Choosing the Right Prosthetic Foot for Cycling, Running, and Alpine Skiing
October 2014 Issue
Refreshment, relaxation, stimulation, excitement, the thrill of competition- sports and recreation offer all this and more. For persons with amputations, the right prosthetic componentry is an integral part of their athletic and recreational experiences. The prosthetic foot plays an important role in performance, and is especially vital in competitive cycling, running, and alpine skiing.
"If you want to bike recreationally, then you can use the prosthesis you use for walking," says Lee Childers, PhD, MSPO, CP, assistant professor, prosthetics and orthotics, Biomechanics and Motor Control Laboratory, Alabama State University, Montgomery. "If you want to be competitive, then you want the stiffest connection possible." Childers explains, "When you push down on the pedal, the power stroke, you want all that energy you're generating to go through the prosthesis and into the pedal. But when your walking prosthesis uses a flexible dynamic energy-return foot, some of that energy is wasted because you're compressing the foot. In walking, you can recall and reuse some of that energy, but in cycling, the foot will recoil off the back of the pedal stroke so you're losing more energy than you need to.... That's why running feet are terrible for cycling and walking feet are not the best choice."
A research article coauthored by Childers notes that the energy lost from the crank cycle by the flexible foot requires the sound limb to exert more and increases pedaling asymmetry. However, the study found that increases in asymmetry are only apparent at higher, more competitive intensities. Thus, for rehabilitation or recreation, the stiffness of the prosthetic foot has little effect (Childers, W. L., R. S. Kistenberg, and R. J. Gregor. 2009. "The Biomechanics of Cycling with a Transtibial Amputation: Recommendations for Prosthetic Design and Direction for Future Research." Prosthetics and Orthotics International, 33:3, 256-71). For competitive cycling, an airfoil-shaped shank in an exoskeletal-type prosthetic design made of carbon fiber provides a stiff connection to the pedal, reduces weight, and makes the prosthesis more aerodynamic, Childers points out. Additionally, some cycling prostheses are custom made to the individual athlete, such as British track cycling star Jody Cundy's lightweight carbon fiber prosthesis, fashioned without a foot, that attaches directly to the crank to assure that all the pedaling power goes directly to the bike.
Solutions for Cycling and Walking
However, when the cyclist needs to dismount and walk, such as in recreational cycling, triathlons, and mountain biking, a regular walking foot is desirable. Childers suggests using a foot with a foot shell that has a really deep arch. "Remove the foot shell, and place the cleat that clips to the foot on the bicycle in the midfoot position." The smaller lever arm between the cleat and the prosthetic ankle joint, in combination with the lower forces during cycling, effectively stiffens the foot, Childers explains. "You can glue a shoe sole to the forefoot and heel sections so when the cyclist is walking off the bicycle, the foot behaves normally."
Another concern is maintaining the prosthetic foot in contact with the pedal throughout the pedal cycle. In the aforementioned study, the authors note that there are several methods that balance security with the need to remove the foot quickly in case of emergency.
For people who may not want a dedicated cycling leg, the easiest, but least secure, method is to use a studded BMX pedal with a soft-sole shoe. Other methods include attaching Velcro® between the pedal and bike shoe, using a neoprene strap to secure the shoe to the pedal, or using a commercially available clipless pedal system.
Although clipless pedal systems have been used successfully by cyclists with transtibial amputations, some are better than others, the authors point out. "[O]ne can get clipless pedals for mountain biking...and use it with a mountain bike-specific cycling shoe," Childers says. Mountain bike pedals are dual-sided so the cyclist can clip into either side of the pedal, and mountain bike shoes have lug soles with the cleats recessed below the lugs for ease of walking while off the bicycle. Conversely, most road pedal systems only allow access on one side. The pedal system should require 20 degrees or less of axial rotation (known as float) to ease cleat disengagement, the authors add.
Since the cylindrical shape of the residual limb can limit axial rotation needed to disengage the pedal, the study authors recommend a technique for using a clipless pedal system: The cyclist straddles the bike and clips in the amputated limb side first, pushes off on the pedal with that limb, and then clips in the sound limb once the bike is moving. Prior to dismounting, the cyclist unclips the sound limb first, stops the bike, places the sound limb on the ground, moves his or her body toward the front of the bike, fully extends the amputated limb, and uses internal and external hip rotation to unclip. "This technique has also been successful for people cycling with AFOs or limited ankle motion," the authors say.
Childers also suggests buying a spacer to move the pedal on the amputated side out far enough to provide clearance or shortening the crank. "For walking, the prosthetic foot is toed out. Cyclists with intact limbs can toe in to keep the heel from hitting the crank as it spins, but someone with an amputation is unable to do that."
The case of whether Oscar Pistorius' high-tech prostheses would give him an advantage against able-bodied Olympic competitors raised the question as to whether this could be the case in other sports. Another research study coauthored by Childers examined the effect of an individual with a unilateral transtibial amputation (TTA) on a 4km pursuit-a cycling time trial performed on a velodrome track (Childers, W. L., T. P. Gallagher, J. C. Duncan, and D. K. Taylor. 2014. "Modeling the Effect of a Prosthetic Limb on 4km Pursuit Performance," International Journal of Sports Physiology and Performance). Using a forward integration model of pursuit performance, the study explores the interplay between power loss and aerodynamic gains in parathletes with unilateral TTAs.
