Lie on your stomach and lift your feet and knees off the floor by flutter-kicking repeatedly as if you were swimming freestyle. Build up to flutter-kicking for 1 minute or keep your feet still, but off the floor.
Running will affect your lower and upper back. Make it flexible and strong using the following exercises.
Keep your back straight and abs tight while placing your elbows and toes on the floor and holding for as long as you can. Build up to 1 minute (advanced is 3–5 minutes).
When you exercise your stomach muscles, make sure to exercise and stretch your back as well. The stomach and lower back muscles are opposing muscle groups and if one is much stronger than the other is, you can easily injure the weaker muscle group.
Tighten abs while keeping your back straight. If your shoulders bother you or your arms are not built up, do this same exercise on your elbows so that only your elbows and toes are touching.
In fact, when you fail at push-ups during your workout, stay in this position for an extra 30–60 seconds each time. This will prepare you well for the long periods in the “leaning rest” as well as strengthening the core for crawling obstacles.
Lie on your stomach. Lift your chest slightly off the floor and wave your arms from your sides to over your head for 30 seconds.
Lie on your stomach in the down push-up position. Lift your hands off the floor instead of pushing the floor. This will strengthen your upper back muscles that oppose the chest muscles (the rear deltoids and rhomboids).
Lie on your stomach with your arms spread to the height of your shoulders. Lift both arms off the floor until your shoulder blades “pinch” and place them slowly in the down position. Repeat for 10–15 repetitions mimicking a bird flying.