Failure to recover properly can result in chronic dehydration, accumulating muscle damage, systemic inflammation, depressed immune function, and changes in hormone levels.
In the short term insufficient recovery causes muscle soreness, fatigue, and poor performance. If a runner neglects recovery continuously more damaging effects occur like loss of muscle tissue due to excessive muscle fiber breakdown, diminished immunological capacity and, worst of all, permanent damages to joints and other structural damage.
Runner’s knee is considered to be an overuse injury, as are many other types of common running injuries. As the term suggests, overuse injuries involve the gradual breakdown of body tissues resulting from repetitive motion over long periods of time. These injuries are quite different from acute injuries such as ankle sprains.
Beginning runners (like me) suffer the most overuse injuries per hour of training because their bones, muscles, and connective tissues are not yet well adapted to the new lifestyle.
Abrupt changes to training pace or style, like suddenly increasing duration, frequency, or intensity of workouts, can make those problems flare up. The solution for that is to instead ramp up conservatively as the body adapts.
An easy way to measure your recovery status is to take your pulse at rest one day that you are fully recovered and rested, and then again first thing in the morning of every running day. A pulse rate that is much above your reference number suggests that your body is still working on recovering from the most recent workout.
The human body has remarkable capacity to adapt, with proper training, resting, and nourishment. But you must keep an eye on the signs it gives you to know when to push forward and when to let it relax and recover.