Everybody’s heard the song that says “the foot bones connected to the leg bone”, many do not know how important the biomechanics of the foot and ankle are in regards to injuries of the entire lower extremity, the lower back, and even the shoulders. Having problems with the motion of the ankles and feet is akin to having problems with the foundation of a house because that’s what really causes the roof leak. It’s crazy to think that the human foot and ankle has 28 different bones connected with 55 different joints that need to function together to allow or block the multiple movements required in gait. You need to learn a little bit about the proper functioning of the foot to understand foot joint issues in a later article. Look on my running blog for an expanded article with anatomy explained and bonus material.
At the bottom of the tibia in the ankle is a bone called the talus and another bone underneath called the calcaneus or the heel bone. The calcaneus and talus form an extremely important joint called the subtalar joint. The foot has specific locking mechanisms in both directions that limit the amount of movement, which improves the ability to walk and is also interestingly a uniquely human trait. Another important part of the foot is the big toe that connects to the first metatarsal and the medial cuneiform bone, which is known as the first ray. The heads of the metatarsals are located in the ball of the foot. Proper functioning of the foot allows the heads of the first and fifth metatarsals to drop down to the ground while the middle three metatarsal heads lift off of the floor. These two contact points in the medial and lateral forefoot along with the heel in the back create what’s known as the foot tripod.
The phases of gait are broken up into the swing phase (when the leg is swinging through) and the stance phase, which is broken up into the contact period, mid-stance period, and the propulsive period. Each of these different periods of the stance phase takes up approximately one-third of the time of the stance phase. A cycle of the human gait starts when the foot makes contact with the ground and ends when the same foot makes contact with the ground again. The gait cycle lasts approximately one second when walking. During the first part of stance phase, the foot is dispersing forces associated with the foot striking the ground while at the same time becoming a “mobile adapter” required for dealing with the possibility of different terrains like concrete vs. open field or sandy beach. Later in the stance phase, the foot becomes rigid again which is necessary to transfer the force from the leg to push off through the ground without causing any power leaks.
The contact period starts with foot strike and ends when the front part of the foot is loaded. Walkers and many runners start with the heel contacting the ground. In runners with a midfoot or forefoot strike, this phase is much faster. When your foot hits the ground while walking, you land with about 110% of your body weight but when you are running you crash with 300% of your body weight. This adds up as people take around 15000 times per day and almost 1,000 times per mile on average. The foot needs to have the ability to absorb shock in order to prevent injury. Pronation at the subtalar joint is very important for the absorption of shock at foot strike. Interestingly, muscles in the front and medial portion of the shin need to be strong to help with this absorption of shock. Near the end of the contact period, the outside part of the front of the foot (forefoot) hits the ground and begins to load the outside part of the ball of the foot. Loads progress on the ball of the foot from the lateral to the medial aspect as the loads begin to travel off the end of the first ray into the big toe. The forefoot becomes wider and engages intrinsic muscles in the foot as another mechanism that helps with shock absorption.
The midstance period starts as the front of the foot becomes loaded and it ends when the heel starts to leave the ground. the main thing that occurs during midstance is that the energy that is absorbed during the contact period is stored in the foot to be utilized during the propulsive period. The energy is stored in the muscles of the foot and the ligaments of the arch and has been shown to store a significant amount of energy, which can be returned during the propulsive period. It is comparable to the energy storage to the way a rubber ball stores energy and returns it when it’s bouncing.
When the heel leaves the ground, the propulsive period starts. What’s important to know about this phase for runners is that you push your body weight forward using your entire foot, even your toes. To accomplish this, the foot needs to supinate to lock into a position just after heel lift by properly positioning the joints of the foot. The articulations of the foot form a bony locking of the foot and if this does not occur, the foot can collapse when under stress.
Supination and the elevation of the arch causes the proper downward flexion of the first ray. This first ray needs to shift downwards to allow the upward extension of the big toe.
Another thing that reinforces the stiffening of the foot during propulsion is a phenomenon known as the Windlass effect of the plantar fascia. This occurs because the toes bend upward as the heel rises which causes pulling on the plantar fascia, making the foot a little shorter, which helps to raise the height of the arch and supinate the foot. It is almost like a self-fulfilling prophecy as the increase in supination and lifting of the arch allows the first ray to flex downwards, allowing the big toe to extend upwards more, creating a higher arch and more supination. Go to this article at BestToledoChiropractor.com to see an example of this that you can demonstrate on yourself. If you continue to force it, then you will lose that pressure on the ball of the foot and the arch will increase in height.
As the pressure comes up onto the ball of the foot, a reflexive contraction occurs of the toe flexors which helps with propulsion. If the pressure came across the medial part of the foot, the toe flexors are shut off and propulsion is lost. Also important is that the toes should assist in propulsion because their contraction decreases the risk for injury to the metatarsal heads in the ball of the foot by spreading out the forces over more surface area.
When measuring the amount of impact on the joint, it is found that multiple impact peaks occurred during walking with the impact of the foot being less of an impact than the force generated during propulsion. Runners have an impact peak and then a peak for propulsion, but in midfoot or forefoot strikers, the initial impact peak generally disappears.
For more information on abnormal foot biomechanics, click here
Drawings courtesy of Tom Michaud – Author of Human Locomotion: The Conservative Management of Gait-Related Disorders