When exercising, it's very important to listen to the signals that your body sends about what is happening. These signals will tell you if you are pushing yourself hard enough or if you are holding back. Unfortunately, interpreting these signals can be difficult, especially if you are new to exercise.
There are three main levels to the signals the body gives before and during exercise. The sensations in level one are sensations that everyone has experienced at some point or another and are minor. These signals can, however, be responsible for whether you keep exercising or even start. Without a doubt, you have experienced the feeling of knowing you want to get a workout in, but your body and mind just don't want to cooperate, telling you that you can do it later; you are too tired now; just go sit down and rest (even if it is 8:00 AM). The human body is very good at sending out signals to get out of work, but that lazy, sluggish, in-a-funk feeling is one that you need to ignore and push through to get to work or to keep at it.
The second level of signals your body can give you spans a large range, which most of you have experienced. Though the majority of exercisers will have had experience with the full gamut of this level, some newcomers to exercise may have experienced only part of it. This level includes minor fatigue during a workout - all the way up to that colossal muscle burn while pushing through a particularly hard workout. The middle of this range is where most newcomers tend to hover during exercise because they don't yet know how hard they can push themselves, while veterans typically get to the top of that range when they are challenging themselves.
Getting stuck in the middle of this level is where most people (especially beginners) get off track by thinking that the burning of lactic acid they feel in their muscles is bad or dangerous and that they need to limit it or avoid it all together. Even the most basic starting exercise should give you a decent amount off muscle burn while doing it. It doesn't matter if you are doing strength training, toning, cardio, or HIIT - you should always challenge yourself to "feel the burn" when you can.
Level three, the top and most intense level, is an area where few exercisers have even been let alone get to during their regular training routine. This level is where your body actually stops functioning the way you want it to. Instead of touching on your anaerobic threshold (using up more oxygen then you can take in), this level puts you past your limits and staying at this level for longer than a few seconds at a time will cause your body to shut down and potentially can lead to injury. Though this is not a level that is functional for prolonged activity, it can be used in certain training styles for very small amounts of time to effectively improve your maximum functioning levels.
Getting into level three is typically reserved for elite-level athletes and is not a particularly safe place to be unless you are under the supervision of an educated trainer or coach. However, it is an experience that is valuable for even the average exerciser to increase their scope of what their own body is capable of doing. When exercising for any goal, whether it is strength, endurance, weight loss, etc., having a complete grasp on what your body is capable of and what it takes to get to your maximums is incredibly important to getting the most efficient workout. Not knowing your own maximums will invariably hold you back from pushing as hard as you can and need to, to really see the results you really want.
So, use this guide in order to learn to listen to your body so that you safely push yourself during your workouts. It's a bit of an art form and it takes some trial and error, but it's very useful once you learn to understand and respect your body's signals.