Advancements in materials, technologies, and an evidence-based approach blending science and anecdotal practice are enabling people with limb loss to push both the boundaries of their physical human limits and the limits of their prostheses. Despite the increase in this population, it is a small segment of the general population and most strength coaches, trainers, etc., may rarely encounter athletes with amputations. Therefore, it is important to discuss and understand the function of prostheses and the ways to modify or otherwise adapt coaching paradigms. This article will discuss adaptations to training that should be considered for athletes with a lower extremity prosthesis.
In the United States alone, it was estimated in 2006 that 1.9 million people were living with limb loss, and by 2050, that number is expected to be more than double (12). Greater than 55% of lower limb amputations are secondary to disease, ∼45% are due to trauma, and the remaining are secondary to congenital situations (12). Amputations due to disease are largely related to diabetes and vascular disease, although other maladies can result in amputation as well. Evidence is widely available that exercise improves health conditions in these populations improving their quality of life, lowering health care costs, and reducing the need for further health care.
Improper training can lead to problems with able-bodied athletes. So, of course, it stands to reason that this could affect adaptive athletes as well. Strength coaches should stick to principles that apply to all populations. Every athlete, whether adaptive or not, presents with specific goals, mindset, stressors, and situations for which coaches plan accordingly. Adaptive athletes still can benefit from the principles of overload, progression, balanced development, recovery, technique, specificity, periodization, and ongoing evaluation. Using a prosthesis does not change the fundamentals and principles of the science and practice of exercise (Table 1).
UNDERSTANDING THE PROSTHESIS
A prosthesis is an artificial body part, and in this discussion, typically refers to a transtibial and transfemoral artificial limb. Lower limb prostheses are typically designed with the function of ambulation with a step-over-step gait pattern as the overall goal. When a person loses his or her leg, the main problem is that he or she can no longer walk. This limits independence, ability to perform activities of daily living, and basic human function. When an amputee meets the prosthetist, an evaluation occurs, and the user reports the preamputation function, motivation, and other inputs. Then a short-term and long-term treatment plan should be developed. At this point, a person's desire to be physically active or at what level he or she has the potential to be physically active may not be clear. Once an amputee is further along in rehabilitation, prostheses can be designed and fabricated to meet other needs.
With the aforementioned objective in mind, the prosthetist designs, fabricates, and provides a prosthesis and other necessary supplies to achieve the goal of ambulation. There are multiple supplies and components that the amputee will be provided, and coaches should be aware of and understand their use and function. The prosthetic socket contains the residual limb and loads soft tissue or can also attempt to interact with boney structures to achieve biomechanical efficiency for ambulation. Most transtibial sockets have a patellar tendon bar. This patellar tendon bar creates another fulcrum on the patellar tendon changing the range of motion (ROM) at the knee along with the restrictions placed on the anatomy by the socket trimlines (8). Posterior transtibial socket trimlines will often not allow much knee flexion past 90–100° due to the need for containment of the tissue and sagittal plane counterforce inside the socket for ambulation (6). Transfemoral sockets often contain the ischial tuberosity to provide greater coronal plane stability during midstance, but this can decrease ROM around the hip joint (10).
Gel liners are commonly used as an interface directly against the skin. They can be made of silicone, thermoplastic elastomers, and urethane, all of which provide different material properties. These can reduce shear forces on the limb as well as compress and stiffen soft tissues for mechanical loading and control over the prosthesis. Suspension refers to how the prosthesis is secured or held onto the residual limb. The most commonly used suspension strategies are a locking mechanism that physically locks a pin, strap, or ratchet that attaches directly from the liner into the socket. Another common option is suction suspension, which may be complemented by a vacuum pump, to use negative atmospheric pressure gradients to hold the prosthesis onto the user. This typically provides the user improved proprioception, reduced pistoning (vertical movement inside the socket), and an improved anatomy to prosthesis interface.
