The Ankle Haptic Interface

The ankle joint is no stranger to injury. "Injury to the lateral ligaments of the ankle are the most common injuries in sports and active life" [16]. Medical research thus devotes much energy toward finding effective ways of treating and preventing such injuries. Physical therapy research investigates the various causes of injuries and develops treatment plans, arming therapists and doctors with better means to rehabilitate their patients. The aims of rehabilitation are to lessen the discomfort associated with injury, promote the healing of damaged tissue, and, perhaps most importantly, to increase patients' proprioception (see "Important Terminology") and strength in order to prevent future injury.

The cause of ankle injury is often insufficient strength and proprioception. For "athletes, individuals presenting acute traumatic joint pathology, and people who have degenerative joint disease . . . the lack of proper joint stability presents the potential for re-injury and progressive deterioration of articular structures" [17]. It is therefore imperative that patients teach their bodies' better awareness of joint position. Chandler and Kibler state that physical activity during rehabilitation "is important in preventing reinjury after the symptomatic healing of the original injury" [3].

Tropp and Alaranta offer a medical explanation of how proprioception training helps patients: "Coordination training on an ankle disc improves postural control and pronator muscle strength, and reduces the feeling of giving way of the ankle. Ankle disc training is not only effective for symptoms caused by functional instability, but is also effective in preventing further ankle sprains" [16].

Rehabilitative exercises do more than just prevent future injuries, though. Evidence also supports the idea that they aid in the healing process. "Ligaments have been shown to have faster healing rates if physical activity is performed as a part of the rehabilitation process" [3]. Donatelli explains how inactivity following injury may actually damage the body. "Active and passive exercise programs are performed to reverse the effects of immobilization on the connective tissues. Several authors have noted that prolonged immobilization of a joint leads to various undesirable biochemical and biomechanical changes in the surrounding connective tissues. Mobilization of a joint through its ROM may prevent them" [5]. Reduced ROM following joint immobilization is common to other body segments such as the elbow and the knee.

Post explains how injury is often a signal to athletes that their bodies require better training: "For patients and athletes to return to their desired activity level, their rehabilitated strength and flexibility must often exceed the preinjury level, since that level was inadequate to support the original loads imposed" [14].

The question still remains: what types of exercises are best for rehabilitation? Donatelli notes the following: "Researchers have pointed out that low-load, long duration stretches are more effective in attaining permanent plastic changes in connective tissues than high-load, short duration stretches" [5]. We can thus see that the ankle device that we design must be capable of accurately supplying low-forces over a long duration without overheating. It is also believed that increased stimulation of joint mechanoreceptors during rehabilitation exercises promotes better proprioception development. It is also true that these receptors "are most stimulated by movement of a joint to the limit of its normal range" [17]. It is thus important that the device we design allow a range of motion at least equal to that of the ankle joint itself.

It is thus paramount that devices and routines be developed to increase the effectiveness and efficiency of ankle rehabilitation. They should specifically focus on lightly exercising damaged ligaments to promote healing and challenging the body to increase proprioception.

There are a couple of terms which are important in order to understand the ankle rehabilitation literature. They are:

* Functional instability - "the recurrent sprains and/or a feeling of giving way of the ankle with cause unclear" [16].

* Proprioception - "a sense of joint position" [12]. It is often a lack of sufficient proprioception that leads to injury. To avoid future injuries, rehabilitation exercises should increase proprioception to levels higher than those before the injury.

* Muscle Strength - the force output of muscles

The ankle joint has three degrees of freedom (DOF). For each DOF, the axis translates as the joint rotates. The following presents estimates of the range of motion of a typical ankle. While a person is standing, the range of motion (ROM) of the joint as it rotates around an axis parallel to the ground with the toes rising in the air is between 30 and 42 degrees (between ground and the plane of the foot's underside) (see Figure 1c). The ROM as the foot rotates around an axis perpendicular to the ground is between 25 and 30 degrees (Figure 1d). The ROM as the foot tilts from side to side is 25 degrees (Figure 1e). [5] Projecting the foot onto the plane of the ground, the ankle joint is located about 25% to 28% of the foot's length from the back of the foot. For the average foot, the distance between the ankle joint and the toes is about 0.21 m. For the sake of simplicity, this distance will be considered to be the moment arm of the foot.

