February 2018

Implications of asymmetry in the treatment of injured athletes

Given that many uninjured, successful athletes have some degree of asymmetry, how do clinicians interpret asymmetry in athletes recovering from injury?

By Cary Groner

Champion track sprinter Usain Bolt has what was known colloquially as a “hitch in his get-along.” Bolt, who retired from competition in 2017 after an extraordinary athletic career that includes gold medals in the 100- and 200-meter events in three consecutive Olympic Games, has scoliosis, and consequently his right leg is approximately one half-inch shorter than his left leg. 1 The result is an asymmetrical stride. Researchers found that Bolt struck the ground with 1,080 pounds of peak force on his right leg and 955 on his left. 2 Asymmetry occurs in healthy athletes as well as those who are rehabilitating from injury. Assessing the importance of asymmetry in injured athletes is, therefore, not straightforward.

A host of variables

Research has demonstrated statistically significant differences between left and right sides in both kinetic and kinematic measures in healthy subjects and have reported that people typically use different legs for stabilization, braking, or propulsion while walking, even controlling for dominance. 3-5 This functional asymmetry appears to be common.

Turning to asymmetry in athletes, Scandinavian researchers reported that 11 of 22 competitive sprinters displayed large or very large asymmetry in at least 11 of 14 variables measured and that all of the athletes had asymmetry in at least three variables—but found no significant correlation between the magnitude of individual asymmetry and sprint performance. 6 Other research suggests that asymmetry does not affect sprint performance because the ankle joint may regulate its effect. 7

Can various measures of symmetry be used to predict reinjury to the lower extremity in sports?

Fatigue-induced changes in gait may increase asymmetry in knee movement patterns. Using both kinetic and kinematic measures to assess asymmetry, researchers found that variables previously associated with overuse injuries (eg, knee internal rotation, knee stiffness, loading rate) were significantly different between limbs in both rested and fatigued states. 8

“We did find asymmetry in a healthy population,” said Kara Radzak, PhD, ATC, the paper’s lead author. Radzak, assistant professor in the Department of Kinesiology and Nutrition Science at the University of Nevada Las Vegas, said, “I think our most interesting finding was that some gait factors become more symmetrical with fatigue and others become less symmetrical,” she said. “The variables that got more asymmetrical were those associated with knee overuse injuries, those based on eccentric control of the musculature. Conversely, variables such as loading rate became more symmetrical.”

Radzak believes such changes indicated how those in the study population modulated fatigue. “As we move from a rested to a fatigued state, we may use up our ability to eccentrically control ground forces and go to more of a bony-absorption mechanism,” she explained. “What that tells us is that we need to be looking at patients in multiple states, because their motion may be different when they’re rested and when they’re tired.”

Sport-specific symmetry

iStockphoto.com 143918199

Other research has produced heterogeneous results about the prevalence of gait asymmetry, as well as implications for athletes in various sports. A 2006 study reported that previously injured runners may remain closer to injury threshold than those without an injury history and that asymmetry may influence only the side on which a subsequent injury occurs. 9 A 2008 paper found that although some degree of asymmetry is normal in runners, those with a history of injury, certain variables (hip internal rotation range of motion [ROM] and peak tibial acceleration) were elevated on the injured side, and that such variables were often bilaterally elevated in injured runners as well. 10

A study of uninjured professional rugby players reported that 55 of 60 variables studied indicated symmetry (the players performed a drop landing, a hurdle hop, and a cut), whereas the other five variables were asymmetrical. 11 Greek researchers concluded that of seven variables studied in young male runners—leg stiffness, vertical stiffness, contact time, flight time, maximal ground reaction force (GRF) during contact, vertical displacement of the center of mass, and change in leg length—significant asymmetry was found only in flight time and maximal GRF. 12

Other research on sport-specific symmetry has found that—

  • vertical-jump asymmetry is typically 10% – 15% in both male and female volleyball players; 13
  • asymmetrical step lengths, possibly resulting from underlying imbalances, existed in 12 of 35 race walkers in the United Kingdom;14
  • professional soccer players tend to display lower isokinetic strength asymmetries the longer they’ve been in the sport; 15
  • in 21 healthy young athletes, 26% exceeded 10% asymmetry in quadriceps and hamstring muscle volume (with 10% considered a normal variation); 16 and that
  • over short distances, gait asymmetry in runners with a history of injury was higher than in those without an injury (the authors did not determine whether the asymmetries preexisted or resulted from the injury). 17

In the clinic

What does all this mean to clinicians? Symmetry measures are often used to gauge recovery after injury, particularly those to the anterior cruciate ligament or Achilles tendon. But if some degree of asymmetry is normal in healthy individuals, how do clinicians interpret asymmetry in athletes recovering from injury?

