Cartilage and Exercise – An Introduction

 In Cartilage, Knee Injuries

Traditional Exercise Programs

Traditional exercise regimes for patients with cartilage damage and osteoarthritis of the knee have emphasized low load, repetitive motion through pain-free ranges performed in supported postures usually sitting or lying down. Typical programs would center in such exercises as include straight-leg raises, knee extensions simulating a kicking motion, and quadriceps isometrics. All of these are joint-specific maneuvers that isolate motion to a single, pre-determined plane of movement, avoiding significant weight-bearing across the involved joint.

Exercise progression is achieved by increasing repetitions and adding external resistance in the form of weights, usually loaded at the ankle. Further progression of the rehabilitation program to more functional activities such as stair climbing, walking, running, or return to sport is frequently determined by lifting loads arbitrarily assigned to the activity in question. While traditional exercises can be joint-specific to muscles supporting a knee weakened by osteoarthritis and are the starting point for patients with significant pain in weight-bearing postures, these exercises are marginal in their carryover to resuming functional activities.


Risk Factors for Cartilage

Current research points to several factors that further the progression of the osteoarthritis at the knee. Obesity, genetics, previous injury, and knee joint alignment have been reported. As genetics and previous injury presently offer little recourse for modification, obesity and knee joint alignment offer opportunity. Both obesity and knee joint alignment can be viewed as behaviors that can be altered. At the joint level, the deleterious effects of weight and alignment manifest in the form of increased joint force. Contact time over contact surface area are factors that define stress concentration in the joint.

Concepts for Cartilage

Why does cartilage wear out?  Understanding cartilage behavior.

Man versus Nature

The objective of Nature and the objective of knee-user are somewhat at odds. Biological systems deteriorate with time.  Joints are extraordinarily ordered systems and Nature will try to undo that order.  The rate of deterioration is governed by the material properties of the tissue and the forces placed upon them. Quantity, direction and frequency of joint forces can be modulated through one’s kinematic behavior.  The objective is KINEMATIC DISCIPLINE.

Laws of Cartilage Behavior

Law 1. Cartilage wear is a function of the concentration of forces, or stress, on its surface.  Stress concentration is bad for cartilage.  In essence, this is the wrong amount of force in a given area within the joint.

Law 2. Joints will tend to take the path of least resistance (and least energy expenditure) creating sub-optimal joint mechanics, which imparts high stress concentrations to the joint surface.  The result, over time, is increased stress concentration, which according to Law 1, may lead to joint surface damage.

How about repairing and rebuilding cartilage?  Cartilage is notorious for its poor healing capacity.  It is under-nourished (avascular) and over-worked. However, the inherent beauty of biological tissues is that they contain the machinery to regenerate given the correct environment.  The key, as in most disease processes, is early intervention. Joints generally abide by a principle whereby they strengthen themselves according to the demands placed upon them, a principle that has come to be known as Wolff’s Law.

Law 3. Cartilage is self-rejuvenating when the cells receive the proper signals.  This is Wolff’s Law of form and function.  The appropriate tissue stress creates the ideal tissue structure.

Law 4. Optimal joint mechanics lead to optimal stress concentrations.

Understanding these 4 laws and how they inter-relate is intention of this post.  A joint that takes the path of least resistance will often lead to increased stress concentration (compression/tension/SHEAR) within the joint.  Pressure will not be optimally distributed, but localized to smaller areas, something known as peak loading.  Stress concentration leads to increased wear on the cartilage surface. This is the path of joint destruction.  However, the reverse of this pathway also holds true, that is;

Optimal joint mechanics Þ Minimize stress concentration Þ Send optimal signals for cartilage regeneration

The key to knee health is to optimize the force vectors that travel through your body; up, down and within. This begins with the position of your head and down through the shoulders, moves through your torso, the core and on through the pelvis, hips, knees, ankle, foot and even toes.  All of the intervening joints are connected in a complicated and elegant way.  It is the ability to link these interconnections in an ideal manner that will protect the cartilage of these joints.

MASTER LAW 4 through kinematic discipline.
Our objective is to master Law 4; create optimal joint mechanics, ie, control the forces that go through your joints.  The goal will be to minimize stress concentration with the joint and maximize the effects of Wolffs law.  To accomplish this, we must teach the body a new positional language; the knee joint must have disciplined movement as not to drift into the path of least resistance.  Optimal joint mechanics, in general requires;

  • The forces going in to the joint must be minimized.
  • The pressure within the joint must be optimally distributed
  • The action of the affected joint must be to maximally absorb and attenuate those forces.

The Theory of 5 Principles

The “code” of cartilage longevity is unlocked by system of 5 principles, a control system of the knee. I will consider this system a theory because research is ongoing, and it is very likely that new information will shift these principles in the future.  The 5 principles are the building blocks of cartilage health by way of proper joint mechanics (Law 4).

The 5 principles are

  • Vectors: directional control
  • Vectors: force regulation
  • Energy
  • Elasticity
  • Power

Much of energy modulation comes from muscles.  Muscles produce power and attenuate shock.  Production of power is their first order of business (particularly in youth) while attenuation of shock is secondary.  The code strives to reverse that.

Muscles and the motion they control, will default to that direction that requires the least exertion and energy expenditure.  The code strives to undo that.  It requires re-programming the circuit that governs joint behavior, or kinematic discipline.  This generally requires more energy, not less.  Incorporating the theory of 5 principles into your daily movement patterns will accomplish this.


  • What are the optimal lines of force across the joint? 
  • How does one control these lines of force? 
  • How can one change the way their body works to improve joint forces over a lifetime? 
  • Will all this, in the end, increase cartilage longevity?  I think so ¾ though at the very least, it’s the best you can.

Vectors – Joint Mechanics – Stress Concentration

The forces that determine joint alignment are called vectors. Force vectors move through your knee in all planes of motion. Vectors have both magnitude and direction. Vectors are a function of the position of your body in space. Muscles are the primary controllers of your body’s position, therefore muscles control vectors. Vectors determine the stress concentration in your joint.

Alignment of the knee is significantly affected by 1) the actions of powerful muscles around the hip that initiate subtle changes to control torso position and balance, and 2) actions of the foot and ankle that compensate for propulsion over ever-changing ground surfaces. Hence, the knee can be viewed as a “differential joint” driven by controlled or uncontrolled tri-plane motions at both the hip and foot. Cartilage wear patterns are a function of the resultant multi-plane motion at the knee. The force that may have the most significant effect on cartilage is shear, a translatory vector.

Excessive shear on knee cartilage is controlled, in large part, by controlling the adjacent hip, ankle and foot joints. Excessive motion of the foot in pronation (foot-flat) or supination (high-arched) will alter knee alignment and weight-bearing surfaces in the sagittal, frontal and transverse planes. Weakness of hip abductors, extensors, and lateral rotators will contribute to excessive shear forces at the knee in similar planes of motion. Thus, instruction in proper knee alignment controlling excessive shear forces influenced by the linked foot and hip joints in weight-bearing postures should be a focus of rehabilitation programs for osteoarthritis of the knee.

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