July 2009

In the Moment: 0&P

Ankle brace effect travels up
Proximal muscle activity patterns change

by Jordana Bieze Foster

Ankle braces are popular with volleyball players seeking a safeguard against both contact and non-contact sprains. Research from the University of Findlay (OH) suggests that ankle brace wear significantly alters lower extremity muscle activation patterns in volleyball players but, athletes will be happy to know, does not significantly affect vertical jump height.

The investigators analyzed eight healthy female collegiate volleyball players during a vertical jump and a step-down test after random assignment to a braced or unbraced condition. All study subjects had been regularly wearing braces for about two years, two in lace-up styles and six in semi-rigid devices.

Vertical jump height did not differ significantly between conditions, which is consistent with the recent literature. In the early 1990s, two studies did suggest that ankle brace wear was associated with reduced vertical jump height, but several studies published since then have found no effect.

The Findlay researchers, however, did observe significant changes in muscle activation patterns between conditions during both tasks. While jumping, the braced athletes exhibited significantly greater mean levels of lateral gastrocnemius activation and significantly lower mean levels of peroneus longus activity. During the step-down test, the braced athletes had significantly lower activation of the vastus medialis oblique and higher activation of the gluteus maximus than those who were unbraced.

Within the braced group, muscle activity in the gluteus maximus was significantly higher than in the biceps femoris, VMO, or tibialis anterior. Within the unbraced group, activity in the VMO was significantly higher than in the three other lower extremity muscles that were measured.
The findings support the idea that immobilizing the ankle joint has effects at the knee and hip, said Lucinda E. Bouillon, PhD, associate professor of health sciences at the university and senior author of the two studies. Bouillon presented the results in late May at the annual meeting of the American College of Sports Medicine.

“As clinicians, we need to start looking more proximally,” Bouillon said. “When you restrict ankle motion, something further up the kinetic chain is going to give.”

A 2004 Clinical Biomechanics study from the University of Kansas also reported proximal effects of ankle bracing, although that study did not look at muscle activation. Ten healthy subjects performed trunk rotation movements while standing on one leg during braced and unbraced conditions. During ball-catching tasks, the braced condition was associated with decreased trunk rotation; during a task in which the subject touched a target with the shoulder, the brace was associated with increased knee rotation.

The Findlay researcher’s finding that ankle bracing affects VMO activation suggests that patellar tracking may be involved, which may not be not surprising given the prevalence of patellar tendinopathy and patellofemoral pain syndrome in this population. The effect on hip muscle activation might also be related to patellar tracking, since recent research suggests excessive hip internal rotation may play a role in patellofemoral pain syndrome (see “Patellofemoral pain takes turn in spotlight,” page 19).

Another study presented at the ACSM meeting suggests that ankle bracing and taping does not affect postural control. Investigators from the University of Illinois at Urbana-Champaign analyzed 19 young adult volunteers who completed the Sensory Organization Test with ankles braced, ankles taped, or barefoot.

They found that postural sway increased with test difficulty but that the bracing and taping conditions had no significant effect on postural sway parameters or on postural dynamics (approximate entropy of the center of pressure signals). The results suggest that healthy individuals are able to compensate for restricted ankle mobility in order to maintain postural control.

Amputee sprinter’s test data match up physiologically but not mechanically
Scientists who tested and analyzed bilateral transtibial amputee sprinter Oscar Pistorius were surprised to find that running on customized prosthetic limbs was similar to able-bodied running physiologically, but not mechanically.

The test results, which Pistorius’ lawyers used successfully last year to defend his right to compete against able-bodied runners, were e-published on June 18 in the Journal of Applied Physiology. The International Association of Athletics Federations had claimed the Flex-Foot Cheetah prostheses worn by the Paralympian from South Africa gave him an unfair advantage over other athletes.

Comparing Pistorius’ laboratory testing data to previously published data on elite runners with intact limbs, a multi-center team of researchers found that the amputee’s metabolic cost of running was significantly lower than elite intact-limb sprinters but that his velocity at VO2 max was nearly identical, as was his sprinting endurance.

His sprinting mechanics, however, exhibited 14.1% longer foot-ground contact times, 34.3% shorter aerial times, 21% shorter swing times, and 22.8% lower stance-average vertical forces than runners with intact limbs. An amputee’s inability to transfer force through the entire limb may be offset by the reduced time required to reposition limbs but, the authors suggest, may ultimately be a significant limitation for speed.

Drop foot AFO improves clearance, varies joint motion with gait phase
A prototype power-harvesting ankle foot orthosis designed for patients with foot drop doubled toe clearance while timing ankle motion with the gait cycle in a test on a healthy volunteer published in the June 16 issue of the Journal of NeuroEngineering and Rehabilitation.

A limitation of many drop-foot AFOs is that their control mechanisms cannot be varied during the gait cycle, so that a mechanism to block motion during one phase may adversely affect foot and ankle function during other phases. Researchers at the University of Illinois at Urbana-Champaign have designed the Power-harvesting AFO (PhAFO) to limit plantar flexion during swing phase but allow it during stance phase.

The key to the device is a bellow located in the sole of the device’s foot component. Compression of this bellow generates power during mid- and late stance, and the stored pneumatic power then enables an actuated cam-lock mechanism to block plantarflexion during swing phase. Activation of a release valve at heel strike disengages the lock and allows free ankle and foot movement during stance phase.

In the study, the test subject achieved toe clearance of 44 mm, more than double the previously reported average of 21 mm for healthy individuals.

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