Disclaimer: I'm almost 50

We were doing the traditional stuff: run one day, lift the next... with moderate success.
We then cobbled together some ideas from MMA, martial arts, navy seals, functional strength etc...
Goal: simulate demads of MX: i.e. 10 mins of high heart rate plus required speed, strength and agility, and don't add (much) muscle mass.
Favorite Workouts
Circuit Training
With light bar (20-60#) cycle thru a series of high-rep lifts with no rest. typical set is 3-4 minutes. rest 1 min and repeat.
Typical set;
Squat: Up row: Good morning: Push up: B/O row
Press: 1 arm snatch; Abs
Favorite exercises:
Box 3 min (heavy bag)- jump rope- box......
Wack repeatedly truck tire(large) with sledge hammer
(Dump truck front tire)
Plyo Hops on truck tire
Shadow box on truck tire: springiness of tire wears out the legs!
1-arm dumbbell snatch: this recruits every muscle, develops coordination and elevates heart rate massively.
Drag a car tire (with rim) for 100 yard reps. Leg killer + severe cardio. Use linemans belt or lifting belt. Lots of variation of this to work upper body too.
Comination lifts:
With a light bar: Squat X 3-curl-pressx2
repeat with no rest/
Anyhow: focus on speed/stength/muscular endurance exercises have really shown benefits on track.
Rider can push harder and does'nt fatigue on long moto's
(Plus I'm losing weight!!)