Will cardio make me less muscular?
Some people - weight trainers in particular - are reluctant to perform cardio because of its' potential to break down muscle stores. Cardio performed at length produces catabolic hormones like cortisol that could interfere with the anabolic muscle building process. However, you can protect muscle from this process by ensuring adequate nutrition before, during and after a cardio session and by keeping aerobic training to less than one hour if you have a goal of building muscle.
Ultimately, it’s all about performance. If we level the playing field by assuming that pre-, during-, and post-workout nutrition is optimal, and that cardio will be under an hour for the given training session, then you know you will have enough energy to complete both cardio and strength and will have the nutrition necessary to recover and build muscle if it is so desired.
Next, we just need to take a closer look at what your goals actually are. Often, our members have goals to improve their weaknesses, but continue training behaviors that emphasize their strengths. A triathlete may want stronger legs for cycling power, but insist on cardio first because they experience greater perceived exertion during cardio after a weight training session. This is more of a psychological issue - “I feel slower” - but the reality is that strength should be the emphasis in this part of their periodized training program. Bodybuilders and powerlifters feel the same increase in perceived exertion when trying to lift weights that seemed “easier” on a day when they did weights first. If they are confident in one area, whether that be strength or cardio, they often take pride in it and are reluctant to enter a training program that negatively impacts this confidence. But let's face it - the definition of insanity is “doing the same thing over and over again and expecting different results." That applies here - continuing to focus your training on your strengths when your goal is to improve your functional weaknesses.