Joint locks feel like magic until you start looking at them closely. Once you strip away the theatrics, they are mechanical systems. You apply forces to bone and soft tissue, create torques around a joint, and control the adjacent segments so the joint itself takes the load. That is the basic idea. The rest is about geometry, lever arms, and how tissues fail when you overload them.
I want to make this practical. I’ll explain the physics in plain language, give examples you use every day, and show why ‘control above and below the joint’ is not just coaching lore — it’s biomechanics.
The basics – joints, pivots and torque
A joint is a pivot. Mechanically it behaves like a hinge or a ball and socket. When you apply a force at some distance from that pivot, you create a moment, or torque. In biomechanics, the two words mean the same thing — a rotational effect caused by force acting through a distance. The simple formula is:
τ = F × r
where τ is torque, F is force, and r is the perpendicular distance from the pivot to the line of action of the force. Units are Newton metres if you want to be precise. The larger the force or the longer the moment arm r, the larger the torque.
To get a feel for this, imagine resting a dumbbell along your forearm while your arm is bent and your forearm held roughly horizontal. If the dumbbell sits near your elbow, it feels easy to support. Slide it toward your wrist, and it suddenly feels heavier, even though the weight hasn’t changed.
What’s happening is simple: the further the weight is from your elbow joint, the longer the lever arm. Torque increases because you’re multiplying the same force by a greater distance.
That’s the same mechanical principle behind an armbar. By isolating the elbow and extending the opponent’s forearm, you’re lengthening the lever and applying torque through a small movement of your hips. A small increase in distance creates a big increase in force at the joint.
Levers and fulcrums in common locks
It helps to map classic locks to lever systems. In mechanics there are three classes of lever, defined by where the pivot, load and applied force lie. Most movements in the human body act as third-class levers, where the force is applied between the pivot and the load. For practical purposes, though, you can just think in terms of where the joint acts as the pivot and where you apply force.
Armbar. The elbow is the pivot. The forearm is the lever. You control the wrist and apply a force at the wrist or use your hips to press the forearm across the fulcrum, producing an extension torque at the elbow. The longer the distance from the elbow to where you apply force, the more torque you get for the same push. That is why a straight forearm gives you mechanical advantage.
Kimura. The shoulder is the main target. You fix the wrist and then rotate the humerus in a way that produces internal rotation and posterior shear at the glenohumeral joint. The lever in this case is the humerus. Controlling both the wrist and elbow isolates shoulder motion so the torque transfers into the shoulder rather than being dissipated through the elbow or wrist.
Kneebar. Mechanically an armbar for the knee. The knee is the pivot. The tibia and foot form the lever. By controlling the hip and ankle you lengthen the lever and force hyperextension at the knee. The longer the tibial lever, the greater the extension torque at the knee for a given applied force.
Ankle lock. This can act on the talocrural joint and the subtalar joint. You create a moment that forces plantarflexion or inversion, depending on the lock. The foot and distal tibia are the lever segments. Controlling the calf or thigh acts to stabilise the proximal segment so torque concentrates at the ankle.
Heel hook. Heel hooks generate rotational torque rather than simple flexion or extension. The key movement is external rotation of the tibia relative to the femur, which stresses the ACL and medial meniscus. Because the knee is mainly designed for hinge movement, it cannot tolerate much rotational load, so even a small twist can produce high local stresses and rapid injury..
The common theme is the same. You create a lever, you fix one side, and you apply force on the other. Geometry decides how much force is needed. If you can increase r, you reduce the force you need.
Controlling above and below the joint – why it matters
You will hear coaches say, “control above and below the joint.” That is not a vague mantra. It’s the only way to make the joint take the torque rather than letting the rest of the body escape it.
Imagine trying to hyperextend an elbow while the rest of the arm is free to rotate. The opponent can shrug, roll, or flex their shoulder and dissipate the torque. But if you pin the shoulder so it cannot move, and you control the wrist so it cannot rotate freely, the elbow becomes the only movable part. All the applied torque now must be borne by the passive tissues of the elbow. That is what creates a real, isolating joint lock.
In practical terms, if you only control the wrist, they can turn their shoulder to escape. If you only control the shoulder, their wrist slips free. You need both sides secured so the joint between them is the weakest link.
