HEEL PRESSURE VS. HEEL/FIRST METATARSAL PRESSURE DIFFERENTIAL


The Balance Axis

The two University of Ottawa pressure studies that used elite ski instructors found that, without exception, every ski instructor started their turns with pressure under the head of the first metatarsal (ball of the foot) and that in some, but not all turns, the pressure moved back to the heel in the last part of the turn. What the studies did not make clear is that when the Center of Pressure (COP) is under either the heel or the head of the first metatarsal, there will always be a secondary center of pressure under some other aspect of the foot. When COP is under the heel, there will be a diffused, but weak, area of pressure in the forefoot across the heads of metatarsals 1-2-3. But when COP is under the head of the first metatarsal, there will always be a well-defined, localized secondary area of pressure under the heel. The relationship of the two COPs (heel or head of the first metatarsal), affects the relationship of the balance axis of the foot and the balance axis of the ski.

The ski has two balance axes. These are the theoretical points where a ski would balance on the head of a pin if the mass of the ski were perfectly distributed. The graphic below shows the two balance axes of a ski. A section below the top view of the ski shows the minimum profile width or waist of the ski in black. The limits of the sidecut are shown in grey. A triangle indicates the Balance Axis.

Balance Axis

(click on graphics to enlarge them)

Heel COP

When COP is under the heel, the load or pressure, will lie on the transverse Balance Axis of the ski where it will be distributed outward from its center as shown in the graphic below. Because of the weak, diffused secondary area of pressure across metatarsals 1-2-3, pressure under the heel will extend more forward, towards the shovel, than rearward, towards the tail. The load on the heel lies on the anatomical center axis of the foot that runs through center of the heel and the head of the second metatarsal. As with the Balance Axis of the ski, the anatomical center axis of the foot is the theoretical transverse balance point in bipedal stance. This is the reason why skate blades are mounted on this axis. LeMaster and others have used the skate example to explain why a ski is hard to hold on edge compared to an ice skate. But the balanced load across the transverse aspect of the ski will keep the ski level on flat terrain and resist forces that attempt to tip it onto one of its edges. Heel COP

What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge. This unbalanced load causes the limits of the ski at the shovel and tail to displace away from the center of the load (COP) on the portion of the inside edge under the head of the first metatarsal. The diagonal load axis across the Balance Axis of the ski sets up components of force that are perpendicular and parallel to the Balance Axis of the ski.

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The re-alignment of the balance axis of the foot that occurs in monopedal stance can be clearly seen in images below of speed skaters and Mikaela Shiffrin balancing on a slack line.

Speed skaters

 

Shiffrin Slack line 1

The re-alignment of the anatomical balance axis of the foot is the end result of the transition to the inside ski while it is still on its current edge and the creation of pressure from extending the leg. This mechanism enables racers such as Shiffrin to Get Over it at ski flat and initiate edge and turning effects simply by balancing on the new outside ski and managing the heel/first metatarsal pressure differential.

The actual mechanics are more complicated than what I have described in this post. But as the saying goes, “One step at a time.”

8 comments

  1. “What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge.”

    You use the term balance axis of the foot and you say it becomes re-aligned. When you say it becomes re-aligned, that implies that it had a different alignment at a different point in time. Could you define balance axis of the foot? Could you explain how it changes its alignment?

    1. A meaningful discussion on issues pertaining to balance and COP and COM require that they be framed in the standard anatomical perspective of position, planes and axes and in the context of balance as CNS mediated postural responses as opposed to consciously mediated forces such as extending the ankle to apply a force to an object. A good reference for anatomical position, planes and axes is Muscles Testing and Function, Third Edition by Kendall and McCreary. Skiing and skating involve an application of balance axis that is not part of the typical gait/balance study terminology. In an erect, bipedal posture as defined in the chapter, Joint Motions in Muscles Testing and Function, the M-L anatomical centre line runs through the proximate center of the heel and head of the second metatarsal. If a skate were mounted on this axis the moments of force across the blade would balance on each foot in bipedal stance. But in monopedal stance, the balance axis shifts postero-medially as COP advances towards the head of the first metatarsal.

      This can be tested by standing very erect in a bipedal stance with most of the weight on the heels. If one tries to lift one foot off the ground while maintaining high pressure under the heel, it will be extremely difficult. If one tries to raise the forefoot off the floor and balance on the heel only, if will be very difficult. But most people can easily stand in balance on one foot. If a person tries to balance on one foot on a slack line with the anatomical center of the foot aligned on the transverse center of the slack line they will find it very difficult because when the foot is pronated, the balance axis runs through the heel and center of the head of the first metatarsal. Even in erect bipedal stance, the first metatarsal always bears 50% more weight than the other metatarsals. There is a good reason why it is so robust compared to the other metatarsals.