"Practical application of these results suggest parathletes with a TTA could improve performance by minimizing pedaling asymmetry and/or optimizing aerodynamic design but, at best, they will have similar performance to intact cyclists.... [P]arathletes with TTA do not have a net advantage in the individual pursuit."
However, the results underscore the importance of having a prosthesis designed especially for cycling, the authors point out. Pursuit times were affected by prosthetic foot stiffness, and lower pedaling asymmetry corresponded with lower pursuit times. "Load sharing (asymmetry) between sound and amputated limbs, due to compensation of other joints to account for the altered musculoskeletal geometry, is affected by the mechanics at the limb/socket interface via suspension, prosthetic alignment, prosthetic design, and positioning of the person on the bicycle." The authors add, "Pursuit performance may be enhanced by optimization of the human/prosthesis/bicycle system through prosthesis design and body positioning; i.e., bike fit, to minimize pedaling asymmetry."
Runners often experience a runner's high-the euphoria following an intense run resulting from a beta-endorphin release driven by nervous system neurons. Running also benefits cardiopulmonary function, helps maintain weight and fitness, and improves sleeping, eating habits, and relaxation, according to various health experts.
Jon Disbro, a prosthetic resident with Prosthetic Consulting Technologies, Washoe Valley, Nevada, underwent a right TTA from injuries he sustained while serving in Iraq. The retired Marine was determined to run again and began competing in running events, cycling, and triathlon, reaching the national level in track and paratriathlon. "I tried different kinds of running legs," Disbro says. "I prefer Össur's Flex-Foot Cheetah® leg over Össur's Flex-Run™ with Nike Sole, even though [the Cheetah is] designed for sprinting, not for distance running." To test both feet, Disbro paced himself on a three-mile course multiple times, keeping his heart rate at a certain level. "The Cheetah was actually about 30 seconds faster on the three-mile course," Disbro says. He attributes the speed increase to the foot's higher level of energy return. Össur has recently noted on its website that the Flex-Foot Cheetah is recommended for longer distance running for users with TTAs who do not have clearance for the Flex-Run with Nike Sole. For sprinting, the company recommends Cheetah Xtreme and Cheetah Xtend®.
Richard "Rick" Riley, CP, FAAOP, owner of Prosthetic Consulting Technologies, points out that feet are about four to six inches from each other during walking; however during running, there is no distance between the foot and midline of the body (the runner runs foot over foot so there is no horizontal spread between feet). "So your pelvis is rotating far more when you're running," he says. "In walking, the toe-out of your foot is set at a certain angle, usually five to seven degrees."
Although that prosthetic foot angle works well when walking, running with the same alignment causes feet to be toed in, increasing the danger of tripping over the toes, Riley explains. The force of the prosthetic foot spring is designed to push with a forward thrust, but changing that direction creates an angular forward thrust. "So when designing a running prosthesis, you turn the toe out so that when [the user is] running, the toe is straight. It's all because of pelvic rotation."
"With running feet, it is very important to fit the patient with a running foot that is the proper category for maximum energy return," says Chad Simpson, BOCP/LP, practice manager, Hanger Clinic, Tuttle, Oklahoma. "A foot that is too stiff can be so rigid that it does not load and release properly; conversely, a foot that is too soft can absorb too much energy and feel very soft and mushy to the patient."
He adds, "Alignment is very important. The more plantarflexed the running foot, the less surface of the foot will be in contact with the track. However, if the foot is overly dorsiflexed, too much of the running foot will be in contact with the track and the foot will not be able to load and release effectively." Simpson recommends using high-speed video analysis to capture the patient's running gait and evaluate the alignment and the amount of deflection the running foot provides. "Typically when you order a running foot, you choose one from the patient's weight category from the manufacturer's specifications. However, it may be necessary to go up or down a category in order to find the most efficient running setup," he says.
Often individuals with amputations have only one prosthesis that must serve multiple purposes. Riley recommends using the Ferrier Coupler for quickly disconnecting and changing feet, which users can do independently. "In about six to 12 seconds you can switch to a different foot with a different alignment and keep on going." However, Disbro points out, in competitive paratriathlons, individuals with amputations are not allowed to swim with a prosthesis, so the coupler would be limited to the bicycling and running portions only. The added weight of a coupling device, although slight, would be another disadvantage; most paratriathletes use specialized prostheses for cycling and running, Disbro notes. "But for recreation it would be fine. Another example...is a client who operates a tractor. His prosthetic foot is too large to operate the pedals of the tractor, so he switches to a peg leg to drive the equipment."