Figure 1: DOFs and Approximate Limits of Motion

"The neutral position for the ankle is with the lateral border of the foot at 90º with the axis of the leg and in midposition as regards to inversion and eversion." For a person standing, movement of the ankle around an axis parallel to the ground with the toes rising in the air involves plantar flexion and dorsiflexion. For a person lying on his or her back, plantar flexion involves decreasing the angle between the bottom of the foot and the ground (Figure 1b). Dorsiflexion is the opposite motion (Figure 1c). W. You found the average maximum angle of plantar flexion to be 35º and the average maximum angle of dorsiflexion to be 20º. [18]

Figure 2: Stills from an x-ray movie of the ankle [9]

You created a realistic kinematics model of the human to calculate the torques that the various joints apply when a person is carrying a load. For the ankle, You simulated the torques supplied by the joint in its plantar-flexion/dorsiflexion plane. For a man 1.75 m tall weighing of 75.0 kg carrying a load of 500 N (51 kg), the maximum torque applied at the ankle is 175 N•m. [18] This translates to a force of 833 N.

Donatelli found the following: ankle "plantar flexors should exhibit between 30 and 72 ft·lbs [between 40.7 N•m and 97.6 N•m which translates to 193.8 N and 464.8 N] of plantar flexion and 15 and 27 ft·lbs [20.3 N•m and 36.6 N•m which translates to 96.7 N and 174.3 N] of dorsiflexion at 90 degrees per second" [5].

Maximum force outputs of the ankle can also be figured out a s follows: Assume that the average person has a mass between 50 kg and 100 kg and can stand on his or her toes while carrying a 30 kg load. The force and torque necessary to accomplish this is a good estimate of the maximum forces that that person's ankle can supply (and support). Therefore, we find that the maximum force that such a person can supply lies between 784 N and 1274 N. This translates to a torque of 165 N•m and 268 N•m. Therefore, our ankle device should be able to oppose such forces and torques.

There are many effective exercises for the ankle that patients can perform at home during self therapy using common objects. The Windows-based software "Exercises Xpress Version 2.0 DEMO" by PhysioTools Ltd. provides information about several such manual exercises.

The following exercises are suggested for strengthening the muscles that control plantar flexion:
* "Bend ankle downward, pointing toes away from your body as far as possible." Hold for several seconds and repeat. (See Figure 3)
* "1. Stand on one foot while holding on to [a] sturdy object. 2. Raise up slowly onto your toes as high as you can." Hold for several seconds and repeat.
* "1. Stand with feet 12 inches apart. 2. Raise up slowly onto your toes as high as you can." Hold and repeat.
* "1. Sit in a chair with your feet touching the floor." 2. While keeping the ball of your foot on the ground, raise your heel. Repeat and hold.
* Repeat previous exercise with a weight strapped to your knee.
* Sit on the floor with your legs out in front of you with your knees slightly bent. Hold the two ends of an elastic strap in your hands. Place your toes in the middle of the elastic strap. Bend your ankle so that your foot pivots about your heel and your toes move closer to the floor. The elastic band should resist this motion. Hold and repeat. [6]

Figure 3 [6]

The following exercises strengthen the muscles that control dorsiflexion:
* "Bend ankle up toward your body as far as possible." Hold and repeat.
* "Walk on heels with toes raised up off the floor. Hold on to wall for support if necessary."
* "1. Sitting in a chair, place [one of your feet] on top of the [other] foot. 2. Without letting the bottom foot move, try to pull it upward against the top foot." Hold and repeat.
* "1. Tie one end of the elastic tubing to a solid object and the other end to your foot." Put you foot between your hips and the object. "2. Pull foot up toward yourself slowly." Hold and repeat. [6]

