BioMechanics
November/December 1996

Custom Orthoses:
Ski Boot Dynamics and Custom Orthoses

by Jeffrey S. Rich, C.Ped and Greg Hoffman, C.Ped

Last spring, Skiing Trade News, in conjunction with Ski Magazine and Master Fit University, sponsored Sports Balance 2000, a research study on stance issues. The Ski Magazine Boot Test Camp, with access to elite athletes and more than 200 different pairs of boots, was our laboratory.

Our study goals included answers to the following questions: Are there particular brands or models that work better for a knock-kneed or bowlegged skier? Are there boots that are better for supinators or pronators? What role does the boot's center seam play in the balance equation? And, what effect does the boot cuff have on balance and performance? (See diagram below.)

Using a group of exceptional everyday skiers, including former World Cup racers and top instructors, we had each subject try various boots several times. We recorded their personal impressions and empirical stance data using commercial canting devices.

Testers initially skied each boot with stock insoles. The second time out we substituted their existing custom, weightbearing footbeds with full-blown orthoses with forefoot control and, if necessary, added toe/heel lifts and underbinding cants. Our goal was to create ideal ski balance: a flat ski with the knee in a slight A-framed position.

The Results

A cuff cant is crucial to proper balance. A cuff cant is the single most important feature on a boot. If the cuff doesn't match the lower leg shape, the ski will not sit flat on the snow. For skiers with excessive tibial varum, it was impossible to create a balanced stance in boots without a cuff cant adjuster. Sanmarco's Symflex is clearly the best canting system, followed by designs with adjustments on both sides of the ankles.

The higher the cuff the more of an impact on stance alignment. In fact, some skiers could not be aligned properly in boots with a tall cuff. High cuff proved to be a major problem for skiers with extreme tibial varum or valgum. A high-rising cuff on a bowlegged skier hits the curve in the lower leg, making it difficult—if not impossible—to achieve a significant cuff cant. The result? The skier catches his or her outside edge.

Transverse plane deformities wreak havoc on the center seam. The center seam on a ski boot is anything but centered. The foot has a theoretical balance line that bisects the center of the heel and runs through the center of the second toe. Most ski boot "center seams" actually are between one and two degrees inside of center. This results in an abducted stance with the foot mass being outside of center. The outside center position prevents pronation and balances the knock-kneed skier somewhat, but makes it much more difficult to achieve an ideal balance for bowlegged skiers. Since all boots have an abducted stance, a skier with an abducted stance will ride his outside edges. Stretching and/or removing liner material changes the center line on the boot and in many cases makes the skier perform better.

Skiers with extreme frontal plane deformities are in trouble. A slight knock-kneed stance is a plus for skiing because it puts the knee over the inside edge of the ski, allowing for quicker turn initiations. An unbalanced knock-kneed stance causes the skier to ride the inside edges. Bowlegged skiers, conversely, will ride the outside edges. When it comes to skiing it's better to be knock-kneed than bowlegged. Measurements of more than 3 degrees of tibia varum are considered excessive. These skiers should always opt for boots with dual cuff cants or a Sanmarco model with Symflex. An extreme bowleg or knock-kneed stance, however, is a detriment and requires cuff canting, foot/orthoses, and possible underbinding cants.

Toe lifts and/or heel lifts can significantly improve balance. Lifts, placed under the toe of the bindings or under the heel, are proving to be a boon to recreational skiers since they help improve fore/aft balance. In some cases, we were able to improve skier balance with toe lifts, eliminating "pot belly" and hunched over stances. Again, the amount of toe lift need varied by model.

Limited ankle dorsiflexion in skiing can stunt ski skill growth. Limited dorsiflexion of the knee, ankle joint, functional and/or forefoot equinus hinders balance and performance greatly. The flexibility of the ankle joint—the ability to drive the knee forward over the toes—helps determine the appropriate boot stiffness and the amount of heel lift necessary to balance the fore/aft stance.

