Mobility is closely related to flexibility. But, the main difference is that mobility is the ability for the body to go through a range of movement actively, and against resisting forces. For example, a person’s shoulder may be able to go through a larger range of flexion when done passively (someone else is holding the arm, taking it through the range), than when done actively. Another example is how increasing the resistance on a movement can result in some joints not being able to go through their usual range of motion.
These discrepancies are usually noticed when a movement involves multiple muscle groups, and even joints. In other words, when a gross motor task is undertaken, such as a squat, many joints (ankles, knees, hips, lumbar spine, and even small joints of the feet) are all involved. And, as such, the muscles surrounding these joints are all involved. While these muscles may display good flexibility and range of motion, individually, when coupled together there is a chance of some limitation of the movement.
Causes of Decreased Mobility
Mobility is generally impacted by two main factors. These are flexibility and strength. While flexibility would seem quite obvious, it is still surprising how this works to decrease mobility. And, while strength may not be the first thing that springs to mind, as a hindering factor, it seems the more straight-forward, when thought out.
Flexibility: active range of motion for any joint is determined by a combination of the boney structures of the joint, connective tissues surrounding the joint, and the muscles that work as agonists, as well as antagonists, in causing movement of the joints. Boney structures quite clearly aren’t a part you can negotiate with. And, similarly, connective tissues like ligaments aren’t seemingly very pliable in stand-point. So, the main way of increasing active range of motion must be the muscles causing the movement. To fully understand how this impacts range of motion, a detailed knowledge of anatomy is needed. But, more simply put, all of the muscles involved with an active movement must be capable of enough stretch, within the plane desired. If we return to the squat as an example, and focus on the hamstring complex to detail this idea; the most common forms of testing and stretching the hamstrings all take place in very much a sagittal plane. However, when performing a squat, the femur (and therefore hamstring complex) is moved into an oblique plane with the knee slightly abducted. In this particular instance, the lateral aspect of the hamstrings now requires less flexibility to achieve the same range of movement in the hip, while the medial aspects require greater flexibility. A lack of specific flexibility can present at any joint, and in any plane. It is not limited to the hamstrings. This is why all joints must be assessed, when observing a gross motor task.
Strength: where strength becomes a limiting factor in mobility is when there is a discrepancy in a force-couple. This is not as complicated as it seems. A force-couple, in relation to muscles, is a grouping of muscles that work together to create a movement. They are often on opposite sides of a joint, but still work together to produce a movement (instead of opposing each other). If we keep with our squat example, an easy force-couple to work with would be the external rotators of the hips, and the internal rotators of the hip. It is also worth mentioning that many of the internal rotators are also adductors; and many of the external rotators are also abductors. When these groups contract together, they bring about extension of the hip (which is part of the movement of the squat). However, when there is a difference in strength between these pairings, the stronger half of the pair is forced to take on more of the load, and has to therefore produce more of the force. With our example, the body allows for this by adducting and internally rotating the hips (knees in), if the internal rotators are too weak. This places the femur in a position where the external rotators are more heavily relied upon. Where the external rotators are too weak, the hips are externally rotated and abducted (knees out). This is not limited to the hips, of course. Each joint has a number of these force couples. And, each will present slightly differently, upon observation of the movements. This is why a complete approach to the assessment is needed.
In both of these instances, the joints are not fully capable of moving through the range of motion required. The result of the strength imbalances or the lack of apparent flexibility in the required plane leads to a decrease in the active range of motion.