The following exercises are meant to stretch the muscles controlling dorsiflexion:
* "Sit on floor with towel or strap around [your] foot. . . ." It should wrap around you toes, and your heel should rest on the floor. "Pull top of foot toward your body so that you feel a stretch." Hold and repeat.
* Stand with arms out. "1. Position your body at arm's length from [a] wall. . . . 2. Point toes directly toward wall and hold heels down. 3. Lean into wall [bending your arms] so that you feel a stretch. Hold and repeat. [6]

The following exercises are meant to strengthen the ability of your foot to rotate:
* "Move your ankle around slowly as if tracing the letters of the alphabet." Repeat.
* "1. Stand on [a] balance disc with [or without] one hand on [a] wall. . . . 2. Keeping [your] foot flat on the disc, use the motion of your ankle to move the disc in a circular motion, clockwise and counterclockwise, and from side to side. 3. Try not to let the edges of the disc touch the floor."
* "Move your ankle around slowly in a large circle." Repeat. [6]

The following exercises strengthen the muscles that allow inversion of the ankle:
"Without moving your hip or knee, turn the bottom of your foot inward as far as you can." Hold and repeat.
* "Sit in a chair with your" foot on the ground and your big toe only touching a wall. "Hold [your] heel down as you press your foot against [the] wall. Hold and repeat.
* "1. Sit in a chair placing [your] fist between [your] knees. . . . 2. Tip the inside edges of your feet upward, leaving the outside edge of your feet touching the floor." Hold and repeat. [6]
This exercise stretches the muscles responsible for ankle inversion:
* "1. Sit with [the] soles of [your] feet facing inward [and touching each other]. 2. Bring [your] knees inward and push downward so that you feel a stretch." Hold and repeat. [6]

The following exercises strengthen the muscles that allow ankle eversion:
* "Without moving your knees or hips, turn the bottom of your foot outward as far as you can." Hold and repeat.
* "1. Sit in a chair placing your fist between your knees. . . . 2. Tip the outside edges of your feet upward, leaving the inside edge of your feet touching the floor." Hold and repeat. [6]

There are currently many devices on the market which have proven effective as tools for rehabilitation. From employing robotics to being made of wood and glue, these devices span all levels of complexity. They all, however, seem to be effective for their respective goals. It is important that the device we design be able to provide similar rehabilitation functions. Obviously, the more rehabilitation exercises our device can supply, the more effective it will be, as long as its complexity is not too great.

Elastic Bands
Perhaps the simplest device used for ankle rehabilitation is the elastic band (see Figure 4). This device is a figure-eight-shaped strip of elastic. Patients place both feet through the holes of the elastic strip. When they move their feet, the elastic supplies a resistive force to exercise the muscles that control the ankle. Many companies, such as DM Systems, Inc., produce elastic bands of different elasticities so that patients can control the amount of resistance the device provides. The bands are color coded according to their resistance levels.

Figure 4 [4]

DM System's web page says the following of their AnkleTough device, "[It] compliments rehab regimens prescribed by physicians, physical therapists, and athletic trainers . . ." [4].

Figure 5 [13]

Figure 6 [12]	Figure 7 [10]

The wobble board plays a central role in Abraham's suggested rehabilitation routine to treat chronic lateral ligamentous strain. "A wobble board and other exercises . . . will restore proper proprioception and gait. This will aid in prevention and recurrence" [1].

