Epidemiological studies link high repetitive loading of the lower back with degeneration of intervertebral discs, and experiments on cadaver spines confirm that repetitive mechanical loading can disrupt the lumbar discs in a manner characteristic of "degeneration". But why do living discs not just strengthen in response to this stimulus, as other musculo-skeletal tissues do? Our hypothesis proposes that the low metabolic rate of lumbar discs (the largest avascular structures in the body) prevents them from keeping pace with adaptive remodelling changes in adjacent tissues, so that large and abrupt increases in a person's level of physical activity may leave the lumbar discs the weak link in a strengthening and heavily loaded spine. Recent laboratory investigations support the hypothesis, but clinical evidence is required to relate recent disc degeneration with recent increases in physical activity, and so test the hypothesis.