Every athlete has done some trainings where energy intake didn’t meet the needs. Sometimes it happens unintentionally, sometimes we do a workout and know it wasn’t enough, so we say: ok, I’ll have extra carbs now to make up nutritional leeway after training. The problem: it will not change (correct) the outcome of the training you underfueled.
Have you ever made bread or a cake? Then you know what the outcome looks like when you forget to add yeast and only realise it when you take the cake/bread out of the oven. You can bake your dough but it won’t rise. You can spread yeast on it now but it won’t change the result. Your bread/cake will be flat, hard and heavy.
The same will happen to your muscles when you don’t fuel when you’re supposed to ingest carbohydrates.
I would say almost 95% of the athlete’s that come to me for nutrition consultation underfuel their workouts. Most use only water. However, carbohydrates have an important role in sports. They are the preferred substrate of our muscles, they delay fatigue and are very important during long and/or hard workouts. There is a reason why we should consume carbohydrates during training. Without sufficient energy supply, your performance decreases earlier, i.e. you won’t do as many repetitions as with more energy intake and your speed will slow down earlier due to lower glycogen stores. At the same time, your RPE will rise making a workout feel harder.
You can still have a recovery drink or recovery meal after your workout and double the serving of carbohydrates. But the carbohydrates you ingest then, won’t change [improve] the quality and efforts you performed during the previous workout. Your muscles would have needed the substrate while they were working, not when the work is done. That’s why: for best performance outcome, make sure you fuel your workout. The better the training quality, the better the outcome.