Downhill backwards walking causes repeated, cyclical loading of the muscle-tendon unit. The effect this type of repeated loading has on the mechanical behaviour of the Achilles tendon is presently unknown. This study aimed to investigate the biomechanical response of the Achilles tendon aponeurosis complex following a downhill backwards walking protocol. Twenty active males (age: 22.3 +/- 3.0years; mass: 74.7 +/- 5.6kg; height: 1.8 +/- 0.7m) performed 60min of downhill (8.5 degrees), backwards walking on a treadmill at -0.67m center dot s(-1). Data were collected before, immediately post, and 24-, 48- and 168-h post-downhill backwards walking. Achilles tendon aponeurosis elongation, strain and stiffness were measured using ultrasonography. Muscle force decreased immediately post-downhill backward walking (P=0.019). There were increases in Achilles tendon aponeurosis stiffness at 24-h post-downhill backward walking (307 +/- 179.6N center dot mm(-1), P=0.004), and decreases in Achilles tendon aponeurosis strain during maximum voluntary contraction at 24 (3.8 +/- 1.7%, P=0.008) and 48h (3.9 +/- 1.8%, P=0.002) post. Repeated cyclical loading of downhill backwards walking affects the behaviour of the muscle-tendon unit, most likely by altering muscle compliance, and these changes result in tendon stiffness increases.