HAND EXTENSION EXERCISER

BENEFICIAL DESIGNS, INC. R RESNA 95 ART (Name) December 6, 1995 Page 1

W. Mark Richter, David T. Eveland, Delia Sauceda Department of Mechanical Engineering, Santa Clara University, Santa Clara, CA Beneficial Designs, Inc., Santa Cruz, CA

ABSTRACT

Function of the hand during every day activities is primarily in flexion. We use the palmer surface of the our hands to grip objects more commonly than we use the dorsal surface to push them away. This pattern of use lends itself to a muscular imbalance about the joints of the hand. Finger flexors are commonly twice as strong as their corresponding extensors [1]. Research on the lower extremity has shown that an abnormal balance of agonist: antagonistic muscle strength is associated with both acute injuries and cumulative traumatic disorders (CTDs) [2,3,4]. Research at Beneficial Designs, Inc. has been proposed to determine whether a similar imbalance is associated with the hand and wrist injuries experienced by wheelchair users. In anticipation that there is a correlation between muscle imbalances and CTDs, Beneficial Designs, Inc has proposed the development of a device which strengthens the extensor muscles of the hand.

This paper describes the research and progress to date on the design of a hand extension exerciser.

BACKGROUND

Cumulative traumatic disorders (CTDs) are neural, vascular or musculoskelatal abnormalities resulting from an overuse of the body. By definition, CTDs occur when the forces acting on the body repeatedly exceed its functional capacity [2]. The prevention of CTDs requires that limb use not exceed its functional capacity. Therefore, there are two ways to approach prevention of CTDs. The first is to limit the function or the use of the limb, such that it does not exceed its capabilities. The second, and most empowering approach is to increase the functional capacity of the limb. This is commonly done through strengthening and conditioning. Strengthening and conditioning allows the limb to function safely within its daily activities and still prevents the occurrence of CTDs.

STATEMENT OF THE PROBLEM

The purpose of this project is to design and develop a hand extension exerciser. The key criteria of the design is to achieve a full range of motion as well as have independent, adjustable resistance to each of the finger and thumb joints. The device will force flexion and resist extension, be simple to use, and have a non-medical appearance.

RATIONALE

The first step in any design process is to undertake a thorough investigation into the state of the art. There are many commercial products on the market which exercise the hand in flexion (usually classified as tension relievers). The availability of hand extension exercisers is very limited, yet there are a few products which can be classified into this general category.

Available hand extension exercise products satisfy only a limited number of the desired design criteria. The most common area in which the current products are deficient is in their capability to exercise the full range of motion for each joint. Generally, these products do an excellent job in exercising one or two of the major joint classifications but ignore the third. Additionally, when two of the joint classifications could be exercised, they would need to be exercised separately, rather than in one simple and complete motion. Another setback to some of the designs was the inappropriate distribution of resistance to the various joints. The functional capabilities of the MCP (metacarpophalangeal) joints greatly exceeds that of the DIP (distal interphalangeal) joints. Therefore, the resistance should be applied to the joints in a similar manner, rather than equally or, in the worst case, opposing this order.

After conducting a thorough investigation into the state of the art, as well as having done a patent search for any ideas that had not yet reached the market, it was concluded that a completely new and innovative design was justified and required.

DESIGN AND DEVELOPMENT

The first stage of the design was to decide upon a motion which would achieve the desired results. The approach to deciding on a particular motion was to first examine what appeared to be the most natural solution and then to have that motion critiqued and modified by both an exercise physiologist and then by a physical therapist, specializing in the hand [5]. The result of this approach defined a motion which maximized the design parameters of being simple, natural, and effective. The motion begins with the hand as a clenched fist (the thumb pressing against the middle phalanx of the index and middle fingers), and ends with the hand completely extended, to a comfortable degree. Inherent in this motion is the premise that exercising isotonically is preferable to isometrically. Isometric exercise results in no motion at all. To address this uncertainty, the design criteria was modified to include an ability to lock the motion at any particular joint, allowing both isometric and isotonic exercise.

Once a motion had been decided upon, the actual design of the device began. Many obstacles arose during the conceptual design phase which were responsible for the discarding of several promising preliminary design concepts. Of the obstacles, the most overwhelming has been the fact that, if the hand is closed into a complete fist, there are many locations on the hand which become completely void of space onto which any device could be attached. On the palm side, the fingers, as well as the space between the fingers becomes completely sealed. On the dorsal side of the hand, the resting place of the thumb obstructs access to the middle phalanxes on the index and middle fingers. Because the phalanxes are short, the angular motion at the joints translate into minimal linear displacements between the middles of the phalanxes as the hand flexes and extends. This minimal displacement rules out the use of many linear resistance devices. When designing angular resistance devices to exercise the hand it is critical to design the device so that its hinges are located at the same location as the hands joints, or to design the device such that it does not inhibit the natural motion of the hand; this will ensure that the exercise does not cause discomfort or injury during use.

