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Stress Shielding And Stress Relaxation

Pendulum equipment

Pendulum Hip Press Isometric Holds Against the Lockout Limiter

The musculoskeletal system is characterized by complex anatomy and apparent muscle redundancy, meaning there is more muscle than mechanical degrees of freedom. This being so leads to individuals exhibiting large variation in biomechanical and neural control activation strategies for the same task.

Having large variable movement strategies is extremely important to function. Having pain or with an injury we are able to substitute movement with unaffected musculature and complete daily tasks. Being able to substitute movement with unaffected fibers allows injured structures to recover, an important function of the skeletal muscle system.

Even a mild pathology can be disruptive; it is always necessary to return the previously strained tissue back to normal through the rehabilitative process, as not only muscle, but neural pathways are altered and lingering atrophy may occur.

Redundancy and substitution is inherent in our muscular system, it is important to remember muscles function as a muscle tendon unit. While a muscle uses redundancy and substitution as part of its repair process, tendons use stress shielding and stress relaxation to recover.

Tendon issues like muscle issues are common in athletics and in daily living. Tendons are postural, position joints for movement and they store and return energy. Energy-storing tendons are subject to high stresses and strains and during loading recoil returning energy to the system. These explosive tendons are involved in locomotion stretch-shortening cycles and have a higher risk of injury.

A tendinopathy is thought to result from gradual accumulation of micro-injuries. In the coaching and training world this is often referred to as an ‘overuse’ pathology. Damage to tendons occurs regularly as a result of a variety of normal activities, but only accumulates when equilibrium cannot be maintained between the rate of damage and the cellular driven rate of repair. When a few of the collagen components of the tendon become disrupted this does not significantly disrupt activity, as it remains relatively normal, yet in the highly active it can lead to problems.

Stress shielding: Stress shielding is the body’s coping mechanism to protect a tendon from further damage when active. As described, tendon damage is reflected normally in small tears.  The strong area of the tendon shields the damaged area by having the stress go around the area that is affected, which is how the tissue safeguards itself from tearing across the entire tendon. Unloading of the damaged area, while protective with a constant day to day continuation of activity can decrease the collagen content and it’s fiber orientation. This inturn is associated with impaired tendon performance.

Stress relaxation: Stress relaxation is time-dependent, it is a decrease in stress under a constant strain and can be used to promote healing. Knowing the force within a tendon decreases over time when a static load is applied and the tendon is held at a constant length, isometric exercise becomes an ideal intervention for load-dependent collagen synthesis. The pulling and holding of an isometric exercise causes the strong fibers to relax which loads the part of the tendon that has been injured and shielded during activity.

The ideal exercise devices for isometric exercise are Pendulum strength training machines. A Pendulum machine with Set Extension Technology (S.E.T.) contains an adjustable safety limiter to control the distance the weight can move before reaching the safety stop. This protects the athlete if he or she suddenly loses control of the weight.  

Not only does Pendulum have Set Extension Technology, but the Pendulum Hip Press contains an adjustable lockout limiter that stops forward progress at any knee angle the lifter desires. This makes the exercise device ideal for rehabilitation of the patellar tendon.

To perform an isometric on the Hip Press, first set the adjustable safety limiter to control the distance the weight can be returned towards the body if an athlete suddenly fails. On the adjustable lockout limiter set it for the appropriate knee angle. The lockout limiter provides a surface to press against during an isometric to get an accurate determinant of exercise time while maintaining position.

Setting the Adjustable Lockout Limiter For Knee Angle

Research has shown that when using a leg press for an isometric stress relaxation movement the user starts by selecting a weight that is initially 80% of the projected maximum. The program begins with 10 second holds, increasing the holding time by 5 seconds during the rehabilitative healing process with a final goal of 30 second holds based on the tendon’s response. Two to four repetitions of this movement are done at the end of exercise twice per week.


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The human body is described as having 6 degrees of freedom for each of its segments. Degrees of freedom refers to the number of ways a rigid body can move in three-dimensional space, up/down, left/right,  in/out and in 3 rotations;...

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