In Navasana (boat pose) you balance on your buttocks while forming a V-shape with your legs and torso. Your arms reach past your knees, straight ahead.
While your abdominal muscles help to keep your spine straight in this pose its your hip flexors that can tire quickly in this basic yoga pose since they are working to keep your hips "flexed" at about 90 degrees.
(Your hip flexors pull your legs towards the front of your body "closing" the fronts of the hips.)
In a sail boat the mast has pulleys at the top through which ropes or "lines" pass so that crew members can pull sails up and tighten them.
In navasana you can think of your torso as your mast. The more rigid it is the better your hip flexors can pull on your legs and keep them lifted. And so with Navasana the place to start is with your spine upright.
While sitting with your legs in front of you (knees can be bent) pull your head up and back so that you neck feels long. Keep your head level from front to back .
As you lift your head up allow your chest to open. Reach your side ribs away from your pelvis.
Reach your arms forwards and make your arms and fingers all feel long.
Keeping your spine and arms long, tilt your pelvis backwards. Tilt back so that your weight is balanced on your sitting bones but keep your heels on the floor.
Keep your left heel on the floor and lift your right leg so that your shin is horizontal. Place your left heel on the floor and now lift your left leg. Keep your spine and arms long.
Replace your left heel on the floor and now lift both feet so that your shins are horizontal.
Touch your heels to the floor and then sit up straight. Relax.
As before, lengthen your spine and arms and tilt your torso back. Keep your heels on the floor. This time lift your left leg and straigthen your knee. Push through the ball of your foot as if pulling on a long pair of boots.
Bend the knee and put your heel back on the floor and repeat with your other foot. Put both heels on the floor and then sit up.
Repeat the exercise but this time try to straighten both legs at the same time. After lifting and straigthening both legs, bend your knees without touching your heels to the floor. Then straigthen them again, but slowly. Bend and straighten until you are ready to rest.
Working with Both Legs and Building Awareness
To help build more awareness while doing navasana you can practice lifting your heels about an inch (or less) off of the floor. As soon as you feel the skin of your heels leave the floor, stop and hold. Then touch your heels back down to the floor. Do this while leaning back with your weight balanced on your sitting bones. Do keep your spine and arms long. Practice slowly lifting your heels a little and then slowly touch them to the floor.
After this you can work towards slowly raising and lowering your shins to horizontal (touch your heels to the floor each time.) Then work at straightening your knees and bending them.
As you work towards holding both knees straight in navasana, concentrate first on keeping your spine straight. That means keeping your neck long, your chest open and your waist lenghtened.
With your spine long you may find it easier to keep reaching with your legs.
Rather than struggling and losing your form, rest before you lose your form. Take your time in building a strong navasana so that it can handle stormy weather.