“It’s a great question,” acknowledged Robin Queen, PhD, associate professor of biomedical engineering and mechanics at Virginia Tech and director of the university’s Kevin P. Granata Biomechanics Laboratory. “Paterno showed that side-to-side asymmetry was a risk factor for a secondary tear, 18 but it’s really hard for us to explain to a clinician what that means. How do we know that a person in rehab with 12% asymmetry didn’t start out that way before their ACL tear? The goal in rehab is usually to get patients to 90% symmetric, at which point most clinicians would release them for return to sport [RTS]. But why do we have so many secondary tears? I think we have to consider that maybe it’s not about distance but about how patients are accomplishing a given task, and that’s why we’re working to find a metric around movement quality.”

Asymmetry can be measured in various ways—eg, strength, ROM, muscle volume, kinetics, kinematics—and Queen thinks opinions are still evolving about the most effective approach.

“Most clinicians have been clearing patients for RTS based on quadricep/hamstring ratio and side-to-side strength deficits,” she explained. “Strength assessment in the clinic is important, but when we ask patients to do something more functional, we need to know if the neuromuscular system is really reacting the way we expect it to. If there is kinesiophobia—fear of movement—we may need to retrain them so that they understand that the limb is strong enough to take that load.”

Queen noted that some variability is not only natural but might also be desirable. “If you only know how to accomplish a task in one way, what implications does that have for a secondary injury?” she asked.

In her own research, Queen has shown that a year after anterior cruciate ligament reconstruction (ACLR), subjects still had significant between-limb asymmetry in several measures. 19 “It’s pretty clear that six months isn’t enough recovery time—certainly in terms of movement, but also in terms of graft vascularization,” she said. “Even at 18 months, at two years, they’re not moving at what we want to term ‘normal.’ I think we need to combine the worlds of injury prevention and postinjury therapy to get them playing again without confusion about the risk of a secondary tear.”

ACLR issues

Additional research demonstrated that hamstring strength asymmetry persists three years after ACLR with a hamstring tendon autograft, and affects involved knee mechanics during gait and jogging. 20 “Even very high-level athletes, like those in the NFL, may take two years to get back—and these are guys who have a trainer, a physical therapist, a strength coach, a physician, a whole team working with them every day, guys who are motivated because it’s their living,” said Timothy Hewett, PhD, director of biomechanics laboratories at the Mayo Clinic in Rochester, MN, and the clinic’s John and Posy Krehbiel endowed professor of orthopedics.

Hewett acknowledged that existing measures of symmetry, often called limb-symmetry indices or LSIs, often aren’t adequate for the task, and research supports this position.21-22

“These measures are problematic, partly because people may naturally have a 10% variation between limbs, and the error in our measurements is 10% – 15%,” he said. “So that difference may not be meaningful, or it may be within the scope of error. Symmetry is important, though—we know that asymmetries in movement and kinematics are some of our best predictors for subsequent injury—so we need other measures as well.”

According to Hewett, the loss of an ACL also entails the loss of proprioception provided by the mechanoreceptors in the tendon, and this has repercussions throughout the kinetic chain—including surprising effects on symmetry.

“Those receptors have multiple functions including spinal-level feedback loops that send information about position, force, and torque on the ligament,” he said. “But an ACL reconstruction doesn’t restore those receptors, so neurosensory deficits arise. Your body can’t increase performance on the injured side, but it still seeks symmetry, so it decreases performance on the uninvolved side.”

Hewett noted, moreover, that the body’s tendency to seek symmetry postinjury may lead to complex adaptations through the chain.

“What’s primarily important is the transfer of force through the ankle, knee, and hip,” he explained. “If you look at total force generation across those joints, it is usually highly symmetric, even though the individual joints may not be. That means that if you have a deficit in your knee on one side, the body increases the force generation at the ankle.”

This may be a good thing, of course, but it may also lead to improper joint mechanics, inflammation, and osteoarthritis, as well as an increased risk of reinjury. 23

“When your hip and knee go through greater ROM during a task, it’s going to dissipate the ground reaction forces, but after ACLR, when you don’t sense that joint as well, the ROM decreases in the hip and knee, and the GRF tends to be higher,” Hewett said. “You also tend to favor the uninjured side because you feel it more, so then the forces on that side go up, creating even more asymmetry, which we think increases the risk of a subsequent contralateral tear.”

