflexion rotation equal in magnitude to that recorded during MVC was then performed (passive test). In the active test, the calcaneum and the skin marker shifted in the proximo-distal direction by ∼13 mm. Moreover, the calcaneal displacements in the active and passive tests took place round different rotational axes, as indicated by a calcaneal displacement difference of ∼10 mm between the two tests. These effects resulted in underestimating by 35% the actual elongation of the gastrocnemius tendon when following the currently suggested procedures. The present results directly invalidate the procedures conventionally followed for assessing the tensile response of the human plantarflexor tendons with ultrasonography.