By Rich Smith
Children with cerebral palsy (CP) quickly become exhausted from walking because the disease’s impact on the muscles forces these youngsters to consume energy at significantly higher levels than would otherwise be the case. But while the energetics of walking may work against kids with CP, therapists have a variety of interventions at their disposal that can help such children use less energy when walking and, therefore, enable them to ambulate farther and faster before the onset of fatigue.
According to researchers, the average walking speed of CP kids is about 40 meters per minute, or 57% of normal walking speed, yet their mean rate of energy consumption as measured by either oxygen uptake or heart rate is roughly twice than normal.1
“The idea underlying the energetics of walking is that the greater the amount of energy required to walk, the shorter the time and distance an individual will be able to endure ambulation without tiring; conversely, the less the amount of energy required to walk a specified distance, the easier that walk becomes and the more functional it is,” says Jessica Rose, PhD, PT, director of the Motion and Gait Analysis Laboratory at Lucille Packard Children’s Hospital at Stanford University Medical Center in Palo Alto, Calif. “Children with cerebral palsy not only consume more energy per minute of walking but, because they also walk more slowly, their consumption is not very fuel-efficient.”
The explanation for the increase in their energy consumption is the posture of their gait. Studies indicate that CP kids customarily maintain the hip and knee in flexion throughout the gait cycle, requiring greater than normal hip and knee extensor muscle effort for joint stabilization even at reduced speeds.1
“They have to activate their quadricep muscles to hold themselves upright against gravity—it’s the effort to hold themselves up that utilizes a lot of extra energy,” Rose adds.
Diane L. Damiano, PhD, PT, associate professor of neurological surgery at Washington University School of Medicine in St Louis, says that children with CP suffer biomechanical misalignment on the one hand and are denied proper use of their muscles on the other.
“Their muscles don’t turn on and off as they are supposed to,” she indicates. “They’ve got a lot of restraint from muscles on the opposite side of the joint. So whether it’s spasticity or co-contraction, that really slows them down and it forces them to use a lot more muscle to accomplish what an unimpaired child does in walking.”
In normal children, the rate of energy expenditure required for walking per kilogram of body weight decreases as the child grows older. However, the opposite occurs in CP children, notably those with spastic diplegia.1
EFFICIENT ENERGY USE
Research now suggests that physical therapy interventions can help decrease the energy consumption that occurs when CP kids walk.1 The study by Waters and Mulroy1 finds that “patients who have increased cardiovascular fitness are able to tolerate higher rates of energy consumption in order to maintain a more normal walking speed” and that “aerobic conditioning exercise increases cardiovascular capacity, which reduces the relative effort of submaximal work loads.”
Or, as Damiano puts it, “If you had to sprint all the time instead of walk, you’d tire out quickly. But through training, you’d gain strength and be better able to sprint and not become exhausted as rapidly.”
One cardiovascular intervention favored by Damiano is treadmill training. “It can accomplish many things,” she says. “Treadmill training can train the CP child to use muscles in the right patterns in a reciprocal way. It can really give children a great cardiovascular workout, if they are able to do it at a hard enough pace.”
A variant of this is light gait-therapy. Here, children are placed in a halter or harness with their weight supported from above while they attempt to walk on a treadmill, Damiano explains. “We know that if we provide partial weight support, then children don’t co-contract their muscles as much,” she says, “so we think maybe we can do something to help their energy, but the studies haven’t really been done to demonstrate how this goal can best be accomplished with this particular intervention.”
Still, adds Rose, “It’s important for children to practice walking. The goal is to make it more likely that the child will be able to walk and will want to walk as a 4-, 5- or 6-year-old.” To this end, Rose is optimistic about physical therapy programs for children with cerebral palsy that include weight relieved treadmill gait training.
Damiano also thinks aquatic-based walking, running, and strength-and-endurance training can be valuable for CP kids.
“Aquatic work makes perfect sense, and there’s good support for it in the literature,” she says. “But, again, the studies that will show us the best way to utilize it have not been done.”
SPASTICITY CONSIDERATIONS
Beyond cardiovascular work, muscle strengthening exercise programs can also help by enhancing the ability of a CP patient to substitute for gait disabilities.
“It used to be thought that vigorous exercise training would worsen spasticity,” says Rose, who also is the senior research scientist in the Department of Orthopedic Surgery at Stanford University School of Medicine. “But, while the effort of trying to contract the muscles may in the near-term bring on some spasticity, there is no evidence that exercise training increases spasticity. In fact, there is growing evidence that it actually decreases muscle spasticity.”
