These exercises have been a mainstay of physical conditioning routines for as long as physical conditioning has been around. We all know that one person that starts their day off with a hundred (or other arbitrary number) sit-ups or crunches. We’ve been told for countless years that the best way to train your ‘abs’ is to do sit-ups or crunches in any number of different varieties…. but is it really that simple?
Let’s look at the exercises themselves. At their heart, sit-ups and crunches are built around the action of flexion of the spine. When we lay supine (facing up), we are using gravity to provide resistance against this flexion action. This places load on our rectus abdominis (commonly referred to as the ‘abs’) and to a lesser degree, the obliques (1). With this in mind, we can correctly draw the conclusion that these exercises can be used to strengthen or condition the ‘abs’ (dependant on rep ranges etc), and even cause hypertrophy (increased muscle size) of the ‘abs’ if the load is appropriate. So far so good, right? But is repeated flexion necessary, or even healthy?
For a number of years now, a large number of trainers, strength coaches, and physiotherapists have been moving away from prescribing or recommending these exercises to clients. The driving reason behind this change is the research conducted by spine biomechanist Dr Stuart McGill and his team at the University of Waterloo, Canada, who used in vitro testing of pig spines (which are very similar to human spines) to demonstrate that repeated flexion of the spine is highly likely to lead to disc pathologies (1). McGill’s findings were that high numbers of flexion movements in the spine ultimately lead to disc injuries such as herniation.
Not all health and fitness trainers have subscribed to McGill’s findings. His major opponents consider his use of porcine cervical spines with no active muscular attachments as not being representative of the moving spine in a living human. One of the major threats to disc health during movements, is the compressive force created by the contraction of muscles acting on the spine. As the muscles pull the spine in various directions, the compression on the intervertebral discs shifts, and becomes uneven which can lead to disc herniation. Biomechanical modelling predicts that up to 18% of this compressive force can be offset by the presence of intra-abdominal pressure (IAP) during spinal flexion (2).
These arguments are for the most part theoretical however, with no conclusive clinical evidence to support them – leading us to the conclusion that right now, the best research we have on the effects of repetitive flexion of the spine is Stuart McGill’s. Factoring in McGill’s research, alongside the correlation of genetics and spinal degeneration (3), it’s difficult to justify the prescription of crunch type exercises when lower risk, more practical training approaches exist.
Some organisations (such as the ADF, Police Departments etc) have a requirement for members to perform sit-ups or crunches in workplace fitness assessments. Our advice in this situation would be to only program the required number of sit-ups/crunches required to get you through your fitness test, and ensure you have prescribed exercises to train your spinal extensors to balance out the number of flexions you are performing (deadlift variations are a great way to do this).
As always, having an experienced professional to develop your training programs is the best way to ensure you are keeping a balance in your exercise prescriptions, and performing the safest possible exercises to address your needs.
References:
(1) McGill SM. Low Back Disorders. Champagne, IL: Human Kinetics, 2002
(2) Stokes IA, Gardner-Morse MG, and Henry SM. Intra-abdominal pressure and abdominal wall muscular function: Spinal unloading mechanism. Clin Biotech (Bristol, Avon) 2010
(3) Battie ́ MC and Videman T. Lumbar disc degeneration: Epidemiology and genetics. J Bone Joint Surg Am 88(Suppl 2), 2006 
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