Residual limb volume fluctuation and perspiration are 2 of the most common daily issues that amputees face (2). Prosthetic socks are made of different cloth materials and are integral to the fit and comfort of the prosthesis. They are provided in different thicknesses so that as an amputee's residual limb volume changes throughout the day due to activity, food intake, or other reasons, they can replace the lost volume and ensure proper fit of the socket. Sheaths may also be provided, which can provide a wick for perspiration and moisture away from the skin or provide an air wick to achieve suction suspension inside the socket.
No two prosthetic feet perform the same. Feet are grouped into categories of similar properties and mechanics. Most prosthetic feet do not have an ankle articulation, but due to the design and engineering of the materials, they deflect, or bend, during loading allowing smooth rollover and translation through stance phase. Materials used are usually fiberglass or carbon fiber composites that store and return energy. These feet deflect upon loading, storing energy in the fibers of the material, which is then released at terminal stance to propel the amputee forward. Capabilities of prosthetic feet that a coach may encounter with an adaptive athlete include but are not limited to the following:
- Dynamic response—this feature is usually found in amputees who can ambulate with variable cadence. Dynamic response prosthetic feet minimally deflect 25 mm when loaded with 1,230 Nm and return 75% of the energy that is loaded. (1).
- Torque absorption—this is a feature used in addition to dynamic response or flexible keels, which provides movement in the transverse plane. These components are heavy and tall; thus, they are used only occasionally.
- Multiaxial movement—this allows movement in the coronal plane. This is a function usually built into most prosthetic feet.
- Hydraulic ankle—articulation that uses hydraulic fluid to control speed of movement. The disadvantage is added weight and that very active users can outperform the ability of the hydraulic units.
Most prosthetic feet have a combination of the aforementioned categories to provide as many biomimetic qualities as possible. Sport-specific feet and componentry are available but are placed into the same category as other prosthetic feet. For example, vertical shock feet are great for everyday use, dissipating impact at terminal stance and sparing the spine and sound side, but most running feet or athletic feet are in the same category due to testing parameters. However, the time at which vertical displacement occurs is at midstance due to the shape of the carbon strut, making it better suited for running. Recent controversy has occurred surrounding whether prosthetic running feet can provide an advantage compared with nonamputee runners; however, research has not shown definitive advantage to date (5). Running and athletic prostheses can be fabricated and provided, but prostheses are not one type that fits all activities. Prostheses can cost between $5,000 and 15,000 for a transtibial prosthesis, with a transfemoral prosthesis having a much greater range and expense. Insurances often attempt to deny provision of an athletic prosthesis. However, with proper documentation and cooperative teamwork, it is possible.
Prosthetic knees also have several categories and many different functions. Most prosthetic knees are made to resist flexion during weight bearing, making many different movements in athletics difficult. There are several prosthetic knees designed for athletic purposes, and some microprocessor knees can be programmed to have modes to be used during certain athletic-type movements. Prosthetic knee joints are also organized into categories that describe their mechanical abilities. It should be noted that different parameters are required to unlock or allow for the knee to bend for each specific knee. Knowing these parameters can help the coach understand how to help modify exercise or to train their athlete.
It is well documented that energy expenditure for transfemoral amputees is elevated compared with both transtibial amputees and nonamputees (3). When walking, speed is held constant; studies also reveal that energy cost is higher for those whose amputations are due to vascular issues compared with traumatic accidents (3). For transfemoral amputees, the knee joint is missing, which is replaced by a prosthetic knee joint, and now the entire prosthesis must be controlled by the hip joint while the intrusive socket trimlines limit the ROM. Many amputees lower self-selected walking speed to keep comparable energy costs as nonamputees (3). Joint kinetics and kinematics of both the prosthetic side and contralateral side are affected by amputation, which can lead to comorbidities such as low back pain and osteoarthritis (7). Prosthetic components attempt to mimic biomechanics, lower energy expenditure, and spare the sound side and spine as much as possible, but they are still lacking in comparison with the human body (7). For example, at terminal stance, the plantar flexors that would create push-off to propel the limb into swing are now missing and are replaced by the energy return of a carbon fiber foot. When exercising, these differences may be emphasized due to increased loading or increased demand placed on the physiology.
