A long time ago, I stumbled onto this idea: by adding a line of force (vector) at an angle to the body, I could increase the amount of work during exercise with only a mild increase in compression loads. This idea is derived from knowing that ground reaction force is the sum of three force vectors: vertical, forward-back shear, and medial-lateral shear.
For example, if you perform a lunge, the force of the foot hitting the ground is the vertical vector and is by far the largest of the three. The little bit of wobble forward or back, side to side are the other force vectors. For a time in my practice, I used Kistler Force Plates to test force production for things like a single leg squat or lunge. I could tell if the client was using compensatory movements on one limb versus the other by looking at the forces in the second and third vectors. By using force plates, I proved that by introducing force into the medial-lateral plane, vertical force increased.
I had the client step onto the force plate (the force plate connected to a computer.). I attached an elastic band to the waist and pulled the band taut. If the test was for the right limb, I pulled to the left. Then, I asked the client to perform five, single leg squats through 70 degrees of knee motion. I used an adjustable lab chair to fix the range of motion so that when the client performed the squat, he/she would stop the motion once the buttock hit the chair.
Following this test, I repeated the test without the lateral force. In every case, vertical force was greater during application of the lateral force.
Here's why.
In the case where the force is opposite (pulling to the left while squatting on the right leg), the client must counter that force in order to remain balanced and does this by increasing force in not just the medial-lateral vector but in the other vectors as well.
But, what if the vector is to the same side instead of the opposite? Pulling to the right instead of the left?
A group of scientists studied the effect of lateral force while walking on a treadmill. Their hypothesis was that actively controlling the small amount of lateral sway during gait actually had an associated energy cost and that if you applied equal lateral force, bilaterally, gait parameters would change and energy cost would go down. That's exactly what they found. By applying springs at the waist that pulled right and left at the same time, they reduced the instability of walking and the result was a 10% decrease in energy cost.* They made walking easier.
So, back to my own "lab". Something that many clients with joint disease (knee, hip, back primarily) struggled with was increasing muscle strength. The conventional methods of strengthening, adding resistance, often exceeded the joint force tolerance. The result is joint pain, stiffness, or aching. But, I found that by adding a vector, clients could perform drills such as squats with much less pain and achieve greater fatigue. Since I could control the vector force (I used a pulley but you could use elastic tubing or springs), I could grade the drills in small enough increments and avoid overloading them.
In the image to the right, the vector is lateral to the left while Christine performs a backslider (slides the other foot back) on the right. This version is slightly easier (more stable) than if the vector was directed to the right (remember the treadmill study).
In either case, the vector creates a rotational force that Christine must counter with the hip and trunk muscles by keeping her hands in front of her. This is a spine-friendly drill that will increase strength and endurance of the core (and if you have shoulder weakness, you'll notice it!).
Some of the more common applications of vector loading:
- A lateral force to the trunk while performing a sit to stand drill.
- A lateral force via the hands while performing a lunge or reverse lunge.
- A lateral force to the legs while lifting something off the ground.
- A lateral force against the hand while reaching up and forward (as in a punch).
Donelan, J. M., Shipman, D. W., Kram, R., and Kuo, A. D. (2004) Mechanical and metabolic requirements for active lateral stabilization in human walking. Journal of Biomechanics, 37: 827-835.
