September 2009

In the Moment: O&P


Oscar Pistorius. (Photo courtesy of Ossur Americas.)

Oscar Pistorius. (Photo courtesy of Ossur Americas.)

by Jordana Bieze Foster

Amputee runners hit the lab

Pistorius analysis generates ripple effect

Record-smashing Paralympic sprinter Oscar Pistorius has inspired amputees around the world, for both his athletic accomplishments and his willingness to challenge the track and field establishment. Now the South African bilateral transtibial amputee has also inspired an elite group of researchers to explore in detail the differences in running mechanics between prosthetic and intact limbs.

Early last year, the International Association of Athletics Federations ruling that Pistorius was ineligible for IAAF competitions (including the 2008 Olympics) because his prosthetic limbs gave him an unfair advantage over non-amputees. Pistorius successfully challenged that ruling, however, on the strength of a detailed biomechanical analysis comparing him to elite competitive non-amputee runners. Those findings were ultimately published in the June issue of the Journal of Applied Physiology.

The JAP study found that, although Pistorius was similar physiologically to the non-amputees, he differed significantly with regard to three variables that contribute to running speed: longer foot-ground contact time, shorter swing time, and lower stance-average vertical ground reaction force.

Researchers from the multicenter team that conducted the JAP study found these differences intriguing enough to examine them more closely, this time analyzing a group of unilateral transtibial amputees in order to compare prosthetic and intact limbs within each subject. They assessed six unilateral transtibial amputees (four male, two female) as they ran on an instrumented treadmill, starting at 3 m/s and increasing to their top speed.

The investigators found that stance average GRFs were significantly lower in the prosthetic limb than the intact limb, both across speeds and at top speed, which was consistent with what was seen in the Pistorius study. However, they also found that swing times did not differ significantly between limbs at top speed in the unilateral amputees. Step frequency was significantly higher in the intact limb, possibly as compensation for the lower GRF on the prosthetic side, according to co-author Alena Grabowski, PhD. Grabowski, a postdoctoral associate in the Biomechatronics Group at the Massachusetts Institute of Technology in Cambridge, MA, presented the findings in August at the annual meeting of the American Society of Biomechanics.

“The prosthetic limb affects force in the affected limb at top speed. Some athletes may compensate by swinging both legs faster,” she said, noting that asymmetry may be more of an issue for unilateral amputees than for Pistorius. “I think they’re trying to preserve some symmetry, but we’re trying to figure out which symmetry that is.”

One reason for the lower GRFs on the prosthetic side may be that the prosthetic device is a constant stiffness. In non-amputees, eccentric stiffness tends to increase with speed, and is correlated with GRF. But in studying Pistorius, the researchers noted that even at a top speed of 10.8 m/s, he maintained the same spring-like gait seen at slower speeds, according to co-author Craig McGowan, PhD, a postdoctoral fellow in mechanical engineering at the University of Texas at Austin.

McGowan and colleagues observed similar stiffness patterns in the unilateral amputees. Eccentric stiffness increased significantly with increasing running speed on the intact side, but did not correlate with speed on the prosthetic side. At speeds of 7 m/s and higher, eccentric stiffness was significantly higher for the intact limb than the prosthetic limb. As expected, eccentric stiffness correlated significantly with stance averaged GRF for both limbs, although that relationship was stronger in the three fastest amputee athletes than in the three slowest, McGowan said.

“It seems this eccentric stiffness in the biological leg is key for developing high ground reaction force,” said McGowan, who presented the stiffness findings at the ASB meeting. “Because the prosthesis is not able to vary stiffness, that limits the top running speed.”

Better gait-work balance in AFOs could help gait in children with CP

Ankle foot orthoses improve walking speed in children with cerebral palsy and spastic diplegia, but the devices also create an imbalance between limbs in terms of work done during the double support phase of gait, according to research from the University of Nebraska.

Investigators studied 11 children with CP and spastic diplegia, as they walked barefoot and while wearing their own AFOs. Three wore solid AFOs with a pre-tibial strap; eight wore hinged AFOs.

On average, the subjects walked faster and with a longer step length while wearing AFOs than while barefoot. But during douple support, the researchers observed an external limb work imbalance, with the negative work performed by the lead limb significantly exceeding the positive work performed by the trailing limb.

External work was more balanced during single suport, suggesting that this is the source of the reduced metabolic costs associated with AFO use. The findings, presented in August at the annual ASB meeting, suggest that improving double support phase dynamics could further reduce that metabolic cost.

Morton’s extension for hallux rigidus offloads plantar pressures medially

A Morton’s extension orthosis appears to reduce pain and improve function in patients with hallux rigidus by redistributing plantar pressures medially, according to research from Ithaca College.

Maximum mean plantar pressures while walking were compared in 30 patients with hallux rigidus and 18 control subjects, indicating that metatarsal loads were distributed more laterally in the HR patients. Pressure was also significantly increased under the hallux in the HR subjects.

Twenty six HR patients were analyzed again following six weeks of wearing Morton’s extension orthoses. The intervention was associated with significant reductions in pain and disability as measured using the Foot Function Index-Revised.

In addition, maximum mean plantar pressures medialized so that the lateral-to-medial metatarsal pressure ratio was similar to that of control subjects; pressure under the hallux also increased by 18%. The implications of this medial pressure shift, which could include gait alterations and possibly the increased risk of secondary injury, remain unclear.

The findings were presented in August at the annual ASB meeting.

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