Fatigue damage of human tendons

The study was designed to examine the effects of partial fatigue on specific mechanical parameters which characterise human tendons in vitro. Specimens prepared from 12 intact Extensor digitorum longus tendons of the foot were subjected to partial fatigue, equivalent to 25% of the median fatigue life, by a cyclic square tension-tension stress waveform at the physiological frequency of 4 Hz. The maximum stress was set at a value corresponding to 20% of the calculated ultimate tensile strength (UTS) of 100 MPa. The minimum stress was set at 1% of the UTS. Dynamic characterisation was performed at stress levels of 10% and 20% of the UTS prior to and following partial mechanical fatigue. Subsequent quasi-static tests were performed on some of the specimens. Comparative analysis of the damage ratios (DRs) of quasi-static and dynamic mechanical parameters suggested the use of the DR for dynamic tensile modulus as a good indicator of damage inflicted by mechanical fatigue. Such an approach might be used for an in vivo assessment of tendon damage.