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The Biomechanics of Motion

How a simple exercise could keep your body moving in harmony


At historic Fenway Park, Red Sox fans don’t sit comfortably. The seats, crammed in during the early 20th century, offer very little wiggle room or legroom. When stragglers idle back mid-inning with drinks and dogs, the whole row has to rise as the latecomers squeeze and sidestep past.

In summer 2010, Cara Lewis and a fellow physical therapist watched the regular up and down of row after row as the Sox took on the Arizona Diamondbacks. As the pitches flew, they started talking about the muscle activity involved in the Fenway side step. The hip abductors were getting a workout, from the posterior gluteus medius, which wraps from the rear to the outside of the thigh, to the tensor fascia lata, a small muscle on the outer hip.

They’re the same muscles that can cause, among other things, iliotibial band problems (leg pain common in runners and cyclists) and patellofemoral pain syndrome (that dull ache in the knee you might feel when jogging or descending stairs). In many clinics, the exercise of choice for such conditions is a resisted side step. Physical therapists loop a TheraBand, an elastic resistance band, around a patient’s legs and ask them to step from side to side. As they step, they have to stretch the sophisticated rubber bands, available in a rainbow of colors and resistance levels, forcing the leg’s muscles to work harder—an effect that’s also made the bands popular at-home exercise tools.

But as they sat watching Fenway Park’s sidesteppers, Lewis and her colleague began to wonder which hip muscles the bands were actually working and whether they were being used in the most effective way.

“When I treated patients in the clinic, I usually put the band around their ankles, but why?” says Lewis, a Sargent College assistant professor of physical therapy and athletic training who started her career in professional practice. Almost every clinic and gym has bags full of the bands, but no common guidance on their usage.

In a National Institutes of Health–funded study, Lewis tested the bands every which way: around toes, ankles, and knees; while standing and squatting. “You have to stop and think about what you’re doing and where it’s actually working—put a little biomechanics back in the picture,” she says of her research aims. “What are the forces, what’s each side of the body having to do?”

In a series of lab-based exercises, she determined the best way to rehab hip abductors, from where to place the band to how to move each leg. She also discovered that the leg doing the moving might not be getting a great workout after all. By using biomechanics to test conventional treatments and exercises, Lewis consistently uncovers new ways of helping patients. And her work isn’t limited to just intervening after a knee gives out or a hip locks up—she’s pioneering rehabilitation as prevention to ensure fewer people get injured in the first place.

The Human Machine

Cara Lewis, Boston University Sargent College assistant professor of physical therapy and athletic training stretches a TheraBand

In a National Institutes of Health–funded study, Cara Lewis determined the best way to rehab hip abductors using the TheraBand. Photo by Dan Aguirre

Fixing things runs in Lewis’ family. When she was growing up, Lewis used to watch her grandfather coax car engines back to life. He was an auto mechanic—and occasional racetrack pit crew member—and Lewis was captivated by the combustion engine’s ballet of valves and pistons.

“I always thought being a mechanic was so cool because I loved looking at how things moved, how they interacted,” she says.

The gadgets assembled in Lewis’ Human Adaptation Lab at Sargent College—digital motion capture cameras, an electromyography system, signal conditioning amplifiers—shed light on how components in the human machine work together. And they help Lewis figure out ways to prevent our bodies from breaking down.

In the side step study, Lewis and her team of researchers placed electromyography pickups on subjects to record “the electrical activity in the muscle, which then tells you how active the muscle is.” Participants also wore small reflective markers. Ten cameras nestled close to the lab’s ceiling followed the reflective markers, recording the images and tracking the movements using a 3D motion capture system so Lewis could determine the subjects’ position and how they moved.

She asked the subjects—all healthy, so results weren’t skewed by existing neuromuscular conditions—to sidestep across the lab and then make their way back with a resistance band looped around their knees, ankles, or toes. This test allowed researchers to see each leg stand and move in both directions. They’d then do it all again (and again) with the band in a different position. They repeated the whole scenario one more time, too: if a subject went through the routine standing up, Lewis would ask them to squat and repeat. Lewis says the standing and band positions were randomized in case one position affected the next.

The step distance was not randomized. As the subjects moved through the room, they were told to step one floor tile at a time. “Some studies will do it on leg length and all these other parameters,” says Lewis. “We just used the floor tiles to be practical. In the clinic, you’re not going to sit there and put all these different side step lengths down on the floor when you have floor tiles and you can say, ‘OK, step this far.’”

She expected to see the hip muscles working harder as the band moved lower down the leg, but wasn’t sure if moving it from ankle to toe would make a significant difference.

“From a biomechanical standpoint, when you get farther from the joint, you would assume you’ve increased your lever arm—that is, the distance from your joint—and that’s going to increase the torque created by the band,” says Lewis. She explains that’s why most doorknobs are opposite the hinge and not in the middle. “But when you go from ankle to toe, we weren’t really sure what was going to happen because the increase in lever arm isn’t a lot.”

The Power Needed to Stand

After collating the results, Lewis found that for most conditions it’s better to step while squatting rather than standing and that the toe is the best place to put the band.

“When you go out to the toe,” she says, “the band pulls your toes in, creating this internal rotation of your legs.” When that happens, the external rotators—the gluteus medius on the outer pelvis and the gluteus maximus, the most powerful hip muscle—activate to stop the toes from turning. But, importantly, the tensor fascia lata doesn’t have to work as hard.

