Injury and age make joints vulnerable to high rates of loading, which contribute to the onset of osteoarthritis. Early research suggests that feedback-based gait retraining can help reduce rates of loading by improving joint kinematics and proprioception.
by Jody L. Riskowski, PhD
Musculoskeletal symptoms and ailments are the number two reason, behind respiratory symptoms, for physician visits in the U.S. Annually they lead to more than two million first-time visits to physical therapy and movement rehabilitation centers,1 totaling an estimated $267.2 billion in direct and indirect costs.2 However, recent advances in engineering and technology have led to new devices that aid in physical rehabilitation and therapy, which are thought to bring a faster recovery while decreasing costs.3 Specifically, gait retraining is one area that has seen a rapid growth in product development.
To support the gait retraining efforts, there are a number of active assistive braces to aid in the restoration and/or enhancement of an individual’s ability to walk efficiently. Active assistive devices and braces as opposed to their passive counterparts use external monitoring and assistance to reestablish neuromuscular control. Though passive bracing may provide stability and maintain alignment of the joint, it lacks the ability to provide real-time feedback and to adapt to the individual.
The body has several intrinsic feedback systems to monitor movement and walking, such as vision, sound, and proprioception, defined as the body’s awareness of its actions. With vision, we watch our limbs move, with sound we listen to how our foot strikes the ground, and with proprioception we feel how our body moves. However, with disease and injury, one or more of these systems may become damaged and may not provide accurate information. In this case, external feedback, whether it is via sight, hearing, or touch, may augment our internal cueing and motion, aiding in motor learning and adaptation.
In our everyday world, many external feedback systems are employed: a coach films an athlete to show her how her hurdling technique changes from the start to the end of race, a musician replays a recording of his concert to listen to his timing, and a child learns to ride a bike through training wheels that guide his balance and movement. Over time, these external systems may develop our internal cueing and movement pattern.
Broadly speaking, internal and external feedback in gait monitors and measures the current movement status against a desired action. The feedback provides a goal for the user, allowing them to focus on their movements and actions. Feedback may also increase motivation by keeping individuals informed on their results and performance. This generally translates to greater effort during training sessions and more motivation for practice, yielding greater compliance and enthusiasm. All of this may lead to improved outcomes.
Feedback-based gait retraining
Feedback-based systems have been developed to assist gait retraining in patients with anterior cruciate ligament (ACL) injury4-6 and arthritis.7, 8 With these systems, feedback is relayed to the user visually,9-11 acoustically,4, 12-14 or vibrotactilely15 as well as in combination with one another.7,16-19 Regardless of the system, they all share a common function: to provide a quantitative measure of performance in real-time, which allows the individual to make adjustments on the subsequent step. These measures include muscular activity,16-18 kinematic,13, 20 or kinetic measures.12,14,16
Arthritis, or more specifically osteoarthritis (OA), is the most common arthropathy and a major contributor to disability in the elderly.21 OA is a degenerative joint disease that affects an estimated 27 million people in the U.S.22, generally targeting individuals over the age of 55, with 12% of the population 65 or older experiencing it in the knee joint.23 A secondary classification of OA, known as post-traumatic osteoarthritis, can develop in individuals much younger as a consequence of a previous injury, such as an ACL injury. Primary or secondary OA develops as the normally slippery articular surface becomes roughened and abrasive causing pain with movement. Further degeneration of the articular cartilage causes changes to the underlying bone,24 limits range of motion,25 and decreases joint strength.26 The associated pain as well as reduced strength and range of motion can lead to a decrease in mobility, independence and quality of life.27
Research has shown that a repetitive high force or rate of loading (ROL) during walking can be a predecessor to the onset of joint degenerative diseases, such as OA.28-34 In walking ROL refers to how quickly forces are imparted on a joint (Figure 1). Animal studies have demonstrated that repeated high impulse forces and a high ROL lead to osteoarthritic changes in the articular cartilage and surrounding bone.30,32,34 In gait, a high ROL tends to arise from improper gait kinematics at initial contact (IC), or heel strike.35, 36
To protect itself and prevent damage from the ROL, the body relies on several intrinsic, passive mechanisms, including the articular cartilage, menisci, and intervertebral discs.37 However, these structures by themselves cannot always withstand the repetitive forces of walking, and over a lifetime, they can experience fatigue failure.38 With the repeated exposure to a high ROL, these shock-absorbing and protective mechanisms become damaged, and the underlying bone at the joint experiences increased stress.29 Thus, simply relying passively on the body’s mechanical properties may not be effective to prevent the onset of OA, particularly if a person has experienced an injury, which tends to acerbate the ROL experienced.
In addition to the passive methods of affecting ROL, the body also utilizes active mechanisms to reduce the effects of IC39 by ensuring the body adequately prepares for IC.13,36,39 These methods include eccentric contraction of the quadriceps muscles and proper flexion of the knee to maximize the contact area of the articulating surfaces and disperse the load across the knee joint.37,40 Regardless of the active strategy the body utilizes to protect itself, an intact neuromuscular system is needed.
The neuromuscular system is the combination of the nervous system and muscles they innervate that work synergistically to permit movement. The nervous system activates the muscles to control the muscular stiffness and actions of the limbs. This system is also responsible for providing the proprioceptive feedback, which informs the body what limb actions occur. However, with aging or injury, some individuals’ proprioceptive feedback systems may be lacking, such as a patient post-ACL injury,41 or with pre-osteoarthritic35 and osteoarthritic42 conditions. These individuals tend to have poor intrinsic feedback systems, which suggest that they may benefit from an external feedback-based training system.
