It constantly surprises me how many coaches are reluctant to allocate time for strength training in season for their athletes practice schedules. Strength training is one of the most important keys to help reduce injuries and perform at the highest level possible during the season. I think a lot of this hesitancy is due to the fact that coaches don’t promote off season strength training as much as they need to be. If an athlete has been practicing intermittently a couple of times a week without any strength and conditioning workouts then the season comes and they shift their practice plan to 5 or 6 days a week of training and then add a strength session or two that is asking for an injury. The reason being is that the athlete is not used to the strength training sessions and then add in an excessive stimulus from practices for their sport the body is going to have multiple stimuli to adapt to and a very low recovery time in season. The athlete needs to be training and adapting to strength training in the off season so once the season comes the strength training workouts are not focused on creating new adaptations but rather maintaining the adaptations we made. The training in season should be familiar training methods that the athlete has been introduced to and adapted well to during off season training so in season the body is aware of how to recover from them properly.
In season training should be a systemic workout training all portions that an athlete needs to maintain for their sport. Upper, lower, back, arms, abs, cardio and these other terms are not a way to describe any athletic development training an athlete should perform in season. Athletes need to be athletic which requires their whole body for the sport so the emphasis on training should be movements that correlate to their sport rather than exercises or muscles the athlete does. A baseball player doing bicep curls vs a baseball player doing rotational med ball tosses is a good example on an exercise vs movement perspective, having bigger biceps might help you hit harder but an rotational med ball slam is going to develop more of a pattern to help you hit better regardless if your arms get bigger or not. A typical in season session should consist of a warm up, corrective movements/exercises for the athlete or sport, followed by some conditioning and plyometrics, then a few key lifts or movements to address total body movement and strength, and finish with stretching or any recovery methods you may need.
As far as lifting methods or total load of weights that should be used it depends on what sport, athlete, and what exercises are being done. A few general premises to think about in regard to strength training in season. First maxing or lifting the largest weight you possibly can is never an in season training session or workout. The head strength coach at Concordia St. Paul had the college golfers under his supervision max out their back squat, bench press, and power clean the same week they had their end of the season tournament to go to nationals. Needless to say the athletes did not perform well and that strength coach was out of a job next season. Secondly the athletes should minimize the eccentric (lowering portion of weightlifting) as much as possible to reduce stress where possible. My sprinters utilize the trap bar deadlift at least once a week in season but as time comes closer to peaking we shift to trap bar drops where they only pick the weight up to full extension standing up then dropping the weight to save the strain of lowering it back to the ground. This is not a seamless transition to remove the eccentric portion with every movement as easily as a trap bar (one of the reasons it works so well and it is used so often in this regard) but a med ball chest pass instead of a bench press would be a similar substitution to lower the amount of eccentric load on the athlete.
Remember the in season training is about honing the athletes skills and maintaining their abilities rather than focusing on increasing their strength and conditioning at the same time that they are training to perform skills for their sport at the highest level possible.