Creep and the in vivo assessment of human patellar tendon mechanical properties.

BACKGROUND: Owing to the viscoelastic nature of tendons it may be that the total excursion and hence strain experienced by the tendon under load may be affected by the duration of contraction. Here we examine the effect of contraction duration on the measured in vivo mechanical properties of the patellar tendon.
METHODS: Nine healthy young men aged 21 (SEM 0.5 years) performed three short (3-4s) and three long (10-12s) maximal ramped isometric contractions on an isokinetic dynamometer, with real-time recordings of patellar excursions using B-mode ultrasonography synchronised with forces to determine tendon mechanical properties.
FINDINGS: Maximal patellar excursion was approximately 42% (P<0.001, effect size (r)=0.9) lower for the short 3.6 (SEM 0.4mm) vs. the long 6.2 (SEM 0.4mm) contractions. Similarly, across the range of forces tested, strain was approximately 42% (P<0.001, r=0.9) lower for the short vs. the long contractions 4.5 (SEM 0.5) vs. 8.0 (SEM 0.9%), respectively. Tendon stiffness however, was approximately 77% greater (4648 SEM 434 vs. 2633 SEM 257 N mm(-1), P<0.001, r=0.9) for short vs. long contractions.
INTERPRETATION: Contraction duration significantly affects tendon strain and associated measures of stiffness at all levels of force. The implications of this finding are twofold in that the results: (a) indicate that in order to compare tendon mechanical properties within or across studies, duration of contraction must be standardised and (b) have possible implications on training protocols and associated injury risks.