© Springer International Publishing AG 2018Arthur Jason De Luigi (ed.)Adaptive Sports Medicinedoi.org/10.1007/978-3-319-56568-2_3
3. Technology and Biomechanics of Adaptive Sports Prostheses
Department of Rehabilitation Medicine, Georgetown University School of Medicine, Washington, DC, USA
Arthur Jason De Luigi
KeywordsAdaptive sportsProsthesisAdvanced technologyBiomechanics
With advances in medicine and an emphasis on maintaining physical fitness, the population of athletes with impairments is growing. There is an estimated 55 million persons with impairments in the United States according to the United States Census Bureau [1–3]. Although the number of people with an impairment who participate in sporting activities is increasing, there are still only about two million recreational and competitive athletes with impairments in the United States [2–4]. Despite an increase in the development of a number of recreational and competitive sports programs, about 60% of the persons with an impairment do not participate in any regular physical activity or sports . Two of the primary factors that limit participation in sports and exercise are lack of awareness and access to all of the opportunities available for athletic participation . It is incumbent upon healthcare practitioners to make every effort to inform these individuals of growing and diverse opportunities and encourage their safe participation through counseling and education.
Participation in athletic and exercise activities is universally beneficial, and several publications have made recommendations regarding the efficacy of regular physical activity [6–8]. It is known that physical inactivity increases the risk of many disease pathologies and is a major risk factor in cardiovascular disease [9–11].
There are clear benefits for participating in exercise and athletic activities. Athletes with impairments demonstrate increased exercise endurance, muscle strength, cardiovascular efficiency, flexibility, improved balance, and better motor skills compared with individuals with impairments who do not participate in athletics. In addition to the physical benefits, the psychological benefits of exercise include improved self-image, body awareness, motor development, and mood. Athletes with impairments have fewer cardiac risk factors, higher high-density lipoprotein (HDL) cholesterol, and are less likely to smoke cigarettes than those who are disabled and inactive [3, 12]. Individuals with limb deficiencies who participate in athletics have improved proprioception and increased proficiency in the use of prosthetic devices [3, 13].
Benefits to Participating in Adaptive Sports for Persons with a Limb Deficiency
Physical activity has many specific benefits for the population with physical impairments, including a decrease in self-reported stress, pain, and depression, as well as a general increase in the quality of life . Participation in physical activity has also shown a positive relationship with improved body image for many persons with limb deficiency . Lower limb deficiency due to amputation or congenital defect does indeed constitute a major physical impairment that can lead to functional and professional disabilities . A commonly referenced survey revealed that persons with lower limb deficiencies had a strong interest in participating in sports and recreation with the majority of respondents indicating that their quality of life could be enhanced if the prosthesis did not limit their ability to move quickly and run . Another survey showed that the 20–39-year-old age group had a similar distribution of interests between high-, moderate-, and low-energy activities. The same survey reported a high ability to perform these activities while using a prosthesis . The information suggests that the prosthesis is no longer the primary limiting factor when participating in desired activities. However, it is imperative to set realistic goals based on the physical capabilities of the athlete of when to begin a transition from a general use to sport-specific prosthesis following supplement endurance training from a deconditioned state . Given the opportunities for participation in sports that are currently offered to persons with a limb deficiency, the demand for new, innovative prosthetic designs is challenging the clinical and technical expertise of the prosthetist.
Limb regeneration of a deficient limb has not yet been accomplished in humans. Although technological advances in prosthetics continue to evolve and may one day surpass human capabilities, a prosthesis cannot exactly replace what the individual was born without or lost in trauma or disease. Aggressive rehabilitation and appropriate prosthetic provision will enhance the ability of limb-deficient individuals to pursue athletic activities. Understanding the biomechanics of the sport and the physical characteristics of the remnant limb is the first step in determining what a prosthesis can provide. The following sections present principles involved in the design of prostheses suitable for sports and recreational activities.
Advances in Sports Prosthetics
General Use Versus Sports Prosthesis: Principles of Design for Prostheses
There can be significant differences between a prosthesis developed for general use compared to a sport-specific prosthesis. Typically, there is planning between the person with limb deficiency, physician, and the prosthetist to determine the appropriate modifications to the general prosthetic design to make it compatible with the given sport.