Damiano says muscle strengthening interventions should focus on improving children’s musculoskeletal alignment.
“You take care of the alignment by stretching out the muscles,” she says. “You increase children’s muscle strength to compensate for some of their other deficits so they don’t have to work at their maximum.”
Rose recommends a regimen that includes hamstring stretches in order to prevent hamstring contractures as the child with CP grows and develops. This, she says, should begin at around age 2.
“Watching and monitoring the length of hamstrings and other muscles as the child develops is very important,” Rose advises. “These kids go through growth phases. Often the growth rate of the muscles, for example, the hamstring length, does not keep up with the bone length. There is a muscle-bone growth rate discrepancy, and you have to be alert to this. It’s critical to treat this very early to prevent contractures of the hamstring muscle, since these cause bent-knee posture, which, in turn, forces the child to use those quadricep muscles and thereby expend more energy.”
ALTERNATIVE OPTIONS
In addition to stretching exercises, Rose also suggests use of casting and, possibly, botulinum toxin A (Botox®, Myoblock) treatment.
“Botox temporarily weakens the muscle and allows the therapist a window of opportunity during which the muscle can be stretched,” she says.
Electrical stimulation therapy also might prove effective in aiding the development of greater muscle strength and endurance, Damiano notes.
“In the CP arena, we’re starting to see more use of electrical stimulation to augment the motor system,” she says.
Then there is a new therapy—constraint-induced therapy. In this intervention, says Rose, “Joints other than the one you want to activate are constrained. The child’s entire attention then can be focused on the lone free joint. This approach has been used in stroke patients with great effectiveness, and now it is being adapted for CP children.”
Another condition that causes excessive energy consumption in CP kids is toe walking. As such, Rose believes that therapists should routinely monitor the CP child’s ankle plantar-flexion contracture.
“Any limitation in joint movement is going to disrupt the fine balance of kinematics and increase the energy costs because it ends up increasing the vertical displacement of the center of gravity,” she explains. “Any time there is an increase in the displacement of the center of gravity—vertical or, for that matter, lateral—you’ll have increased energy costs.”
Research indicates that orthoses and prostheses that substitute for lost muscle function or that reduce lower extremity joint deformities can improve walking efficiency and the associated energy cost.1
“Balance training can be helpful as well,” says Rose. “It’s most important for about a third of the CP children, those with very poor balance. These kids need to be identified early and be offered very finely targeted treatment for their vestibular impairment.”
Rose believes that a growing body of literature points to the likelihood that standard physical therapy treatment of CP children in the future will become more aggressive and more active than is the case today.
“We’re learning that interventions are not really effective if you provide them just two or three times a week for 15 minutes,” she cautions. “A session should last at least an hour (90 minutes would be better) and should occur no fewer than four, maybe five, times a week.”
GETTING STARTED EARLY
Echoing Rose, Damiano contends that therapists have traditionally not been nearly aggressive enough in their interventions, as evidenced by the fact that so many young CP patients experienced muscle atrophy.
“We weren’t getting children moving as actively as they should and as early as they should,” she says. “I predict that, eventually, instead of us worrying so much about movement patterns, we’re going to start focusing more on working at the muscle level, understanding why the muscle isn’t working well and helping children with cerebral palsy strengthen their voluntary muscle activation.”
Soon, the energetics of walking may emerge as the primary outcomes measurement methodology for the treatment of CP kids, Rose asserts.
“It can be used to show whether a treatment and the way you’re utilizing it in these sessions are effective,” she says.
There is, however, some controversy as to which is the better way to measure the energetics of walking. Some say it is best to measure oxygen consumption, while others prefer using heart rates instead.
“There is evidence supporting both sides in this argument,” says Damiano. “But the gold standard for measuring the energetics of walking will probably always be oxygen consumption.” Something Damiano likes in particular about this debate is that it draws on a solid foundation of science.
“We have much clarity about the energetics of walking because of the wealth of literature supporting it,” she says. “But we still have more to learn in terms of which treatments affect energy expenditure and to what extent.
“However, as our knowledge base expands, we must remember to be careful, because walking is not everybody’s goal. For some, walking is just too much work, even if you can improve energy efficiency. We must not lose sight of the real goal, which is always independent mobility, and that may or may not include walking. My hope is that we will take our lead from the patient insofar as setting goals. It should be patients who make the goals, and then it’s up to us as the experts in motor movement and motor function to figure out the best, science-based way to help them achieve their goals."
REFERENCE
Rich Smith is a contributing writer for Rehab Management.
