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Excerpt from the book ‘Being Fit by J. Hoffman, PT.

Essay Five: Stability and Mobility

This essay is based upon the paper "Expanding Panjabi’s Stability Model to Express Movement: A Theoretical Model" (1). The paper was co-written by the author of this book and published by Elsevier Medical Hypothesis in 2013. In this paper, Panjabi's original model from 1992 was expanded (2). In this theoretical model expansion, two observable body-wide movement systems were described: stability and mobility. According to the new model, even though the whole movement system is integrated, skeletal joints tend to specialize in either stability or mobility movement functions, skeletal muscles are defined as either contributing mainly to stability or mobility and the central nervous system conducts separate interactions with both. A total of six movement subsystems are therefore identified: stability and mobility biased muscles (collectively termed 'active' as they have the power to generate movements), stability and mobility biased joints and soft tissue (termed 'passive' as they do not move independently) and separated neural conduction (termed 'neural' as they control the neural activation of the stability and mobility movement structures). It was concluded that for harmonious movements to occur, all six subsystems need to work well both individually and in harmony with each other.


In this essay, the example of kicking a ball is used to depict the expanded model. Stability is provided mainly from the pelvis and the standing leg while mobility comes mainly from the kicking leg. Stability and mobility work together. To safely kick harder, one needs more of both.

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The SI joints are situated in posterior aspect of the pelvis, in the small of the back (circled in red). Their joint surfaces are not smooth and restricted by ligaments and fascia, not allowing for much movement. Instead, they match each other in shape so, when compressed, they lock together like Lego pieces and make the ring-shaped pelvis (as seen from above) much more stable. The lumbo-pelvic stability muscles are anatomically and physiologically optimal to compress the SI joint surfaces concurrently from both opposite directions. The transversus abdominis is wrapped around the lumbo-pelvic region, hugging it like a corset. When activated, this muscle creates external compression and serves the purpose of compressing the SI joint surfaces from the outside in (green arrows) (3,4). Normally, this occurs a split second before the movement (5). This external compression is countered internally by a dynamic air-pressure container, located in the abdominal area between the pelvic floor muscles from bellow and the diaphragm muscles from above. This air-pressure mechanism has been compared to a pressurized soda pop can. When the body expects a challenge to pelvic stability, it uses its muscles to seal the internal container and increase its air pressure. The container then stiffens like a full and closed pressurized soda can, which is difficult to bend (blue arrows). As soon as the can is opened, or as soon as the body releases its abdominal air pressure, the structure returns to be easily bendable for mobility functions (6).

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Once these counter forces concurrently stabilize the pelvis, the leg can safely and accurately mobilize to kick the ball. For this movement, it uses the hip and knee joints, which, in contrast to the rough sacroiliac joint surfaces, have large smooth and round surfaces designed for mobility (blue circles). The power to swing these joints is provided by the long rectus femoris muscle (amongst others), which is ideally attached just beyond both of the mobility joints (red line). This muscle is anatomically and physiologically optimal for short, strong bursts of energy which are required for mobility functions.


The sequence of all six subsystems working together in a precise manner creates functional movements. In the example of kicking a ball, there might be some overlap; however, the subsystems work in essentially this order:

  1. The central nervous system activates the lumbo-pelvic stabilizers according to the predicted challenge (stability-neural subsystem).

  2. The lumbo-pelvic stability muscles are activated, creating compression on the stability joints (stability-active subsystem).

  3. The joint compression results in mechanical locking, significantly increasing lumbo-pelvic stability (stability-passive subsystem).

  4. The central nervous system activates the hip and knee mobility muscles (mobility-neural subsystem).

  5. The long and powerful hip and knee mobility muscles rapidly shorten, leaning on the relatively stable pelvis (mobility-active subsystem).

  6. The mobility joints mechanically swing the leg to kick the ball (mobility-passive subsystem).


Under normal circumstances this split-second sequence of events is automatic. Failure of any of the subsystems will cause others to malfunction, resulting in impaired movements.


In reality, the action of kicking a ball is significantly more complex, involving synergy between all the movement components throughout the entire body. Kicking is asymmetric; therefore, the other knee and hip face different challenges than those of the kicking leg. Additionally, there are also mental and environmental influences on movements, which should be taken into account. However, analyzing a movement from any angle should reveal that fundamentally, movement quality depends upon the quality of interactions between the stability and mobility systems.

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In this image, the active, passive and neural subsystems of both stability and mobility systems are equally represented and are also connected to each other. When the image is symmetrical and the lines meet in the middle, it means that each of the subsystems is functioning individually and in harmony with the others. This provides an environment conducive to harmonious, functional movements.



  1. Hoffman J, Gabel P. Expanding Panjabi’s stability model to express movement: A theoretical model. Medical Hypotheses. 2013; 80(6): 692–697.

  2. Panjabi MM. The stabilizing system of the spine. Part I. Function, dysfunction, adaptation, and enhancement. J Spinal Disord. 1992; 5: 383-389.

  3. Richardson CA, Snijders CJ, Hides JA, Damen L, Pas MS, Storm J. The relation between the transversus abdominis muscles, sacroiliac joint mechanics, and low back pain. Spine. 2002; 27: 399-405.

  4. Mens JM, Damen L, Snijders CJ, Stam HJ. The mechanical effect of a pelvic belt in patients with pregnancy-related pelvic pain. Clin Biomech. 2006; 21: 122-127.

  5. Hodges PW. Is there a role for transversus abdominis in lumbo-pelvic stability? Man Ther. 1999; 4(2): 74–86.

  6. Massery M. Musculoskeletal and neuromuscular interventions: a physical approach to cystic fibrosis. J R Soc Med. 2005; 98(Suppl 45): 55-66.

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