Tendon conditioning: artefact or property?

Isolated tendons subjected to cyclic tensile loads higher than those experienced in the tendons' recent history exhibit 'conditioning', i.e. gradually increasing elongations upon loading and gradually increasing residual elongations after unloading in the first few loading-unloading cycles. The present study examines whether this behaviour is a measurement artefact or an actual time-dependent property. The gastrocnemius tendons of six men who refrained from rigorous physical activities prior to the experiment were loaded cyclically by 10 repeated isometric plantarflexion contractions at 80% of the moment generated during plantarflexion maximal voluntary contraction (MVC). In each contraction, the elongation of the gastrocnemius tendon at 80% of MVC and the residual tendon elongation after relaxation were obtained from the analysis of sonographs recorded during the test. The tendon elongation during activation and the residual tendon elongation after relaxation increased by ca. 5 mm from the first contraction to the tenth contraction, with no changes obtained after the fifth contraction. The behaviour of the tendon in the first five contractions indicates the presence of conditioning. It is therefore concluded that conditioning is a relevant property and not an artefact associated with in vitro testing. This has implications for joint kinematics and muscle excursion.