Power Training
Something I learned from Bill Pearl many years ago is that you have to get out of shape in order to get into shape. You can't maintain excellent shape year-round - your body can't accept the stress of weight training in combination with low bodyfat for extended periods of time.
During the wintertime, slow down and intentionally put on a few pounds of bodyfat or bulk. This building technique provides muscle size and power, contributes to overall recovery and helps to combat winter colds and flus. While letting your body rest, alter your training to give your mental attitude a break at the same time.
Cut back on your aerobic work, and cut the number of repetitions from the 12-15 per set of bodybuilding to the 6-8 per set of powertraining. This will allow you to handle more weight, and it puts the stress of the weight on different components of the muscle cells that are not overloaded during the high reps of bodybuilding training.
Unless you plan to compete as a powerlifter, I don't think it's worth the risk of damage to the tendons and ligaments to lift single maximum reps. It's too dangerous to tempt injury and you won't gain more size doing singles than you will doing sets of 5s anyway.
The overload of winter powertraining will pull your body out of slumps and stabilized periods caused by routine bodybuilding. Expect benefits also as your mind relaxes and you enjoy the challenge of your winter training.
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