Rest days are extremely important. Back after the French revolution there was an attempt made to make the week 10 days long as opposed to 7. The idea of a 7 day week is a Biblical concept. Basically God created everything in 6 days and rested on the 7th. He set aside the 7th as a day of rest and called it the Sabbath.
Okay, a little theology lesson, but to the point. When the rest period was changed from once every 7th day to once every 10th day it was short lived experiment. Not even the horses (an animal with extreme strength and stamina) were able to stay well. They started getting sick and dying. Just like the horse, your body can only adapt if there is rest. I have found that an occasional break from working out (provided it is no longer than a week) sometimes provides gains.
As far as the leg issue, I am building a home gym and about 3 months ago got the equipment to squat and deadlift. I have noticed significant improvement in my UPPER BODY adding these exercises. My legs really have not changed much in size, but the shape and density have definitely changed. What seems to work best for me is heavy compound exercises (squats, deadlifts, rows, dips, bench presses, pullup, chins, etc.)
Another question raised was dealing with cardio on off days. It would be more accurate to say aerobic exercise on off days. I won’t get too technical, but aerobic exercise is exercise that uses a lot of muscle, does so very gently, and is continuous. Under such demands much of the energy needs of the muscles are supplied by utilizing oxygen. Because these work-outs use the muscle gently they can be used on off days.
Be aware that cardio and aerobic are not the same thing. Cardio refers to the cardiorespiratory system. There are two basic ways to exercise the cardiorespiratory system. One is aerobic. The other is anaerobic. Anaerobic exercise is intense; so intense that the body cannot meet the energy demands by using oxygen. It has to use other energy sources - primarily creatine phosphate in the muscle and sugar in the muscle and blood system.
Under extreme conditions it can also start breaking down muscle and use protein. When working out anaerobically the muscle is in oxygen-debt. The body will try to repay that debt, and the heart and lungs will work harder. Watch a sprinter after a 100 meter race or the big dog in the gym that just squatted 500 for 3 reps. Both are extreme examples of anaerobic exercises, but if you watch both guys will be extremely winded.
The reason is they are in oxygen-debt, and the body is trying to repay that debt. In both cases they are working out anaerobically, but are taxing their cardiorespiratory system. One can quite easily condition their cardiorespiratory system through anaerobic exercise IF they chose exercises that use a lot of muscle and IF they do not rest too long betweens sets. In that case the demands of oxygen debt work the heart and lungs.
If one does cardio through the aerobic means then it may be appropriate for an off day. If one does cardio through the anaerobic means, than the muscle is going to need rest between workout sessions.
Here is the cool part about doing cardio through anaerobic means. Which has the potential of requiring a higher workload on the heart and lungs: aerobic or anaerobic? Let’s use an analogy. Which has more potential to drain the money out of your wallet: having a budget where the input of money equals the output in a steady fashion, or being in continuous debt? Obvious it is the latter. If you use exercises that use a lot of muscle and minimize rest periods, anaerobic exercise can provide a greater stimulus to the cardio system.
Alright, I’ve gotten a little long winded, but hopefully this helps.