"When I first started doing ski prostheses, it was more about the alignment," recalls William D. "Bill" Beiswenger, CPO, FAAOP, cofounder and owner of Abilities Unlimited, Colorado Springs, Colorado, and prosthetist for the U.S. Paralympic team. "Carbon fiber feet weren't available. We would stuff the foot into the ski boot, which is difficult with a stiff foot." Sometimes skiers would put a plastic bag over the foot to make it easier to push into the boot and to keep it dry, he adds. "So to me alignment and the proper knee flexion were most important, then whatever was necessary to transfer force from the knee [TTA] through the socket through the boot to the ski to make the turn." At first, exoskeletal prostheses were used because of their strength, durability, and light weight, Beiswenger says, although, of course, no alignment adjustments were possible once the prosthesis was finished.
Competitive stand-up skiers with unilateral TTAs or transfemoral amputations generally compete without a prosthesis, using outriggers for stability (three-track). However, recreational skiers with an amputation often like to spend a day on the mountain and want to be able to walk with a prosthesis to get lunch or be with their group rather than having to go back down the mountain, Beiswenger notes. "Even though they can use the outriggers like crutches, [skiers] like the stability and safety of using a prosthesis, especially in very slippery conditions." Microprocessor-controlled knees with different activity modes that can be changed by the user are another option, Beiswenger says.
When dynamic feet became available, "We started using some endoskeletal components around the ankle, which gave more alignment capabilities," Beiswenger says. "Skiers could be taught how to make their own adjustments. We still used exoskeletal prostheses because the boot had to be tight so the movements would transfer through the knee, socket, boot, to the ski."
However, the weight of the prosthesis was still an issue. "Sometimes people would cut the bottom out of a ski boot and fasten it to the bottom of their prosthetic foot." When the Flex- Foot® became available, some skiers simply removed the foot shell and bolted the bottom of the foot to the ski, reducing the weight. "Everything was endoskeletal. The skier could make alignment changes quickly throughout the day." He adds, "Now there are prosthetic feet designed to clip right into the binding without a boot."
Beiswenger stresses the importance of good suspension: "We don't want to see a ski going down the mountain with a prosthesis and no person." Suction suspension works well, as does suction suspension with a pin system, he notes. "Not many people need auxiliary suspension, so often we use a suction socket with a seal sleeve. Even with a pin-system suspension, we'll put a sleeve over that for better suspension."
For recreational skiers with transtibial amputations, Beiswenger recommends a knee orthosis for mediolateral stability to protect the knee. "We don't want them to crash and ruin that knee or their amputation and perhaps have to have reconstruction surgery." To fabricate the orthosis, a cast impression is made over the socket. The bottom of the knee brace will then clip onto the socket and fit over the socket onto the thigh.
In the past, a knee brace also helped solve another problem by helping suspension when riding the ski lift. Skiers would often complain about the weight of the ski and boot or the ski and foot pulling on their leg, causing discomfort. However, since most resort ski lifts now have footrests, that problem has been largely eliminated, Beiswenger notes.
Since many people can't afford a specialized ski prosthesis, heel wedges can be placed under the prosthetic foot to tip the prosthesis into a more flexed position in the ski boot. The knee can be put in the correct flexed position for skiing. Some of Beiswenger's patients ski with uncovered endoskeletal prostheses. "I teach them how to use a wedge to make some of those changes, and they can lock the knee in the right position for safety. Prosthetists have to be cautious because if something goes wrong, they could be liable. But since patients are going to do this anyway, we might as well teach them how to do it safely."
Prosthetic feet options for K3 and K4 users, especially for sports and recreational activities, abound. For instance, Freedom Innovations' Slalom ski foot clips directly into the ski binding-no need for a boot. Ottobock now offers the ProCarve sports prosthesis, either independently, as a foot that clips directly into the ski binding, or including the ProCarve knee joint for individuals with transfemoral amputations. For running and sprinting, in addition to the Össur products mentioned previously, Ottobock offers the IE90 Sprinter and the custom IC2 S-Sprint®, and Freedom Innovations provides the Catapult running and sprinting foot and the Nitro running foot.
Many higher-activity prosthetic sport and recreational feet accommodate a range of activities and impact levels, including College Park's Soleus® and the Endolite bladeXT foot and Fillauer's Wave Sport from Emotis. New kids on the block include Ability Dynamics' Rush™ foot and BioDapt's Versa foot.
Although specialized sports prostheses are desirable, "There are many prosthetic users that cannot afford a specialized knee or foot, and because of the new designs available, many things are possible with using standard components," Simpson says. "It really all boils down to individuals and their desire, commitment, and determination to achieve their goals. As clinicians, we must step up to the plate and either seek a solution to help patients meet their goals or we must modify or develop a solution."
Beiswenger adds, "It's a good feeling at the end of the day when you put together a prosthesis, and your patient comes back in a week or two and says, 'I'm doing so well and having such a great time.'"
Miki Fairley is a freelance writer based in southwest Colorado. She can be contacted via e-mail at
Editor's note: The content of this article is for informational purposes only and does not indicate sponsorship on the part of the participating companies or endorsement from The O&P EDGE. Unfortunately, space does not allow us to include descriptions of all prosthetic feet available for use in the activities mentioned, and the opinions expressed are solely those of the experts interviewed for this story.