Originally Posted by RedWhiteBlue
I am training to become more competitive in getting into law enforcement. For over a year now, I’ve been working out about nine times a week (run six, lift three). My weak point is my lungs in the two-mile run, where I do about 14:00 (down from over 16:30 a year ago). My weak point is my lungs. I “stumbled” upon cycling and found that after a month of cycling, I can go four to six hours averaging about a 80 – 90% estimated maximum heart rate (not including a ten minute break every hour).
My question is how long (time wise per workout) and estimated maxumum heart-rate is too much? At what point in the workout do I reach diminishing return and should stop? I heard that the body will start to burn muscle at some point.
Also, how often should I go on a four hour and six hour ride? is once a week too long? (I will probably ride once or twice per week in replacement for some of the running.)
RWB, remember also that a bike is more mechanically efficient to a ratio of about 5:1 compared to the same calorific expense of running. When you ride for 4-6 hours you are still mostly in the aerobic domain so you get energy primarily from oxygen. When you cross to the anaerobic domain your energy is provided by glucose. Anaerobic metabolism operates in the presence of oxygen but is not dependent on it. Aerobic exercise can be tolerated for hours but anaerobic becomes intolerable after a few minutes, often much sooner, depending on the intensity and nature of the exercise.
The heart rate issue is less easily defined. I agree with what you say that there is a point where the return on investment as it were is negligible. By the same reasoning, it is physically possible to do 400 sit-ups, but what's the point? The right time to stop is when you think you have reached a plateau where further sustained activity appears pointless. Rather than ride 4-6 hours doing say 50km would it not possibly be better to ride 1-2 hours and aim for 30km. I think (imo) that it is vital to maintain a mix of both types of exercise. Ride for 4-6 hours on a flat road and you'll probably be ok until you come to a steep hill at which point you'll rapidly cross to the anaerobic domain and it might slaughter you very quickly unless you have given time in training to the anaerobic.
How often you ride is a personal thing as well. I could not give the time to doing that, for I am older than you (50) so I need to factor in adequate periods for recovery and tissue repair for example. For myself I try to alternate 70/30 and 30/70 aerobic/anaerobic as that lets me maintain a reasonable balance whilst giving recovery time on alternate days so that muscle groups can adjust.
To what extent will the body burn muscle? That depends. The liver is the primary source of glucose for the body. Once that has been depleted, the body then begins a process called gluconeogenesis where it converts the amino acids in the body fat back into glucose. The thinner you get, the less body fat you have so you will start to breakdown muscle into glucose but by that time you will normally be aware of that. The liver converts stored energy into glucose quicker than body fat can do so by the time you are converting body fat you'll be aware of hypoglycaemia and will usually take on board glucose tablets or high energy glucose drinks to compensate. I don't think you need to worry about muscle burn to be honest. Most of the time your 4-6 rides are aerobic and so oxygen gives you mostly what is needed. Anaerobic however consumes about 14-18 times more glucose so unless you are big into hill climbs etc, you'll find your body will compensate for what you are doing.
How you exercise is very important: stretch, warm-up, exercise, cool down, stretch. Don't neglect this. There is also a concept called the glycogen window where the body consumes more glucose after exercise for about the first 40 minutes or so. As the body heats up, so catalytic processes are accelerated and the body needs that glysogen window to provide the energy that it needs.
I'll close on the old adage which again, imo, is important: drink before you are thirsty, eat before you are hungry.