Strong Legs = More Endurance
Any athlete will tell you that the way to great overall strength are well trained legs. You don’t need have bodybuilder size thighs, but a strong base will help you last longer in your motos. Your upper body cannot support the amount of stress for an entire moto for you to be competitive. It’s pretty simple to prove this. How much can you bench? 
Maybe 170, 180 lbs; however, you can squat twice or maybe three times as much weight. Squeezing your bike throughout the moto is always important and if you squeezed your handle bars as much, your arms would be pumped for the next week.
To get a good, strong base, you’ll need to hit the legs hard. To me, when the legs get lactic acid built up, it is the worst burn ever. But you have to remember, this will pay off on race day. So, to start off, you might want to do this leg work out for a few weeks:
Squats: 15 reps, 12 reps, 8 reps
Lunges: 15 reps, 12 reps, 8 reps
Front Squats: 15 reps, 12 reps, 8 reps
Calf Raises: rep out 3x
Some things to remember about form on the squats: keep your back straight with the weight centered over the feet. When your back is bent, all of the weight puts strain on your spine, resulting in torn and pinched nerves. Don’t squat all the way down, just to the point where your thighs are parallel with the ground. On lunges, to get the most out of them, be sure to explode off your foot. The front squat should be like the regular squat. Except, put the bar on the front of your shoulders, by your neck. Make sure the weight is not on your arms. Grab a few 2x4s for calf raises and make sure you extend all the way up and all the way down. The front squat is vital to motocross because it utilizes your core and back, and obviously your quadriceps. These muscles are the ones used to keep your attack position.
However, don’t just concentrate on this one exercise; you must work on all of the muscles. To work your squats a little different, try to put your feet closer together with your toes pointed out, or you put your feet out wider than shoulder width. This will mix things up and keep your muscles guessing. Again, don’t just go through the motions, work your legs hard and it will pay off when the track gets rough in the later motos.
