Unleash the Power of Propulsion

From heel strike to push-off, the human foot is the first and last contact point in human locomotion. As the foundation to human movement, adequate foot mobility and strength are essential to achieving an efficient gait pattern and proper muscle activation patterns.

This article will explore the relationship between foot function and the propulsive phase of gait. From limited ankle mobility to decreased hallux range of motion, common foot imbalances can lead to decreased power at propulsion and more proximal compensations in the hips. Learning how to integrate foot-specific programming will help you restore your clients’ optimal foot function and unleash the power of propulsion (Sherman, 1999).

The Foot as a Rigid Lever

If we were to capture the foot as it transitions from heel strike to push-off, we’d observe a supination – pronation – re-supination foot pattern. It is the timing of each triplanar motion that dictates whether our foot will act as a mobile adaptor at midstance or as a rigid lever for propulsion.

By transitioning into a rigid lever, the long bones of our foot are able to bear the stress of our bodyweight and increase the amount of force produced during push-off. The quicker our foot can convert into a rigid lever, the more speed and agility can be achieved during sport and gait.

As the forefoot prepares for push-off, there are two key steps that must occur within the foot in order to achieve optimal power at propulsion:

1. supination of the foot through concentric activation of the posterior tibialis, and
2. activation of the windlass mechanism through dorsiflexion of the hallux.

Activation of the Posterior Tibialis

Originating in our deep posterior compartment and traversing posterior to our medial ankle bone, the posterior tibialis muscle dynamically stabilizes the medial arch and plays a critical role in the coordinated triplanar motion of the foot (McMinn, 1985).

As soon as the foot contacts the ground, the posterior tibialis fires eccentrically to control midstance pronation and absorb ground reaction forces. Within milliseconds of pronation, the posterior tibialis must rapidly switch into a concentric firing pattern and supinate the foot to prepare for push-off (Kulig, 2009).

The timing of this eccentric to concentric activation pattern of the posterior tibialis is often dictated by the client’s foot type and influences our foot’s ability to shift onto the ball of the foot in preparation for push-off. One of the most common foot types associated with delayed posterior tibialis function is the over-pronated foot.

An over-pronated foot type (shown in the image to the right) classically presents with increased heel eversion, abduction of the forefoot, and a decreased medial arch height. Functionally, an over-pronated foot tends to strike the ground in an everted heel position. This increase in calcaneal eversion at heel strike means that the posterior tibialis has to work twice as hard to achieve adequate forefoot supination in time for push-off (Kulig, 2009).

When approaching a client with delayed forefoot supination or a weak posterior tibialis, it is important for the fitness professional to be familiar with the biomechanical causes for over-pronation. Two of the most common causes for over-pronation are underactive hip external rotators and limited ankle mobility.

• Underactive hip external rotators. Due to the interconnection between foot and hip joint kinematics, any client presenting with a foot dysfunction requires a more proximal hip assessment. During the gait cycle, the hip rotates externally as the swing leg prepares for heel strike. In the presence of overactive hip internal rotators and weak hip external rotators, the foot will strike the ground in a more everted position. This everted heel strike position creates an over-pronated imbalance that continues through midstance and into propulsion.
• Limited ankle mobility. Another common imbalance that must be assessed for in a client seeking power at propulsion is ankle mobility. Normal gait requires at least 10 degrees of ankle dorsiflexion with maximum ankle dorsiflexion occurring during late midstance. Limited ankle mobility can lead to a myriad of compensations including midfoot pronation, knee hyperextension and an early heel rise during gait.

Any increase in midfoot pronation or weak hip external rotators makes full supination difficult to achieve. Without adequate forefoot supination, a client will push off with an abducted foot and roll off of the medial aspect of the big toe. This means that full hallux dorsiflexion during push-off is never achieved. We will find out that it is the dorsiflexion of the hallux that activates the second step for optimal foot position during push-off.

Activation of the Windlass Mechanism

Originating from the plantar aspect of the calcaneus, the plantar fascia is more than just a passive band of connective tissue. As the plantar fascia travels distally towards the toes, it separates into five slips of fascia. Each plantar fascia slip inserts plantarly onto each of the five digits. As the foot transitions from midstance into push-off, the toes begin to dorsiflex and the plantar fascia is activated.

This activation of the plantar fascia upon hallux dorsiflexion is referred to as the “windlass mechanism” and is the second step in preparing the foot for propulsion (McMinn, 1985). Since power during propulsion is dependent upon the foot’s ability to become a rigid lever, ensuring proper hallux dorsiflexion during the gait is key to achieving full foot supination.

