A computational model for the adaptation of muscle and tendon length to average muscle length and minimum tendon strain

This paper hypothesizes that average muscle length and minimum tendon strain govern muscle and tendon length adaptation in all situations. A model has been implemented to test this hypothesis, and simulations have been performed for normal development, bone lengthening, immobilization, and retinacular release experiments in young and adult animals. The simulation results predict that both muscle and tendon lengthen during normal development, with the rate of tendon growth slowing faster than the rate of muscle growth. The results also predict that muscle length increases during bone lengthening in both young and adult animals, while tendon length increases only in young animals. For immobilization in adult animals, the results predict that muscle length increases when the muscle is immobilized in a lengthened position and decreases when the muscle is immobilized in a shortened position with no change in tendon length. For immobilization in young animals, the results predict reduced muscle growth and increased tendon growth regardless of immobilization position. Finally, the simulations predict that retinacular release which increases excursion of the musculotendinous unit leads to increased muscle length with decreased tendon length in young animals and decreased muscle length with no change in tendon length in adult animals. These simulation results are consistent with experimental findings reported in the literature by other investigators. This suggests that average muscle length and minimum tendon strain may represent general principles that govern muscle and tendon length adaptation.