Standard General Use or Utility Prosthesis
Although the residual limb is undergoing expected shape and volume changes, it is important to consider using the prosthesis for as many activities as possible. In most instances, prostheses that allow the limb-deficient athlete to participate in a wide range of activity, including selected sports, can be designed. Current options, such as elastomeric gel liners that provide socket comfort and skin protection that are required during everyday ambulation can suffice for many recreational activities. Careful choice of the prosthetic foot allows the person with limb deficiency to walk faster and achieve a more equal step length on both sides, thus facilitating recreation and routine walking . The prosthetic foot that has been aligned for comfort and efficiency during walking can still function adequately for intermittent, moderate bouts of higher activity. Although it has been shown that persons with limb deficiency find it difficult to accurately report their activity level versus measured activity level, the focus should be to develop ambulatory skills with a general prosthesis before advancing into sport-specific limbs during the early stages of rehabilitation . Once the person with limb deficiency commits to participation and training for a particular sport, a specific prosthesis or component may be necessary. When a single-use prosthesis is provided, the optimized design facilitates full and potentially competitive participation in the desired activity .
Another approach is to utilize the current daily-use socket with additional foot and/or knee combinations. The socket can be coupled with interchangeable distal components that have been selected to facilitate different tasks. A quick release coupler can be provided to permit interchanging knee and foot/ankle components. This alternative, when appropriate, can be more time-effective and cost-effective than multiple individual prostheses .
When the physician and prosthetist are evaluating the athlete with a limb deficiency for adaptive equipment, it is pertinent to take into consideration several aspects. Compared to the prosthetic device intended for everyday use, sports prostheses incorporate various design modifications that meet the functional demands of the sport. Of particular importance is the weight of the prosthesis, particularly in sports where increased weight may affect speed. Depending on the specific sport, there are other aspects of the prosthesis to consider. There are times where there may be an advantage with a conventional prosthesis rather than a prosthesis with advanced technology for a given individual with limb deficiency. Prescribing prosthetic components that facilitate higher activities is typically based on the experience of the prescribing physician and of the prosthetist . It is useful to clearly understand the functional and biomechanical demands of a specific sport when formulating a prosthetic prescription so that the functional characteristics of the components match these criteria. Participation in most sports can be facilitated by adaptations of conventional socket designs combined with commercially available components, but some activities are best accomplished with unique custom-designed components. Also, during the prescription of a prosthesis, the clinician should consider alignment, prosthetic foot dynamics, shock absorption, and the possible need for transverse rotation . As more persons with physical impairments pursue opportunities to participate in adaptive sports, it becomes imperative that medical providers begin the discussion about these recreational options. The componentry of the prosthesis may vary significantly between the various sports.
Depending on individual choice, patients can opt to participate in sports without a prosthesis. Swimming is one example of an activity in which use of a prosthesis is not always desired. The prosthesis can be used to reach the water and then removed prior to entry. During Paralympic competition, the International Paralympic Committee (IPC) requires that all prosthetics are removed prior to competition. The International Amputee Soccer Association (IASC) requires that all athletes with a lower limb deficiency participate without a prosthesis and use bilateral forearm crutches. In contrast, the Paralympic alpine skiing discipline allows athletes with a unilateral limb deficiency to choose use of a single ski, outriggers, or prostheses; however, athletes with upper limb deficiencies usually compete without using poles. When prostheses are not worn, it is advisable that athletes with limb deficiency wear some form of protection on the residual limb. It can be as simple as using the liner typically used under the prosthesis. However, if the athlete desires increased protection from high-impact falls, then a custom limb protector can be fabricated.
Prosthetic Design Alignment and Componentry
General Alignment for Sports Prostheses
Several studies demonstrate that alignment is not as critical as volume change in affecting skin stress on the residual limb [23–25]. In the context of this evidence, it still remains a critical aspect of optimal sports performance. Alignment of the socket and shank of a lower limb prosthesis critically affects the comfort and dynamic performance of the person it supports by altering the manner in which the weight-bearing load is transferred between the supporting foot and the residual limb. Furthermore, alignment of the lower extremity prosthesis for sports activities may be significantly different than what is optimal for other activities of daily living. Water and snow skiing are good examples of sports requiring increased ankle dorsiflexion. However, when the prosthesis is optimally aligned for these sports, it will not function well for general ambulation. In these instances, either a special-use prosthesis or interchangeable components will be necessary, and it is imperative to educate the limb-deficient athlete on properly change components to protect the limb from misalignment .
