Biomechanics of the Jackknife
- chiewyc02
- 3 days ago
- 4 min read
The jackknife is a martial arts kick that involves two kicks in mid-air. The two kicks would consist of a roundhouse kick and a spinning hook kick in mid-air, and that is what makes the jackknife difficult, like any kicking trick that involve more than one kick. The jackknife can be done out of many set-ups, but this article will use the cheat step as the set-up to make it simpler to understand.
The take off
Creating momentum towards to front side (direction of travel), and rotation is main goal of the cheat-step, hence in the example above, there are multiple steps used in one direction, while spinning before jumping.
The jackknife requires a lot of strength and speed to perform, hence using a set-up is very necessary.
Cheat-step (set up):
Wind up the body in the opposite direction of travel, and the opposite direction of rotation, this helps to use the body like a sling shot, pulling the string back to be shot in the direction you want.
Unwinding the body by externally rotating the left hip, and rotating the torso in the same direction to generate rotational force.
While rotating keeping the arms extended can help increase the rotational forces acting on the body. While rotating, internally rotate the right hip to follow the rotational direction of the body, also it is important for the right leg (jumping leg) to be internally rotated when coming in contact with the ground for one last time.
Still rotating, as the right leg contacts the ground (this is the last contact on the ground), the left leg should then be swung outwards in the rotational direction using the external rotation of both the right and the left hip. At the same time, the extended arms should also be swinging in the same direction as the left leg, prepared to be used to spin then jump.
Lastly, once facing the target, (facing the camera) the left leg, and the arms should be swung up by flexing the hip and the shoulders, this is to generate upwards force on the body to make it easier to jump off the ground.
The jackknife is a kicking trick that visually shows a roundhouse kick and a spinning hook kick in MID-AIR; that being said, "visually" is something important to keep in mind. Yes, a roundhouse kick and a spinning hook kick may look like they are performed in this trick, but as the performer, it might be simpler to think of the two kicks as a "front-raising kick" for the roundhouse and a "back thrust" for the spinning hook. Remember, the body would be carrying the spinning momentum from the takeoff, so any extra "spinning" could throw off the body from whatever it has to do in a specific direction. Keep in mind that the front-raising kick and the back thrust are just movements and thoughts that should be isolated to the specific kicking leg (meaning the rest of the body might not be doing what the name of the move implies, just the kicking leg)
The Kick:
With the momentum from the cheat step, as the body spins towards the target side, "camera side in the photo", the right hip and right knee should be flexing both to about 90 degrees, flexing (bending) these joints are important for two things, first to prep for the first kick; second, to lower the inertia acting on the leg so that it is easier to control.
Then, with the hip and knee flexed, perform a front-raising kick by only extending at the knee to show off the "flick" of the roundhouse kick and externally rotating the left hip by pointing the left knee towards the direction of travel, BUT remember the body is still spinning; hence, as the body spins, bringing the right hip around, the front-raising kick transforms, VISUALLY, into a roundhouse kick.
After the roundhouse is a half twist before the spinning hook; the half twist will be greatly supported by the extension of the right hip (roundhouse kicking leg). Extending (straightening) the right hip helps perform a twisting skill called the "cat twist", where the change of shape of the body helps to spin the body in the air.
After the half twist, the body should now be facing away from the target side, which now allows for the spinning hook OR the back thrust to happen. To perform this special back thrust in the air AND still land, two things must happen: LEFT HIP extension and RIGHT HIP flexion. It is basically moving the left foot back and up behind the body and moving the right foot forward and up in front of the body. This action will happen together to then visually perform a spinning hook kick in the air with the left leg while preparing to land on the right leg.
Note: For the left leg, before the extension on the left hip, bending the left knee would also help, similarly to the roundhouse kick, to help gain more control due to less inertia acting on the leg, and then when performing the spinning hook, extending the left hip and left knee to straighten out the left leg while kicking to make it look explosiving and asthetic.
Lastly, the landing: focus on getting into a single-leg deadlift position, landing on the right leg first, on the ball of the foot.
STRENGTH AND CONDITIONING
Hip airplanes: Great for hip stability and strength, especially on a single-leg stance, which will be used a lot when performing the jackknife. Make it easier by holding on to a stable structure, then progress to balancing only on the one leg, then lastly challenge the body by holding a kettlebell or dumbbell.
Single hops: Build the explosiveness of the one-leg ability to jump and absorb forces and redirect them up to jump again, at the same time training the single-leg strength to protect the joints from the high forces that would come from the jackknife landing.
Core exercises:
Exercises that involve leg movements in the air, like the flutter kick or window wipers, and leg raises are good because it trains the body's ability to pick the weight of the legs up and control them to perform specific actions. Exercises like the Copenhagen side plank could also be beneficial, as it focuses on the lateral abdominal muscles AND strengthens the muscles on the inner side of the hips, which are crucial for movements that split the legs apart violently.
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