Alignment is the process of arranging the components of the prosthesis spatially to optimize function. This is performed to optimize ambulation and is how deficits in movement such as contractures can be accommodated and energy efficiency optimized. If there is a secondary prosthesis for athletic endeavors, this will already be aligned for sport. Progression through the gait cycle is one goal of alignment in the sagittal plane for the transtibial amputee and upon inspection, one may find sockets to be placed into slight flexion to encourage this (6). For transfemoral amputees, alignment is often completed to provide stability and safety and follows the trochanter, knee, and ankle line and places the weight line in front of the knee joint. In the coronal plane, prostheses are aligned to create stability, or balance, during single-limb support (10).
When the user demands more of the prosthesis than basic ambulation, this is where teamwork by those involved with the adaptive athlete is required. A coach should invest time into understanding limitations of their athlete's prosthesis and its basic componentry. For instance, a coach should understand that a nonarticulating prosthetic foot is not going to allow dorsiflexion like a healthy anatomical ankle and should alert the prosthetist to make necessary modifications to movement. The prosthetist can make changes to the prosthesis to meet exercise demands, but only if he or she is informed of those demands. The coach should be willing to interact with the prosthetist for information about the prosthesis to better optimize the exercise prescription. An invitation to the prosthetist to a training session may be warranted to ensure optimal communication, understanding, and optimal outcomes for the athlete. Some athletes use their prostheses during exercise, whereas others choose to adapt and exercise without their prosthesis or use them situationally. These distinctions can lead the coach to better understand how to accommodate their needs. It can also give insight into the mental state of the athlete.
UNDERSTANDING THE ATHLETE
Evaluation and assessment of the athlete is ongoing. If necessary, consult with the athlete's medical team if there are any concerns. Assessments should include but not be limited to strength deficits or imbalances, gait or running deviations, or posture malalignments caused or related by the limb loss. Balance confidence should be assessed because the athlete's connection to the floor and proprioception has been affected. Assessment of cranial nerves may be necessary if the athlete was involved in a traumatic accident as the cranial nerves provide nervous supply to the vision, balance, and base motor function centers. Many prosthesis users may struggle with producing proper weight shift to the prosthetic side due to a lack of trust in the prosthesis. This can cause knee joints and/or feet to not function as intended. Proper and thorough evaluation will reveal whether these things should be corrected, how quickly or gradually, or if it is something that should be accommodated. Compensatory mechanisms should be expected but may not present immediately, once exercise progresses, and therefore, occasional need for re-evaluation is necessary.
After amputation, anatomy is likely rearranged from its nonamputated configuration. Understanding how the anatomy has been reattached or changed can be essential to maintaining comfort and performance. A joint disarticulation, such as a knee disarticulation, which also leaves the femoral condyles, leaves at least some joint surface as compared to a transection, which is directly through a long bone (9). Joint disarticulations are less common, but the advantages are numerous, such as a longer lever arm and the ability to put weight through the distal end of the bone (9). A myodesis results in the cut muscle being tied directly into the bone. Myoplasty results in a muscle being tied into another muscle. There are other types, but these are the most common techniques (4). They may even be used in conjunction with each other. Athletes with an attachment to the bone will have greater control and improved muscular abilities compared to those with myoplasty (4).