      In summary, in physical situations pertaining to the balance axis as a reference for skis and skates that fall outside of the normal terminology of anatomy appropriated terms must be created.

      1. I’m still trying to understand your definition of balance axis of the foot. From your diagrams it appears that balance axis of the foot is a line (or vector) that connects the center of pressure of one half of the foot to the location of the center of pressure in the other half of the foot. Is this correct? Why did you choose to divide the foot into anterior-posterior halves instead of medial lateral halves relative to the anatomical axis of the foot?

      2. As I explained in your last comment, the concept of a balance axis has no relevance in normal gait and balance studies because the multi-axial joint system of the ankle-complex ensures loading of the tripod-like base of the arches of the foot. In fact, this is precisely why the arches are configured as a tripod. Attaching an interface to the sole of the foot with a skate blade or ski that will be placed on one of its edges does not negate the action of the multi-axial joint system. In application to a skate blade, the it is necessary to arrive at a location on the transverse axis that approximates the balance axis. This is where the medial-lateral of weight distribution on the foot is equal in 50/50 bipedal stance. It is not the anterior-posterior balance point because there is no fixed balance axis. Both these issues are misrepresented in discussions of ski technique. The bipedal balance axis only exists in 50/50 weight distribution. In skis, it only exists when a skier is standing still with both feet flat on the snow, moving down the fall line with weight equal on both skis or standing at the bar.

      3. The problem with balance axis of the foot as you have described it is that if you connect the dots between the center of pressure of the forefoot and the center of pressure of the rearfoot is that you don’t take into account the relative loading of the forefoot and the rearfoot. When you do take that into account you will get a single point along that axis and that point is the center of pressure for the whole foot. The point center of pressure of the entire foot can be used anywhere you have used the concept of balance axis of the foot.

        Some articles illustrating that concept.

        Motor mechanisms of balance during quiet standing.
        Winter DA, Patla AE, Ishac M, Gage WH.
        J Electromyogr Kinesiol. 2003 Feb;13(1):49-56.

        Stiffness control of balance in quiet standing.
        Winter DA, Patla AE, Prince F, Ishac M, Gielo-Perczak K.
        J Neurophysiol. 1998 Sep;80(3):1211-21.

        Unified theory regarding A/P and M/L balance in quiet stance.
        Winter DA, Prince F, Frank JS, Powell C, Zabjek KF.
        J Neurophysiol. 1996 Jun;75(6):2334-43.

      4. Do you even read my posts? It doesn’t seem so. Even if you did, the studies you cite on quiet standing have no relevance to skiing unless one is standing at the bar having a few cold ones.

  2. I think you are making this more complicated than you need to.

    “But when COP is under the head of the first metatarsal, there will always be a well-defined, localized secondary area of pressure under the heel.”

    From wikipidea the definition of center of pressure: The center of pressure is the point where the total sum of a pressure field acts on a body, causing a force to act through that point.

    You can have localized areas of pressure on the first metatarsal head and the heel. However, those two localized areas of pressure are averaged together to calculate the point of center of pressure for the entire foot. So, if you want to look at what the foot is tring to do to the ski, all you have to do is look at the center of pressure of the entire foot.

    ” What has not been recognised in skiing, is that the transverse balance axis of foot becomes re-aligned in monopedal stance so that it runs through the center of the heel and the center of the head of the first metatarsal as shown in the graphic below. By placing the highest load on the ski under the head of the first metatarsal, the re-alignment of the anatomic balance axis that occurs in monopedal stance, unbalances the load on the outside ski causing it to tip on edge. This unbalanced load causes the limits of the ski at the shovel and tail to displace away from the center of the load (COP) on the portion of the inside edge under the head of the first metatarsal. The diagonal load axis across the Balance Axis of the ski sets up components of force that are perpendicular and parallel to the Balance Axis of the ski.”

    The reason that the transverse balance of the axis of the foot hasn’t been recognized in skiing is that it is an over complication of a simple consept. To make a turn, you want to get pressure on the inside edge of the ski and to do that you activate your foot muscles to increase pressure under the first metatarsal head to shift the center of pressure, of the foot versus ski force, more medially.

    1. The benefit of publishing your comments is that it serves as a graphic example of the abysmal lack of knowledge within an influential camp in skiing. Aside from the fact that you appear to have no understanding of the issues, you seem to have some sort of reading comprehension issue. As an example, I have clarified the issue of COP on a number of occasions because the reference to heel COP and 1st MT COP in the Ottawa papers inferred that there are no associated secondary areas of pressure. I also explained that for COP to be under the head of the 1st MT in the biomechanics of stance, COP must first be present under the heel and then advance forward (anteriorly) to the head of the 1st MT. In the progression, a point is reached where COP shifts to the head of the 1st MT while a localized area of pressure remains under the heel.

      The preceding explanation is mostly for the benefit of other viewers.

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