Absorption vs Propulsion

Human movement can be broken down into two distinct phases: absorption and propulsion.


During gait we simply alternate between these absorption and propulsion phases in a reciprocal fashion. Absorption can be thought of as a global “flexion-rotation” pattern while propulsion is an “extension-rotation” pattern. In the top right picture, I am demonstrating absorption to the right, or flexion-rotation right. This pattern is followed by propulsion up and across the midline to the left, or extension-rotation left, as demonstrated in the picture to the bottom left. This would be followed by flexion-rotation left and extension-rotation right before returning back to flexion-rotation right.

When this chain is broken, due to a lack of mobility at a specific segment or the inability to coordinate each segment to create dynamic movement, it negatively impacts the next phase of the gait cycle. Dysfunctional movement is the result. This is where compensations, aches, and pains often manifest themselves. For this reason, it is crucial as a clinician to be able to identify dysfunctional movement at both a global and segmental level.

Absorption from the Bottom Up

Upon heel strike, the body begins absorbing force through the foot and ankle. Although we have previously viewed the ankle as simply dorsiflexing at heel strike, it is a bit more complicated than that. The ankle is a tri-planar joint, meaning that it works in the sagittal, frontal, and transverse planes. When we walk, the posterior-lateral portion of the calcaneus strikes the ground, causing the calcaneus to evert (frontal plane) and the talocrural joint to dorsiflex (sagittal plane). As a result, the mid-tarsal joint unlocks, and the forefoot abducts (transverse plane). This tri-planar motion at the ankle allows for pronation. A pronated foot acts as a shock absorber, conforming to the ground to better dissipate the ground reaction forces present during ground contact.

The tri-planar motion at the ankle has a ripple effect up the kinetic chain. As the foot pronates, the tibia internally rotates. Because the tibia and femur run in concert with one another, as the tibia internally rotates, so too does the femur. This femoral internal rotation cues the glutes to turn on and eccentrically lengthen, allowing for absorption to occur.

Propulsion from the Top Down

While absorption occurs from the bottom up, propulsion occurs from the top down. The same process that was described during absorption is now reversed. As the swing leg advances forward, it causes the pelvis to rotate towards the stance leg. The momentum caused by this pelvic rotation causes the femur to externally rotate, which leads to a domino effect down the kinetic chain. Femoral external rotation precedes tibial external rotation, which leads to the re-supination of the foot as the calcaneus inverts, the forefoot adducts, and we begin to plantar flex at the talocrural joint. Supination of the foot locks the mid-tarsal joint, providing us with a rigid lever to push off and produce force during the propulsion phase.

Some schools of thought teach us that the glutes are what initiate the propulsion phase of gait, but electromyography proves this not to be the case (Perry and Burnfield, 2010). The glutes are active from heel strike to midstance but are quiet after that. This means that the glutes work to help us eccentrically absorb force but are not involved during the propulsion phase. Rather, momentum and fascial efficiency contribute most to our gait economy (I should note that this is true when momentum is already present during gait; electromyography findings would differ in the first few steps of gait).

Breakdowns in the Kinetic Chain

So, what happens when we lose motion at a specific joint or our timing is off during the execution of these movements? We discussed earlier the significance of tibial internal rotation during the absorption phase. Tibial internal rotation leads to femoral internal rotation, which cues the glutes to turn on to help us to safely absorb force. But what if tibial internal rotation is limited? A lack of tibial internal rotation means the femur won’t internally rotate adequately, leading to decreased glute activity. When the glutes become less involved in absorption, another structure is forced to step up and pick up the slack. Sometimes this means the quadriceps muscles work harder, other times the plantarflexors are forced to come to the rescue. In either case, we are asking a muscle group to perform at a level it was not built to perform at. In this specific case, it wouldn’t be uncommon for a patellar or Achilles tendinopathy to be the end result of limited tibial internal rotation.

As clinicians we must be able to dissect human movement at both a segmental and a global level. This means we need to be able to assess movement at a specific joint and understand how dysfunctions at a specific joint contribute to the bigger picture. The bigger picture might be walking or running, but it could also be bending over to pick something up off the floor or squatting to sit on the toilet. Regardless of what our clients’ goals may be, a basic understanding of how humans move can help us to better understand why aches and pains have begun to present themselves so we are better able to correct the dysfunction and empower our clients to get back to what they enjoy doing.


References:


1. Earls, J. (2020). The Mechanical Chain. In Born to walk: Myofascial efficiency and the body in movement / James Earls (2nd ed., pp. 65-72). Nutbourne: Lotus Publishing.

2. Earls, J. (2020). Sagittal Plane. In Born to walk: Myofascial efficiency and the body in movement / James Earls (2nd ed., pp. 81-82). Nutbourne: Lotus Publishing.

3. Perry, J., and J.M. Burnfield. 2010. Gait Analysis. Thorofare, NJ: Slack.