Biomechanics and Force Reduction in Sports Prostheses
When the multidirectional forces that give rise to pressure and shear stresses are expected to increase because of athletic activity, a socket liner made from an elastomeric gel is often recommended. Patients with conditions such as skin grafts or adherent scars will have a reduced tolerance for shear . For transfemoral limb-deficient athletes, special consideration should be given to the proximal tissue along the socket brim and the ischial tuberosity. Patient comfort can be increased by the use of a flexible plastic inner socket supported by a rigid external frame. This combination simultaneously maintains the structural support and integrity of the socket while allowing for increased hip range of motion because of the flexibility of the proximal portion of the inner socket .
The heels of prosthetic feet can dissipate significant amounts of energy during loading . Prosthetic feet have been shown to be capable of dissipating up to 63% of the input energy. Once a running shoe was added, the dissipation capacity increased to 73%. Even with the encouraging capability of the foot to absorb energy, once it has reached its limit, the forces are transferred to the socket and then ultimately the limb. Shock-absorbing pylons can be added between the socket and foot if additional impact reduction is desired. They may be an independent component or an integral part of a distal lower extremity integrated system. Some shock-absorbing pylon systems are pneumatic and easily adjusted by the user; other systems must be adjusted by the prosthetist to provide the optimal amount of vertical travel. The addition of a shock-absorbing pylon may show few quantitative kinetic or kinematic advantages with ambulation, but pylons do show a force reduction during loading response. Furthermore, prosthetic users also reported improved comfort, particularly at higher speeds [28, 29]. It is important to consider that when negotiating a descent on stairs or a step, the transfemoral limb-deficient athlete may gain added effect from an energy-absorbing pylon because of the increased lower extremity stiffness secondary to a lack of shock-absorbing knee flexion of a mechanical knee.
A prosthetic torque absorber component can be provided that will allow up to a 40-degree range of internal and external rotation between the socket and foot. Although multiaxial ankles offer some rotational movement, a separate torque-absorbing component performs this most effectively. There are many torque absorber options commercially available, but none can effectively match or be adjusted to the asymmetrical internal and external torque seen in able-bodied individuals . Even given the importance of minimizing transverse plane shear stress on vulnerable soft tissue, this component should still be considered even though it cannot exactly match the characteristics of the intact contralateral limb .
The development and prescription of energy storage and return prosthetic feet instead of conventional feet is largely based upon the experience between prosthetist and athlete with limb deficiency. The clinical decision making for the use of prosthetic feet is not always based upon the comparative biomechanical analysis of energy storage and return and conventional prosthetic feet but also incorporates the feedback of the athlete with limb deficiency. Despite the history of comparative prosthetic literature and continued analysis of prosthetic components, there remains a missing link between the scientific evidence and clinical experience of the medical providers . Although there may not have statistically significant changes in gait or performance parameters, these subtle changes and differences are perceived by athletes with lower limb deficiencies to affect their preferences and perception of foot performance. Variations in prescription may allow for benefits such as greater propulsive impulses by the residual leg that contributes to limb symmetry [32, 33].
Considerations for Various Sport-Specific Prostheses
There are numerous prototypes of running prostheses which have been developed. The most individualized aspect of the running prosthetic is the socket. The socket is formed fitted to the individual amputee to allow direct contact to maximize function and decrease complications. Given the increased energy cost of ambulation in an athlete with transfemoral limb deficiency, there has been limited evaluation of the best knee devices for running. However, running has frequently been performed by lower extremity limb-deficient athletes with single axis, hydraulic, pneumatic, and computer microprocessor knee devices. The majority of the variations are in the type of terminal device. Most running legs will have a flexible keel, which is energy storing. The goal of the flexible keel is to simulate propulsion caused by plantar flexion. There are different tensile strengths leading to a varied amount of kinetic energy stored in the flexible keel, and they can be molded as predominantly linear or curvilinear. There is less kinetic energy being stored in the prosthetic designs with a greater bend in the carbon graphite keel. Therefore, curvilinear flexible keels are utilized for recreational running or jogging. However, the competitive athletes will utilize a linear flexible keel with a less pronounced curve re-creating the foot.