In 2010, altered neuromuscular control of the hip and knee following ACLR were reported as predictors of a second ACL injury, 24 and earlier research found that neuromuscular control and valgus knee loading predicted ACL injury in female athletes 25 and that limb asymmetries persisted at two years post-ACLR. 26 A 2016 meta-analysis in Sports Medicine, moreover, found that knee function is not fully restored five years post-ACLR,27 and several papers have reported similar effects on symmetry in the quadriceps, including measures of strength, volume, and neural function. 28-30

Christopher Kuenze, PhD, an assistant professor of kinesiology at Michigan State University and lead author of several of those papers, told LER that he and his colleagues were interested in a variety of factors related to asymmetry. One question was why post-ACLR patients tended to offload the involved limb, consciously or unconsciously, even after completing rehab.

“Our hypothesis is that it’s likely due to pain, discomfort, and trust in the limb, and I think it becomes a learned behavior over time,” he said. “It would be great if you had a physical therapist in your closet you could take out when you need to, because I think this kind of problem needs to be treated more like a chronic disease than an acute injury.”

Kuenze believes that clinicians and researchers should choose assessment variables based on the long-term outcomes they want to measure.

“If your concern is reinjury risk, then you probably want to measure functional symmetry early in the process leading up to return to sport,” he said. “If you’re more interested in long-term joint health, you may want to assess symmetry during activities of daily living, things like walking gait.”

He echoed Tim Hewett’s concern about the body seeking symmetry by weakening the uninvolved side, as well.

“If your injured limb isn’t functioning well and you’re symmetrical, it means your uninvolved limb is probably not functioning well either,” he said. “So whether it’s reinjury risk or long-term joint health, it’s not an optimal situation.”

The Achilles heel

The Achilles tendon also can affect symmetry, even when uninjured. In healthy athletes without a history of Achilles injury, the amount of asymmetry between legs rose as high as 31% in terms of mechanical and morphologic properties. 31 A 2017 study from Brazil, moreover, found that injured patients who underwent surgical Achilles repair retained long-term asymmetries in ankle stiffness and plantar flexor function. 32

“When you have an injury to the Achilles, the tendon lengthens and you have permanent changes to the ankle that affect function,” 33 said Karin Grave Silbernagel, PT, PhD, an assistant professor of physical therapy at the University of Delaware in Newark.

“Your body compensates by using the knee, the ankle, and the hip, which is probably a good accommodation,” she continued. 34 “The problem is that clinicians often think that if the patient doesn’t achieve symmetry in the ankle, they shouldn’t return to sport. But the nature of an Achilles injury is such that they will never be able to do as high a heel lift as on the uninjured side, so that can’t be the basis of your RTS decision. Instead, you can measure the total amount of reps at a certain height, or how far they hiked. When the patient is ready, he or she should return to running or jumping; they just need to know that the knee will be working harder.”

A relationship between symmetry and fatigue also has a role in the RTS decision. “We guide return to sport based on how long you can keep your running pattern without losing symmetry,” Silbernagel said. “We know that the amount of load matters in the recovery process. If they do too much they get worse, but if they do the right amount they can build and get better. If you don’t have good form, you may be overloading the tendon—and that’s where we should stop you.”

Symmetry in return to sport

In determining when an injured athlete is ready to return to sport, Silbernagel considers several factors. “Overall, we want to use a test battery that includes range of motion, strength, endurance, jumping performance, all those things,” she said. “You need to focus on the task at hand, and if you can’t achieve it, is there a way for the body to compensate? Because if there isn’t, then the patient is put at a much greater risk of reinjury or failure.”

Chris Kuenze and his colleagues consider RTS a staged approach. “Our first goal is to restore involved limb muscle function,” he explained. “Second is to restore the limb’s functional performance on tasks like hops and stair climbing. Third is to restore between-limb symmetry. The hope is that by restoring involved limb function you will have already achieved goal three, but it doesn’t always work out that way. What you don’t want is to achieve symmetry without restoring function in the injured limb, for all the reasons we’ve discussed.”

“We need to look at people more holistically,” said Robin Queen. “We can’t just focus on the part that was injured; we have to consider what’s above and below it in the kinetic chain, as well as the contralateral limb. That’s the way to get the athletes back to feeling confident, to knowing their body will respond when they’re back on the field.”

“We have to come up with multiple ways to create parallel circuits to allow our bodies to sense movement and forces, and to adjust,” he said. “We want to test people before they return to sport using four or five factors that we know lead to increased risk, 35 then use rehabilitative tools to enhance those to decrease risk. The idea is to deal with strength, symmetry, and kinematics, but also to look at proprioception and kinesthesia. We have to consider all options because the risk of re-injury is just too high.”

Cary Groner is a freelance writer.

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One Response to Implications of asymmetry in the treatment of injured athletes

  1. What about heel lifts before, during or after an injury?

    Dennis Shavelson. “The Inclined Posture (Tip): A New Clinical Entity”. EC Orthopaedics 3.4 (2016): 359-364.

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