Depending on the type, intensity and duration of the session, the body responds to exercise in very distinctive ways. You can become very skilled at reading those signals. It's a good idea if you're taking your fitness program seriously, because it gives you the power to manage your program much more effectively. Here are a few "feelings" to watch for with different aspects of fitness training:
1. Cardio
Lungs on fire = too intense to sustain for more than a minute. This is fine to include in short, spaced increments for very fit, regular exercisers, but is generally too aggressive a training style for new exercisers.
Modestly accelerated breathing and a feeling you could keep going for a couple of hours is great for general health but won't significantly improve your stamina.
Deep, rapid breathing but a steady, tolerable level of exertion that can be sustained for 20-40 minutes is perfect for most exercisers looking to slowly build their heart and lung capacity and burn a decent amount of body fat.
2. Strength
Loads that make controlled, fluid movements very difficult and exhaust the muscles in six or fewer repetitions is pure power training, and dangerous for inexperienced weight lifters. This usually has no "burn" associated with it - it's characterized by a rapid loss of force capacity during a short set.
Resistance levels that are easy to move for the first six or eight repetitions, and are only moderately difficult to move past 12-15 reps is strictly for muscle endurance improvement. This is good for first time lifters and athletes who want to avoid even moderate power or muscle girth gains. This is the level at which the burning sensation is most pronounced if you go deep into muscle endurance capacity. Intolerance of the increasing burn is usually the limiting factor - not loss of muscle power.
Weight that becomes very difficult to move in the 8-12 repetition range strikes a nice balance between muscle endurance and strength improvements. This is the range where most people choose to train and has the widest application for functional improvements (day-to-day, sports and recreational activities.)
3. Flexibility training can be static, held positions; or it can be yoga or Pilates, or even pre-event dynamic/ballistic or mildly explosive stretching. It's best to get direction from a qualified fitness professional (e-mail about specifics on this) about what mode or combinations of modes are best for you based on your profile and fitness goals. But, as a general rule, caution is a good idea here. It's much better to be less aggressive with stretch loads on the muscles and tendons (say 60 - 75% of comfort tolerance) and get in longer stretches or more rounds of stretching, than risk injuring the joint and surrounding connective tissue. A good rule of thumb is to be careful to avoid a load that makes the tissue tense back up in defense against the load. You should be able to relax and breathe deeply into the stretch without difficulty.
Learning to read your body's signals will help you get the most out of your program, and you will enjoy the process of training much more as well as you sharpen that critical skill.
As a general rule, no. If you have a virus or bacterial infection and are at the very tail end of it with at least 90% energy, strength training at less than full intensity might be OK. Use your judgment and listen to your body. But aerobics and oxygen-intense cross training should wait. You won't suffer by waiting another day or two. And your body shouldn't have to decide whether to focus on recuperating fully from an illness or rebuilding itself after a tough workout. If it has to, it will do both poorly.
