Just as overload will stimulate improvements, failure to use your fitness (i.e. under-load) will lead to declines in fitness.
Deconditioning begins after a few weeks of inactivity. The good news is that fitness gains can be maintained with less exercise than it took to get them.
In most cases, if intensity is maintained, the number of workouts per week can be decreased. Modest fitness gains may be maintained by staying active through sports and leisure activities.
Golden Rule Recap:
Golden Rule #5 – Energy Balance