Introduction to Kinetics

For Medical Students

The Principle

Kinetics is the study of the forces that produce motion. Kinematics describes the movement and says nothing about its cause. 3.3.1 covers that side.

A force has a size and a direction, so it is drawn as an arrow. Two forces of the same size pointing differently do different things.

A force applied away from a joint's axis also turns the joint. That turning effect is a moment, and it is the quantity most of this chapter is about.

At the Knee

Three kinds of force reach the knee. External forces come from gravity and the ground. Internal forces come from muscles and from the passive structures. The joint surfaces themselves push back.

The external and internal forces are not equal in size. A muscle works at a short distance from the joint axis and body weight at a long one, so the muscle has to pull much harder than the external load weighs.

That is why joint loads are far larger than body weight. Conventional teaching gives tibiofemoral load in walking as roughly two to three times body weight.

Newton's third law applies at every joint surface. Whatever the femur pushes on the tibia with, the tibia pushes back with.

What Changes It

Body weight, which scales almost everything in the chapter.

Activity, since the external forces in landing are several times those in walking.

Flexion angle, which changes the distances the forces act at and therefore the moments.

Muscle strength and co-ordination, which decide how the internal forces are produced.

Clinical Relevance

Joint load is dominated by muscle rather than by body weight, which is why strength and activity change symptoms more than the scales suggest.

Weight loss still helps, because the external load it removes is multiplied by the same lever arms. Section 6 covers management.

A patient asking why a knee hurts going downstairs and not on the flat is asking a kinetic question. 3.3.12 covers the kinematics of stairs.

Describing a treatment as unloading the knee means naming which force it reduces.

MRCS Level

The Principle

Draw a free body diagram before quoting a figure. Isolate the segment, mark every force acting on it, and the problem becomes arithmetic rather than intuition.

The three equations of static equilibrium are the tool: the forces balance in two directions and the moments balance about any point. A knee in slow motion is close enough to static for the estimate to be useful.

Inverse dynamics is the method behind the published figures. Motion and ground reaction force are measured, segment masses are assumed, and the joint forces and moments are calculated backwards from them.

At the Knee

The net moment at the knee is the sum of everything turning it, and it is what inverse dynamics computes. It is not the force in any one muscle.

Co-contraction defeats the calculation. Quadriceps and hamstrings pulling together produce a large joint compression and a small net moment, so the net moment underreads the load.

Joint reaction force is the compressive force across the surfaces and is the quantity cartilage cares about. 3.4.3 covers it.

Moment arms change with flexion, so a moment calculated at one angle does not transfer to another. 3.4.4 covers lever arms.

What Changes It

Segment mass and inertia assumptions, which are taken from population tables and are a stated source of error in every inverse dynamics result.

Co-contraction, which is invisible to inverse dynamics and is greater in a painful or unstable knee.

Speed, since acceleration terms grow and the static approximation fails.

The measured ground reaction force, which is the one quantity in the chain that is measured directly rather than assumed.

How It Is Measured

Force plates measure the ground reaction force directly, in three directions, and are the foundation of the whole literature.

Inverse dynamics combines that with motion capture to compute joint moments, and inherits the error of both.

Electromyography measures when a muscle is active and not how hard it pulls, which is the gap co-contraction sits in.

Instrumented implants measure tibiofemoral load directly in vivo, in a small number of replaced knees, and are the only direct measurement of joint force in a living person.

Clinical Relevance

Read a joint moment as a net quantity rather than as a muscle force.

Expect inverse dynamics to underestimate joint load where co-contraction is likely, which includes most symptomatic knees.

Treat instrumented implant data as the direct measurement and the rest as computed.

State the activity and the method with any kinetic figure, since both change it by more than most clinical variables do.

FRCS Level

At the Knee

Treat the kinetic account as the one that explains disease progression, since cartilage responds to load rather than to motion. 3.5 covers contact mechanics and 6.77 the post-traumatic natural history.

The clinically actionable kinetic quantity is the knee adduction moment, because it is modifiable by osteotomy, by gait retraining and by a brace. 3.4.6 and 3.13 cover it.

Almost every surgical decision in the knee is a kinetic intervention described in anatomical terms. An osteotomy moves a lever arm, a meniscal repair restores a load path, and a reconstruction restores a restraint that limits a moment.

What Changes It

Alignment, which is the largest modifiable determinant of compartment load.

Body mass, which scales the external loads and is modifiable with difficulty.

Activity selection, which changes the number and size of loading cycles.

Muscle capacity and co-ordination, which set how much of the load is muscular and how well timed it is.

How It Is Measured

Instrumented gait analysis with force plates is the clinical research standard and measures a laboratory version of an activity.

Musculoskeletal modelling estimates individual muscle forces by resolving the redundancy that inverse dynamics leaves, using assumptions that are the model's main limitation.

Instrumented implant telemetry validates those models in replaced knees and cannot be applied to an intact one.

No routine clinical measurement of joint load exists, so load is inferred from alignment, mass and activity.

Clinical Relevance

Describe a planned operation by the load it changes rather than by the structure it touches, since that is what the patient's cartilage experiences.

Correct alignment where compartment load is the problem, because it is the largest modifiable term.

Address muscle capacity alongside any mechanical correction, since co-contraction and weakness both raise load.

Read a modelled muscle force with the model's assumptions in mind rather than as a measurement.