The topic of the comparative kinetic energy in the flexible keel to the human leg is controversial. There is specific concern as to the length of the prosthetic limbs and subsequently the amount of energy stored in the keel. Typically the longer, more linear flexible keels will store the most energy. The more energy which is stored in the flexible keel, the greater the force of propulsion. This has led to controversy when an athlete with lower limb deficiency has attempted to compete against an able-bodied athlete.
Transtibial Running Prostheses
Prior to running, it is helpful to understand the goals of the adaptive athlete. If the individual’s primary desire is to jog for cardiovascular fitness, a slow jog occurs at about 140 m/min. The heel has minimal effect because the primary initial contact point is the middle portion of the foot at this speed. It may be advantageous, to use a specific running foot without a heel component, because the heel is minimally used or virtually eliminated as speeds up to approximately 180 m/min . Prosthetic limb kinematics has been shown to mimic this able-bodied data . The running foot is light and responsive with a significant amount of deflection on weight bearing that adds to the shock-absorbing qualities. Further weight reduction can be achieved by the adherence of running shoe tread to the plantar surface. The heel is more important if a decrease in speed occurs (e.g., jogging with intermittent walking), and a more versatile utility foot should be chosen. A sprint-specific foot should be considered if the limb-deficient athlete desires to sprint and short bursts of speed are the goal. In general, the sprint foot is designed with a much longer shank that attaches to the posterior aspect of the socket. The longer shank provides a longer lever arm for increased energy storage and return. For sprinting, the socket/limb interface should be a more intimate fitting that will maximize the transfer of motion from the limb to the socket. As with any running, use of gel liner interfaces is recommended. Choosing a thinner (3-mm-thickness) liner will reduce the motion of the tibia in the socket. For jogging, the liner should be 6 mm thickness or 9 mm thickness to maximize the shock-absorbing capabilities over a longer duration of the activity . Suspension is a key factor in movement and shear reduction on the limb, and an airtight sleeve and expulsion valve can give the best limb stability .
Transfemoral Running Prostheses
The guidelines to design of a transfemoral running limb are similar to the transtibial running limb. The component choices are based on defining the goal of the athlete with regard to jogging and sprinting. There are not any specific differences or criteria with foot choices for either transtibial or transfemoral limbs.
The major variant in the design decision involves whether to incorporate a knee or begin with a non-articulated limb. It is a viable option to begin without a knee when stability is a concern or in individuals with limited cardiovascular endurance. Training begins with a circumducted gait to allow foot clearance in swing. If a patient intends to participate in distance running, a non-articulated system eliminates the mental concern of inadvertent knee flexion and is less demanding for a longer run.
A knee component is generally recommended when sprinting is the goal. There are several good choices available that use hydraulic control of flexion and extension resistance and that interface well with the sprint feet . The overall limb alignment must be fine-tuned to the individual needs of the patient. Interlimb asymmetry has been shown to increase significantly when an athlete with transfemoral limb deficiency runs. Therefore, special attention to alignment, component adjustments, and training is particularly important [37, 38]. Maximum sports performance may require specialized components or significant deviations from standard alignment techniques to help improve interlimb symmetry and running velocity .
Adaptive Cycling Prostheses
Adaptive cycling is a very popular sport and recreation for limb-deficient athletes. Adaptive cycling is an excellent exercise that is non-weight bearing and indicated for individuals who may have impact restrictions or cannot tolerate higher-impact activities. There are significant variances in the adaptive cycling equipment for limb-deficient athletes given their specific level of limb deficiency which may also lead to which type of cycle they will utilize. In competitive cycling for limb-deficient athletes, the types of cycles used in competition are the bicycle, tricycle, tandem bicycle, recumbent cycle, or handcycle. Once proper fitting of the bicycle has been completed, the prosthesis will need some accommodations if more than recreational cycling is intended.
The most significant differentiation is whether the limb deficiency involves the upper or lower extremity. Athletes with upper extremity limb deficiencies usually compete with the traditional upright bicycles but may also choose to compete with recumbent or handcycles. The upper extremity limb-deficient athlete can utilize a specialty terminal device for hand breaking or upper extremity propulsion. It can either be a terminal device which can be opened and closed or can be directly clipped into the handle bars or hand crank. Other than the specialized upper extremity terminal devices, it is much less frequent for a limb-deficient athlete to utilize adaptive equipment when competing with either a recumbent or handcycle.