When patients suffer from ligamentous and capsular injuries, Helal also recommends use of the wobble board. After patients have rested their joints for some time through supportive strapping or in plaster casts, "physiotherapy consisting of gentle mobilization exercises [should be] carried out routinely." "At the same time, proprioceptive toning exercises such as 'wobble board' exercises [should be] instituted." He suggests that this routine be followed for about 3 weeks before a two-week gradual increase in training and an ultimate return to sports participation. [8]

The ARTU
The "Active Reflex Treatment Unit" (ARTU) was produced by Universal Gym Equipment, Inc. of Cedar Rapids, IA. It is somewhat similar to the device that we are currently developing. It works as a passive motion foot and ankle rehabilitation device which also can massage and cool injured muscles. "The extremes of all motions can be preset by the therapist to limit the degree of movement allowed by the ARTU" [5]. Donatelli expresses his favorable opinion: "Theoretically, joint mobility should improve through the use of a mobilization device such as the ARTU because of the positive effects of mobilization on connective tissue" [5]. Unfortunately from a research standpoint, it appears to no longer be in production.

Figure 8 [7]

Figure 9 [2]

Figure 10 [2]

Donatelli discusses several case studies in The Biomechanics of the Foot and Ankle, 1996. These examples of common injuries and suggestions for their treatment provide a framework for the requirements of a rehabilitation device.

Patient 1 suffered a postermedial skin splint. At first, a regimen of ice, resting, and exercise was introduced to promote healing and increased proprioception. Later, "a progressive resistive exercise program initially used elastic for open kinetic chain strengthening of the ankle joint plantar flexors and invertors. As symptoms decreased, the program was progressed to bilateral calf raises and then to unilateral calf raises." It is thus important that the device we design be able to apply forces similar to those experienced by a person doing calf raises. The patient was told to perform aerobic exercise for 1 hour each day, every couple of days. "This schedule precludes overstressing of tissues in lower leg and allows for tissue recovery and repair." [5]

Patient 2 suffered a lateral ligament sprain. He was initially given electrotherapy, active joint mobilization exercises, and a home exercise program. The "patient [then] performed 45 repetitions of active plantar-flexion-dorsiflextion exercises against manual resistance, as tolerated. Next, a 10-minute light effleurage massage. . . ." At home , he was to do exercises consisting of "active dorsiflexion and plantar flexion of the of the ankle without resistance for 45 repetitions, two times per day, icing throughout the day." [5]

During the second week, Patient 2 was told to begin proprioceptive exercises to "rehabilitate the damaged mechanoreceptors in the joint capsule and accompanying ligaments." He was to use a wobble board in a circular pattern for 10 minutes three times during the week. "Following proprioceptive training using the tiltboard, position sense improves." [5]

By week three, more active treatment methods were used. "A warm-up period, consisting of cycling on a bicycle ergometer for 10 minutes, was followed by an isokinetic workout of 90 repetitions at 90 degrees per second with right ankle dorsiflexors and plantar flexors." At the end of the active treatment regimen, the patient worked on a wobble board for 10-minutes. [5] By week 4, the patient began to exercise the evertors and invertors of the ankle for "120 repetitions at 90 degrees per second." At home, the patient did evertor and invertor resistive exercises for 45 repetitions using an elastic band for resistance. He also took part in agility skills involving "running figure-eight, back-pedaling, and zigzag patterns at varying speeds." By the eighth week of treatment, the patient was discharged. [5]

Patient 3 had a bimalleolar fracture. He was told to move his hallux (big toe) to trace out letters while in a whirlpool. At home, he was to do flexibility and resistive exercises. His flexibility exercise was gastrocsoleus stretching (patients put one foot forward, bending their knees, and pivot over the extended foot). "The stretched position was held for 30 seconds for five repetitions, twice per day." For resistive exercises, he was told to do "eversion, plantar flexion, inversion, and dorsiflexion [exercises] using an elastic material for resistance, performing the exercises for 45 repetitions for each motion twice per day." [5] It is important that the device we design be capable of allowing patients to trace patterns with their feet.

For week 2, joint mobilization therapy was used. Then, "resistive exercises against moderate manual resistance were applied by the physical therapist. The straight motions of eversion, inversion, plantar flexion, and dorsiflexion were resisted for 45 repetitions in each direction." The forces were applied manually because the patient was experiencing great pain and required the lowest, safest forces. The ARTU was also used for 20 minutes. At home, the exercise resistance was increased. Again, we see the importance of designing a machine that is capable of supplying low forces.