From a physics perspective, you are changing boundary conditions. Fixing the proximal and distal ends converts the segment into a single-input lever, concentrating rotation about the joint. That concentration increases the local stress and the strain rate in ligaments and capsule structures.
Tissues and thresholds – what actually fails
The body has passive and active stabilisers. Passive stabilisers are ligaments, joint capsule, cartilage and bone geometry. Active stabilisers are muscles and tendons. When you apply torque, muscles contract and ligaments stretch.
Two key things happen.
First, if you apply torque slowly, muscles have a chance to activate and resist through reflexes and voluntary contraction. That can increase the joint’s effective stiffness. Second, if you apply torque quickly, you can overload passive structures before muscles can respond. That is why sudden cranks are so dangerous.
Ligaments have a nonlinear stress-strain curve. Initially they stretch easily, then they stiffen, and finally they fail if the load is high enough. The exact numbers vary by joint and by individual, but the qualitative idea holds. When the shear or rotational moment exceeds the tissue limit, structural failure occurs. In training the aim is for voluntary submission before any structural damage.
Two more physiological points. Pain and proprioceptive feedback act as early warning systems. Pain usually appears before structural failure. A tap is often a response to nociceptive input rather than a direct mechanical sign of tissue rupture. Second, neuromuscular inhibition can occur. If a joint is loaded into a dangerous position, muscles may suddenly shut down reflexively, making the joint more vulnerable.
Geometry, angles and mechanical advantage
Angles change everything. The moment arm r depends on where your force is applied and the joint angle. For example, in an armbar, if the opponent keeps a bent wrist close to the elbow, r is small and it is harder to generate torque. If you extend the forearm straight and place force at the wrist, r increases and torque goes up.
That is why precision in grip and limb position matters more than brute force. Small changes in angle lead to big differences in the torque you produce. The best locks use body mechanics to maximise r and use skeletal alignment to apply force efficiently. Your hips, core and legs are often the real engine of the lock, not just the limbs you see working.
Rotational versus bending moments
Not all joint locks are pure extension or flexion. Some create rotational moments. Heel hooks are prime examples. A rotational torque at the knee produces complex loading patterns in the ligaments because the knee is primarily a hinge with limited rotational capacity. Small rotational inputs can therefore produce high local stresses.
Similarly, shoulder locks like the kimura involve both rotation and translation of the humeral head within the socket. Translation is dangerous because the joint relies on capsule and labrum integrity to restrict it. That is why shoulder locks are effective but also why they can injure quickly.
Practical coaching points
A few actionable things to think about when teaching or drilling joint locks:
- Control both sides of the joint. Wrist and shoulder. Ankle and knee. Hip and tibia. If one end is free, the lock is mitigated.
- Use geometry before strength. Body alignment and lever length are more important than brute force. Teach students to increase moment arms safely.
- Consider rate of application. Slow controlled pressure lets partners tap. Fast violent cranks risk injury. Teach pacing.
- Train awareness of force vectors. Show how a perpendicular push versus a rotational twist changes the effect at the joint.
- Be explicit about which tissues are being stressed. If you are applying a rotational torque, emphasise knee and ACL risk. If you are applying hyperextension, point out elbow or knee capsular stress. That helps partners understand the danger and tap earlier.
Safety and ethics
A science-based view should not be an excuse to crank. Understanding how tissues fail is a responsibility. Use that knowledge to teach control, not to test limits. Encourage early tapping, controlled progression, and communication on the mat.
Heel hooks and other high-risk rotational locks are highly effective because they exploit mechanical vulnerability. That is also why many academies restrict them to advanced classes. Be clear with students about risk, and never use anatomical knowledge to force an unsafe submission.
Final thought
Joint locks are applied mechanics. They are physics and biology working together. If you think in terms of pivots, lever arms, torque and tissue thresholds you stop guessing and start engineering outcomes. That makes your coaching clearer, your students safer, and your own technique more efficient.
So next time you teach an armbar, focus less on squeezing and more on creating a long lever and fixing the shoulder. Once you see joint locks as applied physics, every submission starts to make more sense.
About the author
Chris Ward is a personal trainer, sports science graduate, and purple belt in Brazilian Jiu Jitsu. He combines his academic background with years of coaching and training experience to break down how strength, conditioning, and smart preparation translate directly to better performance on the mat.
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