Due to the limited space available for a device to be incorporated onto the hand, developing a method to apply angular resistance to the hand's extension became the primary focus of the design. As shown in Figure 1, the resistance system developed is attached to the dorsal surface of the hand. It occupies minimal distance from the surface while the hand is clenched, and is allowed to extend from the surface as the hand is extended. The device depends upon the hand for pivot points, and therefore is not uncomfortable to operate.

This type of motion is achieved by placing rigid members between hinges, located at each joint of the hand, as well as between each of the joints. A coil spring between each of the hand's joints forces the adjoining rigid members to remain flat. Figure 2 is a layout drawing of a single resistance mechanism. This orientation of the rigid members occurs when the hand is fully extended. The displacement of the coil spring during exercise is approximately 90 degrees. The coil spring is preloaded when the two adjoining members are flat, in order to ensure resistance to extension throughout the entire motion.

The base plate in Figure 2 is attached to the finger between the joints. The attachment of the base plate to the fingers is presently accomplished using a glove. The base plate is sewn onto the dorsal side of the glove at locations correlating to the midpoints of each phalanx. A specific glove has been chosen to minimize hindrance of the hands motion, while maximizing rigidity around the hand.

The resistance's are adjustable by the changing the coil springs. The stiffness of any particular spring will proportionally effect the difficulty by which the hand can be extended. The size of the device follows the standard fitting guidelines for gloves, where small, medium, and large are the described fits.

EVALUATION

Thus far, the prototypes are performing as we had predicted. Mounting the base plates tightly to the finger has been a critical issue in ensuring that the resistance device is effective throughout the range of motion. The present prototype appears to be useful on various size hands, reducing the number of required sizes for multiple users. The hinged resistance mechanism is a significant design innovation which will lead our future design efforts.

DISCUSSION

Future Work

The early prototypes of the design have shown their potential for success. Future efforts will be focused on refining the resistance mechanism and improving the interface between the resistance mechanism and the hand. Additionally, the devices performance will be assessed by both the staff at Beneficial Designs, Inc. and by a mechanical engineering advisor at Santa Clara University.

The resistance mechanism will be refined by careful selection of coil springs and member lengths. Coil springs must be linear within a small range of motion (90 degrees or less). Also, the coil springs must be available in many slightly varying stiffness, in order to allow for gradual strength development and thus reduce the chance of injury, due to over exertion.

The use of a glove as the device / hand interface may not prove to be a necessary structural element. In the event that the resistance hinges and hand joints create a sufficiently stable structure, the glove may be replaced with a series of fingerloops and a larger piece which fits over the back of the hand. The advantage of eliminating the glove is an increase in ventilation to the hand and an increase in the aesthetics of the device. These interface components are rigid thermoplastics, which have been molded into the shape of the hand. The inside of the interface is a soft spongy material (i.e.. neoprene). A strap will wrap around the palm, securing the device to the hand

Significance

The hand extension exerciser holds promise in the prevention of CTDs. The relatively high rate of CTDs occurring in the last decade shows us that there is a need for such a device. The approach to preventing CTDs using the hand extension exerciser is to increase the functional capacity of the hand to a level beyond its daily demands. The device is designed to be versatile, aesthetic, and easy to use. Such characteristics will make it a desirable product to a wide variety of consumers and its versatility will allow it to be used in such places as physical therapy clinics, while traveling, exercise gyms, and at home.

ACKNOWLEDGMENTS

We would like to thank: Peter Axelson, Patricia Longmuir, Sharon Gordon, Arne Folkedal, and Tim Hight, for all of their criticisms, support, and advice. This project could not have reached the heights it did without them.

REFERENCES

[1] Nordin Margareta, Frankel Victor H. "Biomechanics of the Hand." Basic Biomechanics of the Muscular Skeletal System. Lea & Febiger, US, 1989.

[2] Baratta R., Solomonow M., Zhou., Letson D., Chuinard R., D'Ambrosia R. "Muscular Coactivation: The role of the antagonist musculature in maintaining knee stability." Am J Sports Med, 1980, 16(2): 113-122.

[3] Irrgang J. J. "Modern trands in anterior cruciate ligament rehabilitation: Nonoperative and postoperative managment." Clinics in Sports Medicine, Oct. 1993, 12(4): 797-813.

[4] Prentice W. E. Rehabilitation Techniques in Sports Medicine. Mosby Publishing Co., Toronto 1994.

[5] Patricia Longmuir, exercise physiologist, Toronto Canada. Sharon Gordon, physical therapist specializing in the hand, San Jose Medical Center, San Jose CA.

W. Mark Richter Santa Clara University 641 Circle Dr. Oroville, CA 95966 (916) 534-1120 wrichter@scuacc.scu.edu