Biomechanics of Repetitive Movements in Group Exercise Classes
The way we exercise and work has changed in the last few generations. Our bodies were designed to be hunter/gatherers. Our very physical make-up enables us to hunt, escape, harvest and gather by performing an almost unlimited number of movements like walking, running, throwing, bending, twisting and turning. As the technological revolution has enveloped our lives, these tasks are no longer necessary and our bodies have begun the process of de-conditioning in physiological and biomechanical terms. Problems then arise when we ask our bodies to deliver group exercise classes in this biomechanically de-conditioned state; our bodies become adept at compensating for a rotated pelvis, leg length discrepancies, tight thoracic spines, stiff sciatic nerves and many other issues, which are significant factors in why we get pain despite being ‘fit’.
An often-unrecognized problem for us is that our classes are repetitive in nature. Where our muscles were once responsible for performing a variety of different movements throughout the day, they are now performing small semi-static, repetitive movements when we work on our laptops or larger repetitive movements when we are in our classes. The body does not respond well to repetitive movements; nerves in particular go through a process of pre-conditioning. Pre-conditioning is a mechanical phenomenon where the nature of structures (in this case, nerves) actually changes and as they do, your muscles go into a protective spasm. Remember this is all while performing exercises; this is another reason for us having pain despite being fit.
Predisposition to injury
When you look at Meeuwisse’s work in 1994, he showed that the injured individual is predisposed at a number of different levels. If you apply this to you as group exercise instructors you can see the loop that you will commonly go through from pain to returning to classes (and indeed working through pain).
These same principles apply to you too. If you do actually stop for a while to let your injury settle, even with therapy, you are still predisposed to injury if you are not correcting the biomechanical problems.
Let’s look at some more of the specific problem with repetitive movement:
If a joint is subjected to prolonged pressure when repetitive movements in your classes are performed, the cartilage begins to lose its elasticity and therefore becomes vulnerable to damage, (Oleyede & Broom 19942). A biomechanically efficient joint will cope better.
Tension, stress and relaxation
Tendons are visco-elastic in nature and are therefore subject to the mechanical phenomena of preconditioning, creep and hysteresis when subjected to repeated stresses and strains. Synovial fluid which lubricates your joints and tendons is a non-newtonian visco-elastic fluid, which means the visco-elasticity of the fluid changes with continued shearing of joint surfaces or tendons, leading to reduced viscosity and escape of fluid from the stressed structures. The reduction in viscosity then is directly related to the repetition of movements you perform in your classes.
High repetition work in your classes can lead to muscle fatigue and in some cases, Delayed Onset Muscle soreness (DOMS) and overuse syndromes. DOMS is associated with increase in muscle tone and faulty excitation of muscle, (Janda 1983). This phenomenon is termed "The Calcium Paradox". In this state the muscle is no longer able to contract and relax properly which means the muscle is in spasm i.e. it cannot relax. In a normal individual this chronic state is avoided because we naturally move giving the muscle a normal sensory input and allowing it to relax, if however we ignore the warning signs and continue to exercise in our classes, the continual irritation of the now-shortened muscle leads to physical changes within the muscle. Up to 40% of muscle can be replaced in a shortened muscle by connective tissue (Tardieu & Tarbary). Once this occurs normal mechanics are no longer possible without biomechanical correction (Janda 1983).
Nerve is visco-elastic in nature which means that the greater the velocity of stretch applied to your nerve, the less stretch is available and the greater the stress on the nerve. It can be seen when a nerve is subjected to repeated stretching in classes as it begins to react in an abnormal manner. When a nerve is first stretched it begins to lengthen as the stress is removed and then reapplied, however, it reaches a certain point when repeated applications of stress cause no increase in nerve length. Sometime after this point has been reached if tension is continually removed and reapplied the nerve under stretch begins to show a greater resistance to elongation, (Barrow 1994). This is similar to the mechanical concept of pre-conditioning found in other materials. As the nerve becomes pre-conditioned, local muscles go into a protective spasm, which can cause pain and certainly compromises the quality of the movement you perform in your classes.
So what can you do? There are key areas that you can work on to improve your biomechanics:
Reduction of muscle spasm associated with repetitive movements
Muscle release exercises are designed to encourage the muscles in spasm to relax. Studies by Ribot-Ciscar have established that a muscle relaxes maximally after it has been contracted sub-maximally and for a prolonged period. So, if you think one of your muscles is unduly tight or is not responding to stretching, then try low grade endurance isometric contractions of that muscle to release the spasm.
Prevention of postural abnormality
Many of you will have fundamental intrinsic biomechanical problems and these need to be addressed to have a good chance of your posture improving. Even if you work on your posture, if you do not have the capacity to adopt and maintain good posture, that work is less likely to be effective. Ensure you have good biomechanical foundations and your ability to adopt and hold a better posture will be enhanced.
In the fitness industry we sometimes get ‘hung up’ on what type of stretching we should be doing; static, ballistic, dynamic etc. But it has been shown that nerves can be at least as important; as a tight nerve can cause muscle and joint injuries. So as part of your warm up and cool down, it’s helpful to mobilise your median and sciatic nerves in particular.
• Meeuwisse, W.,H., 1994, Clinical Journal of Sports Medicine 4 (3) 168
• Oleyede & Broom. Complex nature of stress inside loaded articular cartilage. Clinical Biomechanics 1994.
• Janda V. Muscle function testing. London, Butterworths, 1983.
• Barrow N. Recent advances in the use of robotic dynamometry - Measurement of Neural Tension. European Isokinetic Congress 1994
• E. Ribot-Ciscar1, M. F. Tardy-Gervet1, J. P. Vedel2 and J. P. Roll1 Post Contraction Sensory Discharge.Laboratoire de Neurobiologie Humaine, Université Aix-Marseille I, URA CNRS, 372, Avenue Escadrille Normandie-Niemen, F-13397 Marseille Cedex 13, France. Laboratoire de Neurosciences Fonctionnelles, CNRS-LNF3, 31 Chemin Joseph Aiguier, B.P. 71, F-13402 Marseille Cedex 9, France
• Edwards RHT. Muscle fatigue and pain. Acta Med Scand Supplement, 1987.