Balance is an internal response to an external disturbing force
Recently, I came across an excellent article by Ian Griffiths titled, ‘Putting the mechanics back into ‘biomechanics‘. Ian is a Sports Podiatrist in the UK. His article was published on http://www.running-phyiso.com/mechanics/ on February 4, 2015.
According to the introduction by PhysioTom, Griffiths has been challenging misconceptions around pronation and foot function for some time. Griffiths states that the aim of writing his blog is to try and put some mechanics back into discussions of biomechanics. According to Griffiths if those discussing biomechanics, “don’t do physics”, then it may be time to choose another speciality. I whole-heartedly agree. From what I have read in the literature over several decades, it is not so much a case of putting the biomechanics back into discussions of the biomechanics of skiing, but more a case of putting the mechanics into discussions of the biomechanics of skiing where little or no component of mechanics currently exists. It is not possible to engage in a meaningful discussion of the biomechanics of ski technique, especially at the World Cup level, without including the components of mechanics and physics and especially the opposing forces acting across the inside edge of the outside ski of a turn.
Here is an article that I wrote on skier balance in February of 2002.
Good balance is everything in skiing. Few would argue the point. So why is it is that no one seems to be able to describe what good balance is? It’s pretty hard to know when a skier has good balance if no one knows what good balance is. It’s even harder to learn good balance for the same reason. If you watch skiers coming down a hill it’s obvious that some have better balance than others. How can you tell? They are typically much quieter and more fluid in their movements than less balanced skiers. And they don’t get tossed around as much in the bumps. More important, they determine where and how they want to move. Skiers who have good balance, are in control.
The ability to stand upright without falling over is a form of balance. But good ski technique is more than simply not falling over. Good balance allows a skier to resist the external forces of skiing as efficiently as possible by controlling their position on their skis. The muscle effort required to do so with good balance is a fraction of that required when balance is poor. The stress on a skiers’ body is equally reduced. If you want to become a good skier, then you have to know what good balance is and what you have to do to get it.
Webster’s dictionary defines balance as equilibrium or to remain in equilibrium. Not much help here. No wonder the term balance is nebulous in skiing.
The definition of equilibrium is more involved. But it provides clues as to what balance is (or should be). Webster’s defines equilibrium as: a state of balance between opposing forces or effects, the system involved (that’s the skier) undergoing no total change. This provides a better picture of balance because equilibrium in this context means that a skier can initiate a movement pattern and then return to the same body position from where the movement started. This implies the ability of the body’s balance system to maintain control of the movement of the joint system.
From a perspective of physics, Webster’s defines equilibrium as the state of a system in which the net force is zero and which may be either static or dynamic. Skiing is a dynamic activity because the external forces acting on a skier are constantly changing. Here, balance is a process and not a static condition because the body must change in response to changes in external force. Balance in this situation is the equivalent of a stalemate in a tug-of-war. Both sides are pulling at each other. But neither side is moving. In the human system, balance is an internal response to an external force that challenges equilibrium because the body has no direct control over the external force. But in the balance tug-of-war scenario, the balance system must have the reserve power to give up or take territory at will in order maintain equilibrium.
Balance is controlled by the balance system of the CNS. Its job is to maintain an upright position and prevent falls that could cause injury, especially to the brain. It does this by gathering information from a wide variety of sources distributed throughout our body. Vision is important to the process. So are the tensions sensed in the muscles and joints. But in standing upright, some of the most critical information about balance comes from the feet. In skiing, the feet are where everything happens.
Since walking and running are a series of intentional falls and recoveries, the balance system has a complex job. It has to continuously analyze the flood of information it receives and then compare it to movement patterns stored in its memory bank. In fractions of a second it has to decide if movements are putting the body in danger of falling. If so, it must respond with appropriate action.
