Periodization
Phase One: Loading
At the beginning of the offseason, focus on building power by lifting high weight at low rep counts. Aim to fail between 4-6 reps each set, and recover fully before you go again. Along with proper diet and rest (which we'll discuss below), this will help you gain muscle. The first 6-8 weeks are called your "load phase."
If you need to gain weight, this is your best time to do it. Nutrition is the biggest part of weight gain or weight loss, and we'll cover that below, but workouts are important as well. You'll use many of the same techniques bodybuilders and strongman competitors use to grow muscle mass.
Phase Two: Muscular Endurance
After about six weeks of low-rep, high-weight work (maybe three weeks of an upper/lower split and three weeks of a push/pull split) you will start phase two. In this phase, increase your rep target to between 6-10 per set. As you do this, the rest period should decrease from 90 seconds to around 60 seconds between each set. This will help develop muscular endurance. You'll be able to lift the same amount of weight more times and therefore keep your strength up over the course of a game.
Phase Three: Tempo
As the season approaches, you enter phase three, where you move away from isolation splits like the push/pull or upper/lower you've been working so far and move toward full-body workouts. You will reduce the rest even further through a principle called reciprocal inhibition -- where you work opposing muscle groups on alternating sets.
For example, after you warm up, you might start your workout routine with alternating sets of bench press and bent over rows with 15-30 seconds of rest between each set. When you flex your chest, your back muscles are forced to rest. When you flex your back, your chest is forced to rest. This is reciprocal inhibition, which keeps your heart rate high while still allowing each muscle group adequate time to rest and recover before you put a load on it again.
This phase is all about cardiovascular endurance. The season is almost here -- phase three will get you in "football shape."
Endurance training for a football player isn't about long-distance running, swimming, biking, or any other steady-state cardio. An endurance football workout is more about repetitions. Lots of sprints. You want to exhaust yourself physically and mentally, since that's where you'll be in a game.