Not only are the attachments of muscles different after surgery but also the strength and cross-sectional pull. In a transfemoral amputation, if the adductor magnus is cut, adduction strength has been shown to decrease by 70% (4). The hamstring group becomes weaker after they are cut, and the hip flexors overpower the extensors and can cause a contracture, which may be worsened by prolonged sitting in a flexed position while waiting for a prosthesis to be fabricated (4). Surgical techniques such as the Urtle procedure, which builds a bridge between the tibia and fibula to improve weight bearing, can be performed but are not the standard. Osseointegration is a technique that places implants directly into the bone of an amputee. This technique has only recently been approved for trials in the United States and is therefore rarely seen in the United States. Anecdotally, osseointegration allows for improved use of the residual muscles resulting in less comorbidities, less atrophy and potentially hypertrophy, and improved ROM while using a prosthesis.
An easy way to monitor the area of concern is the cut end of a residual limb. This area typically is the first to be an indicator of detrimental activity. The cut anterior distal end of the tibia and the cut distal end of the femur were not made to bear weight (4,6). They are typically beveled but still are likely to be painful to bear weight upon. If these high stress areas begin to show signs or symptoms or distress, then the athlete should stop and make an adjustment to their fit before it becomes too late, and they must cease prosthesis use or risk causing an ulcer or extreme soreness. The strength coach must consider those boney prominences in their exercise selections. For a transtibial amputee, weighted knee extensions if loaded below the cut end of the bone could be poor exercise selection. This could turn the cut end of the tibia into a fulcrum/pivot point for the load potentially causing skin breakdown.
Assess the condition of the residual limb, check for remarkable observations such as a wound, sore, adherent scar tissue, callous, long lasting redness, or palpable pain. If these are present, they should be addressed before high activity.
When inquiring about the adaptive athletes' goals, coaches must also not be afraid to inquire about how the amputation occurred. This may lead to insights into mental state, purpose, and the goals themselves. One must weigh the cost of training to achieve a perfect technique back squat versus accommodating, modifying, or moving to a different exercise to achieve stress on the same physiological system or engage and activate the same muscle groups. Unless someone is endeavoring to compete in an exercise-specific realm, for example, powerlifting, then the goal is not the exercise, but may instead be the stimulus needed. People with amputations are at a greater risk of osteoarthritis in the contralateral side knee and hip than those without amputation and a greater risk for low back pain as well (11). Therefore, it is important to consider the long-term as well as the short-term goals of the athlete and modify and scale to elicit the necessary stimulus regardless of modality.
BILATERAL TRANSTIBIAL ATHLETE CASE EXAMPLE
Figure 1 shows a bilateral transtibial amputee who desired to learn how to squat and improve her overall strength. As an untrained adaptive athlete begins to squat, the nonarticulating foot does not allow dorsiflexion ROM as in an able-bodied athlete causing the heel of the foot to rise off the ground as can be seen in Figure 1. Also observe where her weight line, or the vector that would extend downward from her center of gravity, would fall. Owing to a lack of balance when initially performing the movement, a stability ball was initially required behind the back for stabilization and a kettlebell held in front providing further manipulation of her center of mass. Use of these implements shifted the weight line forward and provided greater stability while allowing her security and stability to allow her to practice the motion. In addition to the intrinsic instability, the athlete has mechanical vacuum pumps that provide shock absorption, which the athlete reported, also caused her to feel unstable initially. After cueing to improve mechanics to push the hips backward, accommodation of heel rise by adding a wedge beneath the heels was still necessary, but the kettlebell could be removed as can be seen in Figure 2. The athlete was better able to shift her weight line forward to engage the hip extensor muscles and eventually gain control and cease use of the stability ball.
There are exceptions to every rule; however, for beginners, it is important for the heel to stay on the ground if possible. The residual limb is placed inside the socket which has contours that are placed around the anatomy. When the angle of the knee changes, so does the entry point and angle of the tibia inside the socket, but the socket does not change the angle because it is anchored to the foot. The movement of the anatomy inside the socket can cause the tibia to extend or push forward into the socket wall. Attempting to keep the foot flat on the floor can reduce this issue, or the use of a heel wedge to bring the floor up to the foot can be attempted if necessary. This is where the strength coach can use their knowledge to modify the movement as needed until strength can be developed to provide intrinsic control by the muscles. Once the athlete has mastered the movement and can control the residual limb inside the socket regardless of the position, then the weight line can be manipulated into more positions that require increased muscular control and strength. Modified movement may be necessary and is at the discretion of the athlete and coach in these conditions. A suboptimal fit may also have an effect on this movement in a detrimental way, and fit could be assessed to ensure that is not the issue.