The problem with standing erect (Figure 1) is that humans are vulnerable to toppling. This makes balance strategies intended to prevent falls extremely complex. The brain tells us the body that we are standing perfectly vertical in a static position. But this is an illusion. The human system incorporates an ingenious strategy to address the problem of standing upright. Instead of trying to maintain a perfectly vertical position, the body is configured with our weight slightly in front of our ankle (Figure 2). Although we don’t sense it, we are in effect leaning slightly forward. In this position, there is a constant tendency for gravity to cause us to fall on our face. The balance system counters this tendency with the muscles in the back of our leg that push down on our forefoot. This action pushes us backwards; just enough to prevent a forward fall. But not so much that we would fall over backward. Staying upright involves a constant cycling of this back and forth movement called sway.
Activities like skiing make the job of the balance system even more complicated because it has to use movement patterns from hard-wired activities like walking and running to assess the risk of falling on skis. The important thing to know about the balance system is that it will always produce the best balance solution it can for a given situation. So if a skiers’ balance is poor on skis then the balance system is probably doing the best it can with what it has to work with. Simply trying harder to have better balance will usually make things worse. The solution is to find out what is causing problems for the balance system and take steps to correct the situation.
In Figure 1 the ankle is shown with cross hairs through a circle. This is the main joint the body rotates about in an upright posture. The ankle is the key joint in balance in skiing since this is the point where the most significant external forces act.
Figure 2 below shows the ankle extended up to the center of mass (black and red circle). The center of mass or COM represents the net position of the weight of your body. It is where you are in relation to the ground.
Figure 3 below shows a simple model of the foot with a strut extending upward from the ankle to the center of mass. This model is similar to the one the human system uses to maintain an upright position. If we were to turn the model upside down it would become apparent that our body is configured like an upside down pendulum hinged at its base at our feet. This arrangement was first suggested by Dr. David Winter at the University of Waterloo in Canada.
An external force W (gravity) pulls the weight of the COM forward and downward, towards the ground. In effect, gravity is trying to topple COM. An internal force M (muscle) pulls against the rotation of the body caused by gravity W. The opposing arrows through the center of the COM show the direction of forces and relative strength. The external force W tries to rotate the body clockwise. It is shown as a negative (-) force. The action of these forces is shown at the ankle. Force W is said to tend to disturb equilibrium. The internal force M is controlled by the balance system. It acts to oppose force W by pulling COM counterclockwise. It is shown as a positive (+) force. Equilibrium exists when M – W = 0. Therefor M =W. This is called the balance equation.
When an external force acts on our body it tends to cause rotation at the joints of the foot, knee and hip. Our balance system senses the direction and strength of the external force and signals muscles to pull in the opposite direction with equal force. This prevents the joints from rotating. If the external force changes in any way, the balance system responds to match the change. The important point is that for balance to exist in this relationship, the balance system must be in control of the relationship of the 2 opposing forces. This requires that the balance system sense the slightest change in the direction and strength of an external force and respond immediately with opposing muscle action. One factor that helps balance is that the pull of an external force usually tends to stretch a muscle as it is contracting. This is called eccentric contraction or what I wrefer to it as elastic tension. The reason it is so important to good balance is that the harder and faster an external force pulls against muscle in eccentric contraction the harder the muscle pulls back. In other words the pull of an external force can actually make muscle stronger and faster. In eccentric contraction muscle can produce up to 8 times as much power as it can in concentric contraction.
The important point in the balance process shown in Figure 3 is that force M represents an internal response to an external force W. If the balance system can successfully oppose an external force it can usually protect the affected part from injury in addition to maintaining balance. Ski equipment, but especially ski boots can disrupt the balance system and cause a serious loss of muscle power. When the balance system controls the balance equation (equilibrium), it has the ability to adjust the position of the COM in relation to the base of support at the feet. It does this by either increasing or decreasing muscle force so as to create movement at a joint. When the intended position of the body has been attained, the balance system adjusts the muscle force to maintain the balance equation. When external forces exceed the opposing internal forces of the skier, equilibrium is lost.