The human strain

Typically, tendons can experience a strain of up to 8% and stresses of 100 Nmm^-2 before breaking. For a 70 kg man running a marathon, peak ground force can be assumed to be around 2.7 times his body weight, a force of 1900 N. The Achilles tendon has a cross-section of around 90 mm² and during typical running, the tendon might stretch by around 5% and experience a force approximately half the typical breaking stress. The length of the Achilles tendon is around 30 cm and therefore, at peak strain during a run, may store around 35 J. One calculation assumes that 7% of this energy is converted to heat and that the specific heat capacity of the tendon is 3500 J/K/kg leading to a temperature increase of 0.02 K during running. However, if it were assumed that the tendon was perfectly thermally insulated, its temperature would rise by 2 K for every hundred strides.