“As you go from knee to ankle to toe, the gluteals all increase,” says Lewis. “The tensor fascia lata increases from knee to ankle but not from ankle to toe. A lot of these musculoskeletal conditions that we try to rehab, we’re trying to get more gluteal activity and not more tensor fascia lata activity. By putting the TheraBand around the toe, you’re getting that.” Most home exercisers using resistance bands to strengthen leg muscles should put the band around their toes for the same reason.

For some, the most surprising conclusion from the study could be Lewis’ finding on which leg is getting the biggest workout during the side step: the one standing still. “As a biomechanist, it makes perfect sense to me,” says Lewis. “The leg you’re standing on has to stabilize your trunk; all you’re having to do is move the other one. You’re really strengthening the leg you’re standing on more than the leg you’re moving.” The result is one she hopes reminds clinicians—and fitness fanatics—to think more often about the body’s mechanics as it goes through common exercises.

Diagram showing the most effective way to use a TheraBand for hip abductor rehabilitiation and strengthening

In a study using electromyography technology, Cara Lewis determined that the most effective use of the TheraBand is to step while squatting with the band around the toe, as opposed to positioned around the ankle or knee. Graphic courtesy of Cara Lewis

It’s not unusual for Lewis’ research to bring such practice-changing conclusions. In a concurrent study, she tested the mechanism of another popular drill, the single leg squat—one leg off the floor, the other bending at the knee and pushing back up again. What nobody seemed to have thought about before, says Lewis, is the nonstanding leg’s position. And yet it has a big impact on how the exercise works.

“If I stand on my right leg and hold my left leg out in front of me, just to hold my leg out there, I’m going to tip my trunk back a little bit, so I squat differently,” she says. “The second I put it behind me, I lean forward more as I squat and I also drop my hip a little bit more.”

For example, if a clinician were treating someone with femoroacetabular impingement, a common cause of hip osteoarthritis, they would want to avoid holding the air-bound leg back: it would force the hip into an injurious position. “A single leg squat with your leg behind you seems to work the quadriceps muscle more,” says Lewis. “A single leg squat with your leg in the middle or out in front works your gluteals and hamstrings more.”

More Than Treatment

At Sargent, Lewis has the opportunity—albeit between teaching courses (she also holds an assistant professor position on BU’s Medical Campus) and directing a lab—to study the efficacy of exercises and treatments many take for granted. Clinicians in the field aren’t always so blessed, says Lewis. To give her students the best start and “encourage a spirit of inquiry,” she urges them to play an active role in her research.

“It gives you a better appreciation for interpreting evidence,” says Lewis of involving students. “If we’re all going to be evidence-based practitioners, we’ve got to understand the process and we’ve got to be willing to take part in that.”

Two students, Hanna Foley (SAR’15) and Theresa Lee (SAR’15), did much of the work on the side step study and were listed as coauthors when it was published in the Journal of Orthopaedic & Sports Physical Therapy. After being involved in the project, Foley changed her plans and now hopes to work toward a career as a researcher. The study results have already shaped her clinical work.

“My clinical experience is in the outpatient orthopaedic setting,” says Foley. “I implement our findings with patients who need gluteus medius strengthening. It is very rewarding to have been a part of the research that supports the exercises I’m prescribing for real patients.”

Lewis also hopes her students take away another important lesson: physical therapy is about diagnostics as well as treatment. When she was first studying the subject, Lewis remembers, “we were talking about how people walk, and I was one of the ones put up onstage to walk in front of everyone. Not because I did it correctly, but because I didn’t know how to push off with my feet.”

In a study with her postdoctoral mentor, Lewis discovered that pushing off harder from the feet when walking reduced stress on the hip joints. She changed the way she walked, lessening her chances of future hip and back pain.

“I really think physical therapists should be playing a bigger role in prevention. If you think about it, you go to the dentist twice a year, you brush your teeth every day; why don’t we take better care of our musculoskeletal system in a similar way? Why don’t we go to physical therapists at least once a year to find out what we could be doing better?”

When it comes to hip pain, most think about seniors creaking along until they need a joint replacement; Lewis aims to ward off the problems before they begin. Other projects in her lab include a study of how male and female hip structure changes in adolescence and a five-year investigation of young athletic adults with hip pain. She’s also planning to collaborate with a mechanical engineer to model bone growth to better understand the forces that may cause bone to develop abnormally.

“It’s exciting to think we don’t have to injure ourselves to see a rehabilitation specialist,” says Christopher A. Moore, dean of Sargent, of Lewis’ impact on the wider field. “This approach broadens the appeal of Dr. Lewis’ work to many more people—young and old, injured and not injured.” Moore adds that the analytical and mechanical approach taken by Lewis “requires the skills of a counselor, a therapist, an engineer, a physiologist, and a good bit of Dr. Emmett Brown,” Back to the Future’s resourceful inventor. “She’s one of the rare scientists who has all of those capabilities.”

Andrew Thurston can be reached at thurston@bu.edu.

A version of this article was originally published in the 2015–2016 edition of Inside Sargent.


One Comment on The Biomechanics of Motion

  • Francie King on 07.28.2016 at 10:34 am

    Andrew, this was terrific. I learned a lot (some of it surprising, too), and understood what I’ve been doing wrong with the bands! Great article!

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