Training correctly and getting results is all about the intensity that you can train with. This is something that has now been scientifically proven to be correct and studies have shown that if you follow up an intense cardio exercise with another workout like lifting weights, the second workout suffers and the intended muscle gains are not achieved. So you may feel fit and have that after workout euphoria but you could actually be overdoing your cardio workout.
The reason why this happens is that the human body can only handle so much exertion in a given period of time. If you just finished an intense game of basketball or a 10 mile run your body will not have the needed energy to put towards weight lifting workout. But there is a simple solution, give yourself time to recover from one workout (about a four hour window) before you move on to your second workout and you will have the energy to attack it even if you have already completed a cardio workout earlier that same day.
The scenario above is very different than total exhaustion, in which you do cardio until you physically cannot take anymore. It will depend on your general endurance and level of intensity but after hours of cardio on end your body will just shut down. When you get to that phase of total exhaustion any activity will not be possible. For one, your body will be too tired to lift the weight and internally your muscles are busy in repair mode and will not have the capacity to reap the effects of the lift.
When talking about overdoing your cardio workout we also need to talk about doing constant and repetitive motion and the injuries one can get. Often it’s the injuries that result from the excessive cardio that pose the biggest problem and not the actual cardio itself. If you move any part of your body in the same motion for over 4 hours at a time you put yourself at high risk for injury caused by repetitive motion. The activity doesn't matter if it could be jogging, biking, rowing, swimming, or anything else. As soon as your joints start wearing down and feel the pain, you can forget about any hope of gaining muscle mass.
Overtraining is a vitally important issue when it comes to looking at too much cardio. This is important because once your muscles are maxed out from a cardio workout, the same major muscles that were predominantly worked during your cardio session are going into overdrive with a continued repetition from the second workout.
For example any lifting workout that would have you do the same exercise over and over for 4 hours you would have suffer severe fatigue and you would experience diminishing returns, meaning your strength and endurance would decrease substantially. It’s the same concept if you run consecutive marathons in a single day. These are two very extreme examples however there was a specific study that was set up in order to examine this.
In this study the participants were runners in the Trans-Europe race in which they completed 2,800 miles in a mere 64 day period. The average runner clocked roughly 43 miles a day and the outcome of the study had a very interesting conclusion.
Interestingly enough each runner studied all had decreased muscle mass in their legs but none in their upper body. The simple reason why this happened is because of simple overtraining. Therefore it is easy to make a case if these runners could complete over 40 miles a day and not lose their upper body muscle mass the average lifter can definitely fit in 30 minutes to an hour of cardio without suffering any muscle loss. Just keep in mind it's all about balance.