It's time to work your legs harder...After performing sufficient sets of shoulderstand squats you should be flexible enough and your joints and tendons should be strong enough to move on in the squat progression series. The second step is the Jackknife Squat. Jackknife squats are also the first squatting exercise that really works the muscles of your legs strongly, because they have to manage a much higher weight load than they had to in the shoulderstand squat exercise. I've already talked a lot about the reasons why leg training is so importent in this post, so I won't repeat it here again. The jackknife squats are undoubtedly a great exercise to tone your legs in the beginning and also to help you gaining even more flexibility (which you gonna need in the further steps, too) and stronger joints/tendons as well. A side-effect of squats, in general, is an improving endocrine system. As many big muscles are involved, the body starts to produce more anabolic (muscle-building) hormones during squats than in any other exercise. They could help you to build up stronger muscles (in the whole body) and prevent you from amyotrophia (loss of muscle mass).
Let's start with the how-to-perform explanation:
Stand in front of a robust object that is about the hights of your knees (or slightly below). A side-table or a bench should be fine. Your feet should be shoulder-width apart (or a bit wider) with your feet rotated a little outward. Keep your legs fairly straight and bend at your hips until your palms contact the object (like in the video). Tilt forwards so that some of your bodyweight is carried through your hands. Now bend at the knees and hips until your hamstrings reach your calves and you cannot go any further (this should last 2 seconds). Your torso should remain as parallel as possible to the floor during the exercise. Simultaneously you have to bend your arms as well. Hold the finish position for 1 second, and then push yourself up again by using combined leg and arm power (again 2 seconds). Repeat the movement without a rest. If you can't manage to meet the target instantly, do as much reps as possible until you can do the requested amount.
Target: 3 sets of 40 reps
Caution: Do not raise the heels at any point during the exercise and, even more important, try to avoid that your knees travel forward past your toes (leads to less strain on your knees and this could prevent knee injuries)!
Last to say:
Shut up and squat! (famous saying in strength training)