Kinematic and Kinetic Chains

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In engineering, a series of linkages is referred to as a kinematic chain. If the two ends of the series are fixed, the chain is said to be closed. If the terminal end of one link is not fixed, the chain is said to be open. A functional consequence of a closed chain is that movement of one joint will cause every other joint to move in a predictable fashion. Open chains are not subject to these constraints: Movement at one joint will not necessarily cause movement at another joint.

In the mid-1950s, Steindler (28) suggested that the body operates as a kinetic chain. He described an open kinetic chain as “a combination in which the terminal joint is free” and a closed kinetic chain as one “in which the terminal joint meets some considerable external resistance which prohibits or restrains free movement.” Steindler noted that a chain is only “strictly and absolutely closed” when no visible motion is produced, yet he believed it was acceptable to “apply the term in all situations in which the peripheral joint of the chain meets with overwhelming external resistance.” This is often interpreted as the terminal segment (hand, foot) being fixed.

Note that Steindler used the term kinetic (forces), whereas engineers use the term kinematic (motion), yet they appear to be talking about the same thing. “Kinetic chain” is not used in engineering or robotics, because kinematic chain is the technically correct description. Yet the term kinetic chain is used more often in exercise publications, if not interchangeably with kinematic chain.

Steindler acknowledged that the chain is rarely “strictly and absolutely closed.” This is apparent if we look at the physical description, yet his definition is still confusing because he never described “considerable” external resistance. For example, the squat and leg press are kinematically and kinetically similar (10), yet the terminal segment (foot) is fixed during the squat and moving during the leg press. Similarly, the bench press and push-up have similar muscle activation patterns throughout the range of motion (4) even when the resistance could hardly be characterized as “considerable.”

One could garner more utility from the terms by looking at the functional consequences rather than physical descriptions of the motion. An open chain is one in which movement of one joint is independent of the other joints in the chain, while a closed chain is one in which movement of one joint causes the other joints in the chain to move in a predictable manner (20). Thus, an open chain movement usually involves a single joint moving (e.g., arm curl, leg curl) against some form of angular resistance. A closed chain movement involves multiple joints moving, usually against a linear resistance (e.g., bench press, squat).

The importance of recognizing a closed chain activity lies in the fact that the motions of multiple joints are coupled (36). For example, during standing in a weight-bearing position and performing a squat motion, flexion of the knee cannot occur without simultaneous flexion of the hip and dorsiflexion of the ankle. A limitation in the range of motion of any one of the joints will affect the range of motion of the entire exercise. Similarly, the torques at the joints are also coupled: As one lowers in the squat position and increases the flexion angles of the joints, all of the muscles in the chain increase their internal torques. Weakness at any one joint will consequently limit performance of the entire movement.

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