During the third week, he began a "submaximal isokinetic workout [which] consisted of plantar flexion and dorsiflexion for 45 repetitions at 90 degrees per second. . . . Eversion and inversion exercises continued against moderate manual resistance for 45 repetitions. . . ." Following the exercises meant to strengthen muscles, came the proprioceptive retraining regimen. A wobble board was used for 10 minutes. "All treatment sessions ended with a 20-minute session of the ARTU using only the cooling and passive motion features of the instrument. At home, he was to do more proprioceptive exercises and use a stronger elastic material during strengthening exercises" [5].

The following discusses suggestions for the ankle force feedback device's design based on the results of the preliminary research previously discussed.

Two imperative goals of the design must be simplicity and modularity. Simplicity yields easier maintenance and control as well as more efficient construction and replication. The device must contain the fewest number of actuators, for example. Modularity is important because the device must interface with the lab's existing virtual reality rehabilitation systems. The device must allow movement and supply force feedback for all 3 of the ankle's degrees of freedom. This requirement is essential to being able to emulate the various exercises that are currently used for ankle rehabilitation. It must also allow users to move their ankle through its full range of motion. This is important because, as explained earlier, proprioception is best improved when exercises involve movement near the limits of motion.

Low forces are also important. As Donatelli suggests, low forces are the best for many rehabilitation exercises. It is also important, however, to supply moderately high, continuous forces as well. Wobble-board-like proprioceptive exercises require the device to support the user's weight as well as counteract the torques his or her weight can supply while pivoting on the ankle.

As discussed earlier, there are many devices and exercises currently available for ankle rehabilitation. It is important that our device be capable of emulating as many of these as possible. It must be capable of wobble board and calf-raise exercises. It must also allow coordination exercises such as the letter trace exercise described earlier. In addition to these exercises, however, the device must also take advantage of its uniqueness as a human-computer interface and allow other, special exercises.

Like the Multi Joint System3, this device must allow users to limit the range of motion through which the ankle joint can move. This permits therapists to make exercises concentrate on specific motions.

Based on the data discussed in this report, the device needs to be able to supply at most forces that lie within the range 784 N to 1274 N (here, we assumed the maximum ankle force output of a person with a mass between 50 kg and 100 kg carrying a 30 kg load). This means that the device must be able to supply torques ranging between 165 N•m and 268 N•m.

There is currently a preliminary design that incorporates the issues discussed above. This design is based on the Stewart platform. A Stewart platform is a device using six double-acting cylinders capable of moving a platform with 6 degrees of freedom. The device consists of two platforms connected by a zigzagging array of actuators arranged in a circular pattern. Its kinematics is well documented and well understood. [15]

The device will consist of two platforms. One platform will rest on the floor. Between the platforms there will be six double-acting cylinders as specified by the Stewart platform design. Each actuator will be connected to the platforms by spherical joints. Linear resistive transducers will be connected in parallel with each cylinder to provide continuous measuring of the cylinder's expansion. The user's foot is placed on (or strapped to) the upper side of the top platform.

The device will be able to function in two modes: sitting mode and standing mode. In sitting mode, the user is able to sit in a chair, strap his or her foot to the upper platform and perform exercises while the device supplies varying degrees of resistive forces. Standing mode is for proprioceptive exercises. In this mode, the user stands on the device and tries to maintain his or her balance as it tilts the platform in various directions.

To obtain a rough estimate for the required piston forces, one can assume that the pistons are able to split up the force load nearly equally. Because the maximum forces lie within the 784 N to 1274 N range, we can assume that each piston's maximum force must be in the 131 N to 212 N range. Since at most 100 psi (689475.7 N/m2) is available, the piston's diameter must be between 1.55 cm and 2.03 cm. Further investigation into the piston requirements is necessary.

Research reported here was supported by National Science Foundation REU Grant #BES-9708020 and by the CAIP Center at Rutgers University.

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