This was not one of Newton’s laws but has been introduced by Noakes. There is the temptation just to pile on the mileage and that fitness will increase as mileage does. Even those runners with perfect running form who are remarkably resilient to injury will find that there diminishing returns with this approach. The point at which this occurs will be different for each runner but it is safe the say that once past a certain point, each additional mile run will not accrue the same fitness benefits.
By alternating hard and easy training, it is possible to keep pushing fitness limits during the hard sessions. The danger with this law though is in the interpretation. There is a tendency to interpret it as 1 hard day, 1 easy day and so on ad infinitum. I would use training and recovery as more appropriate descriptions. Dependent upon the severity of the training depends upon the nature of the recovery. For a masters runner, it might be that a speed session requires three days of recovery.
For some runners, it might be that they get more benefit from back to back hard sessions followed by a longer recovery eg tempo session followed next day by a long run and then three days recovery. It is also important to consider alternating hard and easy training when considering an annual plan – building in recovery weeks and low keys races.
In conjunction with law 4, I would generally identify the keys sessions for the week and give the athlete the flexibility to incorporate them into their own life. For some athletes it might be two sessions a week, for others, three. In addition, the recovery sessions need to be sufficiently easy to allow recovery. One of the biggest faults many runners make is to train too hard when they are supposed to be recovering. This goes not induce an additional fitness benefits and hampers the benefits that could be achieved from harder training.
Train Smart