For many years caged canaries would accompany coal miners into the pits and act as an early warning system for the buildup of harmful gases. Being acutely sensitive to airborne poisons a canary showing signs of distress would immediately alert the miners something was wrong and evasive action was necessary.
So the idiom canary in a coal mine was born, an early warning signal for impending danger!
In the fitness world our sensory system serves a similar purpose. Unlike acute injuries which occur without warning, chronic injuries can bubble just under the surface for a long period of time until eventually they pass a tipping point and manifest as long-term pain.
During the buildup our body will often send little signals in the form of niggles, tweaks and aches. These are the physiological equivalent to the canary in the coal mine. An alarm telling us something is amiss with the way we are moving, exercising or training! Perhaps we are using bad biomechanics, perhaps we are training too frequently, perhaps we are overusing a particular movement, perhaps the loads are beyond our current connective tissue strength.
Whatever the problem ignoring these warning signals (something this author has been guilty of) is a bad idea. Much better to take a step back, listen to the canary and take evasive action before you pass the point of no return.