In Figure 3, the athlete is on a plate-loaded machine mimicking a free weight squat. The platform's angle allows the athlete to squat with the foot in full contact with the ground. The athlete's back is in a more advantageous and safer biomechanical position keeping her residual limbs in a better position within the socket and engaging the hip extensors while the weight line remains within the center of mass. By modifying the movement, progression can occur, and a significant stimulus to the same muscle groups can be achieved and the training goal achieved.
Further trial and error revealed that a trap bar deadlift also allowed the athlete to produce needed stress to the large muscle groups of the lower body by allowing her to begin in a more biomechanically advantageous position. With most adaptive athletes, it is this type of trial and error built upon the foundation of a scientific principle that will lead to great success.
TRANSFEMORAL ATHLETE CASE EXAMPLE
The athlete for this case example aspired to perform free weight movements such as the squat, deadlift, and other variations to improve overall fitness but also to improve her ability to progress in high-intensity style weightlifting classes. In Figure 4, the athlete is trying to maintain proper position of the pelvis and hip, knee, and contact of the foot with the ground. The athlete again must attempt to accommodate for the knee center discrepancy and the proximal and distal effects of that discrepancy. The photo shows that she can squat only within a limited ROM. But also, that this caused her to create increased lordosis to feel stable over her prosthetic limb as well as a compensation at the pelvis and hip that is nonsymmetrical. Foot placement of the prosthetic limb required modification and placement forward compared to her contralateral side.
In Figure 5, several issues can be found with attaining a normal front squat. Shift to a front squat was attempted to change the position of the athlete's weight line but still works a related movement in a similar pattern. Even with a decreased ROM in this squat by using a box, the long residual limb of the athlete creates a knee center discrepancy when wearing the prosthesis. The long residual limb provides a long lever arm, which is great for ambulation and control over the prosthesis, but still causes ROM issues as can be observed now in Figures 4 and 5.
Figure 6 shows a change in exercise selection to a limited ROM deadlift. This provides a different stimulus but still allows work to be placed on the larger lower-body muscles. The athlete's alignment is improved with a more symmetrical knee, hip, and pelvis while foot is flat on the floor despite a small amount of excessive lordosis. For the athlete at this time, selecting a different exercise put the body in a better biomechanical position to preserve long-term wear and tear from accumulating as quickly.
In this case, no acceptable solution could be found to both accommodate keeping the heel of the foot in complete contact with the ground while also keeping the pelvis and hip level while performing a traditional front or back squat without excessive lordosis at this time. The solution was to change the movement altogether or decrease the ROM to the point before the movement deviations. Another approach was to build the musculature with auxiliary exercises and mobility work and the return to the movement pattern. For this athlete, after further cueing, targeted mobility exercises and completed continued coaching; squatting for this athlete was improved.
In conclusion, it should be the goal of the strength professional to not only challenge the adaptive athlete but to protect them as well. Modifications must be evaluated and matched to the athlete's goals and choices made with the discretion of the coach and the athletes educated. These 2 case examples show how, though not perfect, trial and error movement can be adapted to prioritize function and optimize performance. When working with an adaptive athlete, the process is likely to be longer and more arduous than working with an able-bodied athlete. Goals and expectations may need to be modified, and patience is of high value. However, with careful consideration, the strength coach can play a vital role in the development both short term and long term of an adaptive athlete.
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Keywords:© 2019 by the National Strength & Conditioning Association
adaptive; prosthesis; prosthetic; amputee; prosthetic leg