Any coaching or teaching requires planning.
Some folks confuse planning with being rigid. Planning actually means having prepared ideas and the appropriate resources to implement them before an event, and having contingencies available. You need to have flexibility in what you plan. You need to have examined potential scenarios, and have Plan Bs ready.
This applies especially to strength and conditioning training. Obvious example: if you are planning conditioning work for a season, you need to have contingency plans if you, for example, roll your ankle and cannot run. How will your season plan be different if that happens?
But there are more subtle flexibilities, and that includes adapting the workload to the athletes' condition. Are they strong enough to increase their sets? Are they tired? Are they flexible enough?
Aaron Coutts gave one of the talks at my Level 2 Coaching General Principles course back in 2005, and he explained his solutions for this challenge. Aaron has worked for the Essendon Bombers and the Parramatta Eels.
He is one of the authors of this paper.
The guts of it is that if you ask athletes to rate their perceived exertion on a scale of 1-10, it is a good indicator of how hard they physically worked. The key idea that Aaron uses in his work is to use this surveying after training sessions, and use it to vary the intensity of trainings. If you ensure there is a constant variation in training intensities, athletes are less likely to fatigue or disengage. Following harder competitions/trainings with relatively easier ones, and vice versa, keeps athletes focussed, fitter, and more able to achieve the goals of trainings.
The adoption of these concepts means that AFL teams no longer belt their players with a hard fitness session on the Monday after a hard weekend game. Recovery and variety in training load is essential to peak performance.