Strength will come when training for muscular endurance. Stick to the 20 rep range and start with two sets and add a set when ready, until you reach 3 sets. Then add weight to your resistance.
Do not worry about the "beach" lifts, bench and curls. For riding you need to focus on the muscle groups used for your sport, quads and back.
You need to squat and deadlift. These are the staples of lifting. Yes they do appear to be olympic strength movements, but think about he fact the they really are full body lifts. This is exacly what you do when riding....use your whole body with focus on the quads and back. Isn't that what's sore monday morning?
Don't use straps when deadlifting and it will work your forearms sufficiently to build strength.
Cardio is CRITICAL, you will beat more riders with endurance, than strength. When they can't hang on and get all sloppy you'll rosst past them. When you do late in a race, they loose mentally also.
Train hard, ride hard, live long.
vetwfo'er
If you don't think I know, ask Slo'Mo. He'll vouch for my knowledge.
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99 WR400f, White Bros E-Series silencer and tapered head pipe. YZ seat and tank. Thumper rad guards, works frame guards, scots steering damper. Soon to be MX-Tech suspension. SETRA, AMA, Happy Ramblers MC.
97KDX220, FMF Rev pipe and silencer, renthals.
Courage is not the absense of fear. It is the judgement that something is more important, than fear.