KER, R.F.: The implications of the adaptable fatigue quality of tendons for their construction, repair and function

Different tendons are (i) subject to very different stresses from their muscles and (ii) show a corresponding variation in their susceptibility to fatigue damage. The balance between the functional demand (i) and the material property (ii) was demonstrated by in vitro tensile fatigue tests using tendons from wallaby and sheep limbs. With the maximum stress set at a value which could have applied in life, each tendon ruptures after a time of the order of 3 hours. The discovery of major differences in fatigue quality among tendons, which had previously seemed similar in their mechanical properties (elastic modulus, ultimate tensile stress, hysteresis), raises a wide range of new questions. (A) In life, routine repair is required to avoid fatigue damage becoming symptomatic. What structures within the tendon are damaged and how is their repair organised? (B) What structural differences underlie the differences in fatigue quality? (C) Each tendon appears to have the minimum acceptable quality for the task it is set. Why not use the best available material for all tendons, even those which are subject only to low stress? (D) How do tendon cells adapt the fatigue quality of their extracellular material?