This is a good indicator of how much water you may need to replace to stay well hydrated. Always remember that re-hydrating after a workout is a crucial step in helping your body recover faster.
If you gain a few pounds for a day or two after a workout:
This is most likely due to some water retention due to the inflammation from the workout. This is especially true if you are also feeling sore and stiff. It might be a sign that you’re giving yourself some new information instead of the same old routine.
If your weight doesn’t change but your cloths fit differently in a good way:
Your body composition is shifting, but more importantly so is your shape which is far more influential in how you actually look. After all, would you rather be 20 pounds lighter but still the same shape and size, or weight the same and look and feel different?
If your weight doesn’t change but your cloths fit differently in a bad way:
This is also when your body composition is shifting in a way that is changing your shape. Chances are it’s a shape you’re not looking for.
If your weight increases as well as your performance:
This one can be tricky. You might be gaining muscle, but who’s to say that muscle isn’t also coming along with fat? Plus a certain degree of athletic improvement can happen without adding or gaining any muscle or fat at all because of changes in the nervous system. When measuring body weight along with performance also include a body fat test to clear up any confusion.
If your weight decreases but your performance increases:
Again this can mean a number of things. If your performance is more due to activates where you are actually moving your body through space it’s a good bet you’ve lost some fat. It is possible to improve some aspects of performance while losing others. If your marathon time goes down but your bench press also goes down, it’s a good bet you’re running faster because of less upper body muscle.
If your weight drops and you still feel softer:
While you might have lost some fat, it’s a good bet you lost some muscle mass as well.
This is probably more so if your strength and endurance is going down as well. Be sure to keep on with the strength training to preserve that muscle!
If your weight drops and you see more definition or tone:
While some definition can come from adding muscle chances are you’re dropping fat. As you get leaner you can see more definition and weight loss by water weight as well, but this only accounts for a small amount. For the most part more tone means less fat especially if you’re noticing tone where there wasn’t any tone before.
If your weight increases and you maintain definition:
Your gaining muscle and possibly water as well. Seeing as there is a lot of water within the muscle cells the two do go hand-in-hand.
Chances are it’s a combination of fat, water and muscle. If your strength or endurance is not also increasing chances are it’s mostly fat.
There are a bunch of other scenarios but these are some of the most common. Also keep in mind that none of these are set in stone. There is some grey area with all of them. For instance you could increase your weight and muscle tone while gaining a bit of fat as well.
Once again though it’s important to keep in mind that no single piece of data can tell a full picture of what’s really going on. It’s important to keep this in mind if that one piece of data is the only thing you care about. But that information won’t tell you if your leg strength is better, if your VO2 max increased or if you’re running technique improved. You’ll need more information to clearly know that.