Hit the wall

I did the lower-body workout yesterday. After four days of intense leg-related cardio (biking, volleyball, and running intervals), my quadriceps finally gave up during jump squats. I was still able to do barbell squats and extensions, but the explosion needed for performing a jump just vanished about 30 seconds into the second set. I finished out the set just doing deep squats and raising to toes, but it's just not the same.