October 2010

EMG characterizes FAI

In the moment: Sports medicine

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Altered activation isn’t just at the ankle

By Jordana Bieze Foster

Two studies presented at the second International Foot and Ankle Biomechanics conference add to the evidence that treatment of chronic ankle instability should focus on improving feed-forward neuromuscular control mechanisms.

Whereas a feedback response is based on sensory input, a feed-forward pattern is more anticipatory. Studies from Brigham Young University and the University of Toledo both demonstrated evidence suggestive of feed-forward mechanisms at work during jump landings and lateral hops, respectively, in subjects with chronic ankle instability.

Researchers from BYU analyzed electromyographic activity in 10 subjects with functional ankle instability and 10 control subjects as they jumped over a 20-cm barrier onto a force plate, then performed a contralateral hop.

They observed statistically significant differences between groups with regard to gluteus medius and fibularis longus (or peroneus longus) activity. Mean gluteus medius preactivation occurred 57 msec earlier in the control subjects than in the FAI subjects. And the mean fibularis longus EMG signal had almost twice the amplitude in the FAI subjects as the control subjects during the landing phase.

Researchers from the University of Toledo observed similar differences activation patterns between 16 subjects with chronic ankle instability and 16 healthy subjects as they performed a series of five lateral hops. The lateral hop test was designed to combine the biomechanics of landing from a hop with those of a lateral cutting maneuver—both of which have been associated with ankle sprains. Average EMG was collected during the 200 msec prior to landing.

As in the BYU study, the peroneus longus demonstrated significantly greater muscle activation (as a percentage of peak muscle activation) in the CAI group than the control group. The same was true for gluteus maximus muscle activation, although the Toledo researchers did not find significant differences for the gluteus medius.

The findings suggest that subjects with ankle instability are actively changing their muscle activation patterns in anticipation of landing, in such a way as to lower the risk of an ankle sprain, according to Kathryn Webster, PhD, ATC, who presented the Toledo findings at the i-FAB meeting. Webster, who completed the research as part of her doctoral thesis at the University of Toledo, is now a clinical assistant professor in the Sargent College athletic training program at Boston University.

The distal musculature is likely working to evert the foot, while the proximal musculature is likely working to prevent femoral internal rotation, Webster hypothesized.

The changes in proximal activation may also be related to hip adduction or repositioning of the center of mass over its base of support, according to Mark Coglianese, a graduate student in the department of physical education at BYU, who presented his group’s findings at the i-FAB meeting.

Both presenters agreed that their findings supported the use of feed-forward focused neuromuscular training, emphasizing both distal and proximal muscle groups, in patients with ankle instability.

“Clinicians should continue closed kinetic chain rehabilitation, including gluteus medius and gluteus maximus exercises, in patients with chronic ankle instability to support their neuromuscular ability to stabilize the ankle,” Webster said.

Interestingly, the BYU study found that the significant between-group differences occurred only during anticipated landings. The study protocol also included unanticipated landings, in which subjects received an in-flight visual cue telling them whether to land on the right foot, left foot, or both feet. No significant differences were observed during the unanticipated trials, however.

Similarly, the Toledo study found no significant effects of fatigue on the observed activation patterns. All subjects performed a functional fatigue protocol that included a circuit of 30 hops over a 10 cm bar, followed by a running segment that required the subject to negotiate a series of large plyometric blocks.

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