Halliday SE, Winter DA, Frank JS, Patla AE, Prince F. The initiation of gait in young, elderly, and Parkinson’s disease subjects. Gait and Posture 1998; 8: 8-14.
This study is not new, but we can apply the findings to our interventions for the elderly and those suffering from Parkinson’s disease. The authors built on the existing knowledge of how two parameters – center of pressure and center of mass – allow for the taking of the first step. They compared the initiation of gait in healthy young subjects to that of elderly subjects and subjects with Parkinson’s disease. The measured variables were the center of pressure (COP) from a force plate under each foot, center of mass (COM) from motions of the body segments, and electromyography recording (EMG) of the muscles. Their results showed the same pattern in the elderly and Parkinson’s patients, but the shift of the COP (and the resulting drive of the COM) was reduced. The COP-COM movement was less in the elderly compared to the young subjects, and less in the Parkinson’s subjects compared to the elderly.
First, we need to discuss the relationship between the COM and COP. COP is where the weight of the body is distributed based on the forces under the feet. COM is the place in the body where the sum of the mass of all the body segments would be. Another way to look at COM is the point around which of mass of all the segments is equally distributed. In quiet stance, it is just anterior to the second sacral vertebra. As the limb segments move, the COM will change dynamically. If the COM is aligned over the COP, the body will remain in place. If we want to move, there needs to be a separation of the COP and the COM. To start walking, we must alter the COP in order to drive the COM in a certain direction that will allow us to move. We learn this as a baby and continue to leverage this subconsciously for moving ourselves in 3-D space.
The sheep dog and the sheep analogy becomes instructive when applying this research. In the analogy, the sheep dog is the COP and the sheep is the COM. When we stand at rest, our COM sways slightly forward and back. If the COM goes too far in either direction, we restore balance by sending the COP beyond it to drive the COM back. When the sheep (COM) moves to the edge of the boundaries, the sheep dog (COP) runs ahead to send it back to keep us in balance.
However, if we want to move, the COP must be displaced to drive the COM in the direction required. The sheep dog moves to the edge to drive the sheep in the opposite direction. In order to lift our foot to take a step, the COM must move over to the stance leg. The COM must be over the stance foot so that when we lift our foot we don’t fall over. The paradox of the biomechanics is that to lift the foot, the pressure under that foot must first be increased. With pressure under the stepping foot, the COM will move towards the stance leg. In this study, the young subjects shifted 68% of their weight onto the stepping leg, while the elderly shifted 64% and the Parkinson’s subjects only reached 58%. Less weight shifted means less driving of the COM towards the stance foot.
Even if the foot can be lifted, for the body to move forward the COM must move forward, which is driven by the COP moving back. Again, the results showed decreased posterior translation of the COP in the two groups compared to the young subjects. This results in a slower and shorter first step because the COM is not accelerated forward. How does the COP move back? As documented in this study, it requires a combination of “shutting off” the calf muscles followed by a slight contraction of the anterior tibialis muscles to move the pressure towards the heels.
With our patients / clients we can use weight shifting side to side to increase the percentage of weight under the stepping foot. Using the pelvis as a driver will accomplish the weight shift. We can also have our patients / clients rock forward and back to get the automatic activation of the anterior tibialis muscles. Returning to the lateral weight shift, the direction of the pelvis drive can be tweaked to a slightly posterior-lateral direction to get the muscular reaction we are looking for combined with the stepping leg load. It would be very futile to tell a client to “turn off” their calf muscles. Likewise, instead of consciously asking the patient / client to lift his / her toes to activate the anterior tibialis, it would be much more functional to drive the pelvis back slightly and have the muscles activate subconsciously. Understanding the kinetics of taking a step “instructs” us how to use task-specific movements to enhance function.