Foot-Specific Programming for Propulsion

For any client or athlete seeking optimal power during propulsion, foot posture and strength must be considered. Any limitation in joint mobility or proximal hip instability must be assessed before implementing corrective exercise techniques. When approaching the foot and ankle, a solid foundation of mobility is essential before progressing to foot stability and integrating foot-strengthening exercises.

Step 1: Maintain Foot Mobility

Due to the daily stresses that are placed on the foot and ankle, restricted posterior group mobility is common. With each step, the muscles of the lower leg fire eccentrically to decelerate and dissipate ground reaction forces. Studies have shown that eccentric contractions create adhesions and trigger points at a higher rate than concentric or isometric contractions, and that there is a direct correlation between trigger points and joint hypomobility (Fernandez de las Penas, 2009).

When addressing foot and ankle hypomobility, it’s a good idea to integrate myofascial and trigger point release at the beginning of each session. Just five minutes of trigger point release to the gastrocnemius, soleus and peroneals has been associated with an immediate increase in joint mobility and a decrease in foot and ankle compensation patterns (Fernandez de las Penas, 2009).

Step 2: Maintain Foot Stability

The next step in our foot-specific program focuses on achieving foot stability. Foot and ankle stabilization requires a balance in strength between our larger extrinsic muscles and our smaller intrinsic muscles. The imbalance between the two often leads to foot dysfunction and decreased power during propulsion.

Lying deep within the foot and inserting into the proximal aspect of our digits, the primary function of the intrinsic muscles (shown to the right) is to stabilize the digits during propulsion (Sherman, 1999). Any weakness in the intrinsic muscles and our digits causes the foot to become destabilized and contract inward forming hammertoes.

One of the most effective techniques for increasing intrinsic activation is through barefoot balance training. By integrating barefoot balance work, the intrinsic muscles are activated for increased foot stabilization, and the extrinsic muscles are activated for ankle joint stabilization.

When introducing barefoot balance work into a client’s program, start with static exercises and slowly integrating more advance dynamic exercises. The single-leg stance position in barefoot balance exercises (shown in the three images below) also activates and strengthens the gluteus medius and lateral hip musculature, which are essential for optimal foot function.

Standard Single Leg Balance	Variation #1: Leg Abduction	Variation #3: Hip Extension

Step 3: Maintain Foot Strength

After ensuring a strong foundation in foot mobility and stability, you can introduce foot-strengthening exercises for the posterior tibialis into the foot-specific program. Since this muscle fires both eccentrically and concentrically with every step, we must ensure adequate strength in both phases. Any weakness in eccentric posterior tibialis strength will increase the amount of midfoot pronation and delay concentric activation and forefoot supination (Kulig, 2009).

One of the most effective exercises for increasing eccentric posterior tibialis strength is the reverse single-leg calf raise (Kulig, 2009), shown to the right. Begin by standing on a step with one foot (A). Slowly lower the heel down until it is below the level of the step (B). Use the other foot to push up into the start position and repeat. The success of any eccentric strengthening exercise depends on performing an adequate number of repetitions at high-enough intensity with an increased stretch at the end range of motion.

To increase concentric strength of the posterior tibialis muscle, studies have shown the highest muscle activation occurs during a closed chain heel rise with a ball held between the heels (Kulig, 2005). Again, adequate repetitions and intensity are essential to the success of this exercise.

Conclusion

As the foundation to human movement, optimal power during propulsion is dependent upon proper foot posture and muscle activation patterns. With every step, the human foot must convert from a mobile adaptor at midstance to a rigid lever for propulsion. Integrating foot-specific exercises sets the foundation for sufficient foot strength and forefoot re-supination. The quicker the foot can become a rigid lever, the greater the power that is unleashed during propulsion.

References

1. Aquino, A. et al. (2001). Function of the windlass mechanism in excessively pronated feet. Journal of the American Podiatric Medical Association, 91(5): 245-250.
2. Fernandez de las Penas, C. (2009). Interaction between trigger points and joint hypomobiity: a clinical perspective. Journal of Manual & Manipulative Therapy, 17(2): 74-77.
3. Kulig, K. et al. (2005). Effect of foot orthoses on tibialis posterior activation in persons with pes planus. Medicine & Science in Sports & Exercise, 37:24–29.
4. Kulig, K. et al. (2009). Effect of eccentric exercise program for early tibialis posterior tendinopathy. Foot and Ankle International, 30(9): 877-885.
5. Kura, H. et al. (1998). Quantitative analysis of the intrinsic muscle of the foot. The Anatomical Record, 249(1): 143-151.
6. McMinn, R.M.H. et al. (1985). McMinn’s Color Atlas of Foot & Ankle Anatomy. Chicago, IL: Year Book Medical Publishers.
7. Sherman, K.P. (1999). The foot in sport. British Journal of Sports Medicine, 33: 6-13.