Tearing, rupturing or damaging the ACL can disrupt the joint mechanisms and may interrupt the proprioceptive signaling.43 Even after the ACL injury is rehabilitated and/or surgically repaired, the proprioceptive feedback may not be fully corrected.43 This means the body may be unable to provide appropriate feedback on the limb’s actions, which may result in further damage to the knee joint caused by incorrect movements. Studies analyzing gait post-ACL injury show that patients have disrupted joint mechanisms, altered force transmission, and unique muscle activation patterns compared to non-ACL injured subjects.44-52 This chronic knee instability can lead to further joint degeneration,53-56 changes in the meniscal properties,57,58 and alterations in the joint cartilage properties.59 The knee joint instability and subsequent altered joint mechanics that result from inaccurate proprioceptive feedback may be one reason that 14% to 60% of the ACL-injured population shows radiographic evidence of degenerative changes at the knee joint.53,60-67 As stated earlier, patients with poor intrinsic feedback may benefit from an external feedback-based gait training system.
There are a limited number of external monitoring systems that have been used for gait retraining in the post-ACL injured populations. One system is the force–driven harmonic oscillator (FDHO)4 that uses an individual’s body weight and thigh, shank, and foot lengths to determine the preferred walking cadence. This cadence is set with a metronome, and the patient is instructed to step to its frequency. Once the clinician calculates the preferred cadence, the metronome FDHO system can be used at home, with no need for the patient to be at the clinic for gait training therapy. Manipulating the step frequency is thought to alter the sensory and motor neuron signaling, as well as enhance the training stimulus, and the metronomic signaling increases directional attention and focus.4 The motor learning that results from this type of device is thought to promote greater quadricep activity and encourage focused attention on the actions of walking.4 Though this system has shown favorable results with regard to increasing knee flexion and improving gait speed post-ACL surgery, it does not account for the inherent variability of normal, healthy gait, nor does it account for differences in walking speed as the individual is recovering.4
Audio signaling for gait retraining has also been show to effectively increase muscle activation.17 In this work, Petrofsky developed a biofeedback training device to re-educate the neuromuscular activation in patients with Trendelenberg gait secondary to incomplete spinal cord injury. The device provided feedback, in the form of an audio signal, if improper muscular activation of the gluteus medius was noted. The system used electromyography (EMG) to monitor muscular activation, and after two months of home training, the subjects’ gait pattern was nearly fully restored with regard to hip drop.17 In a clinical setting, this muscular activation feedback also showed positive results with regard to developing quadriceps femoris strength in patients following ACL reconstruction.5 Although the quadriceps experience significant functional loss after ACL reconstruction, restoring strength to these muscles can be a challenge if patients have difficulty contracting the knee extensors and instead compensate by recruiting the hip musculature during rehabilitation exercises.
In recent work with gait retraining in ACL-injured and pre-osteoarthritic populations, a new feedback-based monitoring system (Figure 2) has been designed by our research group (described in detail elsewhere).13 This gait retraining system monitors an individual’s gait kinematics and provides an auditory signal if there is limited knee flexion ( < 5°) or high vertical acceleration (greater than 8.6 m/s2) at initial contact (IC), both of which are factors that may affect ROL.36 This gait retraining system is unique as it encourages the subject to develop awareness for the gait kinematics used in locomotion. In 30 minutes of training with this gait monitoring system, healthy subjects were able to effectively reduce their rate of loading (ROL) by 25%, without reducing their gait velocity. Previous research showed no significant kinematics changes by simply wearing this brace without the feedback component compared to normal walking without the brace, but with the added feedback significant changes in gait characteristics were detected, such as a reduced ROL and increased knee flexion. 13 Pilot data of the gait monitoring system suggest individuals are able to gain short-term neuromuscular changes through training. We found that training with the system was associated with an increase in proprioceptive acuity from baseline to post-training.68 In changing the knee joint kinematics, such as knee joint angle and vertical acceleration, and augmenting body awareness through the gait monitoring system, individuals effectively decreased the ROL experienced during walking.13,68
Currently, the potential long-term benefit of this system and the post-ACL injury gait retraining protocols is not just in developing a gait pattern that reduces rate of loading during walking, but also in preventing the early onset of osteoarthritis. Though the results from many of these gait retraining methods are positive, the efficacy of this training is still unknown. Although training with our group’s feedback-based monitoring system was associated with notable changes in the treatment subjects’ gait post-training, it is not feasible to state that this type of training is beneficial post-ACL rehabilitation. Moreover, there are still many questions that remain unanswered: When is the ideal time during rehabilitation to introduce gait retraining devices into the rehabilitation process? Are there specific patient or ACL injury characteristics that are more suited to using the gait monitoring system? Lastly, what are the effects of this type of gait training on an individual’s OA? Future large-scale and long-term studies are needed to address these questions.
Jody L. Riskowski, PhD, CSCS, is an assistant professor in the department of kinesiology at the University of Texas at El Paso, El Paso, TX.
Conflict of Interest Statement: None.
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Figure 1. Typical ground reaction curve demonstrating the heel strike transient. Point A represents the local maximum force generated within the 50 ms after contact. The dashed line represents the portion of the curve used to determine the rate of loading (ROL), and the graph is normalized by body weight (BW).
Figure 2. Subject wearing knee brace with feedback.