Minimal ankle dorsiflexion in some ski boots is 8-10 degrees; and that is just for stand, no flexing (see Ideal ski stance). Most boots have between 12-16 degrees of total forward lean. Total forward lean angle is the sum of the ramp angle (6-8 degrees) minus the forward lean of rear cuff (9-14 degrees).

A skier with less than 12 degrees can get immediate performance improvement with a heel lift. For a foot with a low or normal instep, heel lifts center the body mass over the foot while relieving tension on tight calves. A skier with a high instep may not have enough room in the boots for heel lifts. In that case, the bindings may be moved to position the boot a half to full inch forward of the standard mounting point on the ski.

Leg length discrepancies are common and must be corrected for proper stance. We were surprised to find that nearly half of our subjects had one short leg. The largest length discrepancy was 1/2 inch. We found that without correctly lifting the short leg, ideal stance alignment was not possible. As little as a 1/8-inch discrepancy can affect balance. For differences greater than 1/2-inch, underbinding lifters should be installed.

Orthoses make a big difference. Boot performance often went from mediocre to excellent with the addition of orthoses. As underfoot support was increased, from stock insole to custom footbed to custom orthosis, so did overall comfort, control, and performance. Orthoses greatly enhanced skiing posture, sometimes eliminating the need for canting.

Since the heel bone is the "control center" for proper foot balance, problems here greatly affect stance. The calcaneal stance position helps determine the degree of pronation and supination.

The preferred calcaneal stance for skiing is 2 to 4 degrees of valgus. Measurements outside this range indicated a need for orthoses. The orthoses should maintain a calcaneal stance just shy of perpendicular to the ground. The calcaneal stance less than 2 degrees of valgus may result in catching an outside edge while more than 5 degrees of valgus may make it difficult for the skier to get off his inside edge.

All forefoot imbalance should be corrected to achieve optimal stance alignment. Without posting the forefoot it was nearly impossible to keep the knee tracking straight. With the addition of a forefoot post (when needed) the dynamics of the turn were constant and rhythmic.

Like being slightly knock-kneed, we also discovered that a slight forefoot valgus is a plus for skiing. With the orthotic forefoot posted, the valgus stance helps position the knee medially, increasing leverage power to the inside edge of the ski. (Note: Before installing an orthosis into a ski boot check that there is no varus or arch support built into the plastic shellbed. If there is, grind it flat.)

Interestingly, our test subjects—all tremendously accomplished skiers—felt significant improvement after undergoing their intensive balance sessions. Our coaching observers also noted marked improvements in every case. We also did some placebo testing, setting some of our skiers off balance to see if they'd notice. They all came in whining like first time ski kids.

Remember, the biomechanical needs of skiers are dramatically different from those of gait sports athletes and go far beyond just constructing an orthosis. The leg, orthosis, foot, and ski boot must function together as a unit. Rigid or over-corrected orthoses often are not tolerated. The skier's optimal stance is slightly knock-kneed with at least 14 degrees of dorsiflexion. A good specialty ski shop should understand the performance tuning protocol, the skier's needs, and the biomechanics of the ski boot. So should any practitioner serving a skiing patient.

Before attempting to install a ski orthosis ask yourself these questions. Do you know how to adjust the cuff cant? Forward lean? The position to mount the bindings? Which center seam is best? How much heel or toe lift is needed? Do you even know which boot fits the foot the best? If you can't answer any of these questions, seek some boot fitting knowledge or align yourself with a C.Ped. or ski shop specializing in ski boot dynamics. Placing an orthotic device in a ski boot without checking the boot's alignment may offer little benefit and can move the skier even further out of alignment or even cause injury.

Jeffrey S. Rich, C.Ped., is Technical Editor for Ski Magazine and Education Director of Master Fit University, a training program for ski boot technicians. He owns Custom Sports Lab, an orthotic and sports balancing facility in New York City. Greg Hoffman, C.Ped., Assistant Education Director of Master Fit, is also the owner of Green Mountain Orthotic Lab in Vermont.