Foot orthosis experts explore role of muscles

[Guest guest]

September 2005

http://biomech.com/showArticle.jhtml?articleID=170701855

Foot orthosis experts explore role of muscles

By: Charlie Kupperman

With the American Orthotic and Prosthetic Association and Pedorthic

Footwear Association annual meetings coming up, this is an

appropriate time for our second O & P focus section of the year.

The value of foot orthoses is widely known and accepted.

Practitioners have long thought the devices helped patients by

affecting their joints and skeleton, yet new research suggests they

may in fact work on the patient's musculature. While more research

needs to be done, it's a fascinating, healthy research-oriented

debate.

Although people may have differing opinions about the situation in

Iraq and Afghanistan, the need to treat the returning soldiers and

provide them with the best technology and rehabilitation is

unquestioned. The way practitioners are using their experience

working with military patients and taking it back to help their

civilian patients is pushing the profession forward. And, as always,

the issues of reimbursement and L-codes are covered.

(Next month, we have a special section on footwear, complete with

source list. - R. )

The conventional wisdom says that foot orthoses align the skeleton

and affect joint motion, but a new generation of researchers is

challenging that view.

The most recent body of research is finding that foot orthoses have a

much greater effect on muscle activity than on joint motion,

according to Anne Mundermann, PhD a professor of mechanical

engineering at Stanford University.

Jay Hertel, PhD, ATC, an assistant professor of kinesiology and

director of the athletic training research laboratory at Pennsylvania

State University, confirmed Mundermann's analysis.

" What we're seeing is an effect on the neuromuscular system as

opposed to the skeleton, " Hertel said.

Recent research certainly backs up the idea that the skeletal effects

of foot orthoses are minor and not consistent.

A study in the March issue of the Journal of Biomechanics found no

significant differences in joint coupling pattern or variability when

11 subjects with a history of lower extremity injury wore either of

two orthoses (standard or inverted) or between those subjects and 11

healthy controls. The researchers concluded that foot orthoses don't

cause significant changes in rearfoot-tibial coupling.

A January 2000 study in Clinical Biomechanics used medial foot

orthoses and found that effects on foot eversion and tibial rotation

were small and not systematic in the stance phase of running.

Differences among the five subjects in the study were much greater

than differences between the orthotic and control conditions. The

study found significant orthotic effects only for total internal

tibial rotation.

But Mundermann and other researchers did find small and systematic

results in a group of 21 pronating runners wearing three orthoses

fabricated for different effects (Tables 1 and 2). The posted

orthoses reduced maximum foot eversion and maximum foot inversion and

increased vertical loading rate and maximum knee external rotation

moment. Orthoses with molding and posting reduced vertical loading

rate and ankle inversion moment and increased maximum foot inversion

and maximum knee external rotation moment. The results were published

in the March 2003 issue of Clinical Biomechanics.

The major difference between this study and studies that failed to

find a systematic difference is the rigorous inclusion criteria,

according to Mundermann. Researchers should exercise great care when

making any generalizations or conclusions based on other studies that

had fewer-or no-inclusion criteria, she said.

Preferred movement path

A few theories try to explain the changes in muscle activation with

foot orthoses. Researchers led by Benno Nigg, PhD, a professor of

biomechanics at the University of Calgary, have advanced the theory

that every joint has a " preferred movement path. " If an orthotic

device supports a joint's preferred movement path, then it will

reduce muscle activation. But if an orthosis counteracts the

preferred movement path, then muscle activation will increase to

maintain it, according to the latest article by Mundermann and Nigg,

currently in press with Gait & Posture.

Another possible explanation is that differences in footwear material

may alter the input signal into the body and require different muscle

activations to damp down the soft tissue vibrations of the lower leg,

Mundermann said.

In the Gait & Posture study, the same group of 21 pronating runners

successfully maintained their " preferred movement path, " while

wearing the three orthoses. The study found significantly greater

muscle activation, including an increase in global electromyographic

intensity, with the test orthoses than the control condition. But the

runners typically showed less than 2 degrees of change in their

pronation, according to the researchers.

" The relatively small differences in joint motion between foot

orthoses in our studies do indeed suggest that our subjects

successfully tried to maintain the preferred movement path, "

Mundermann said.

But she noted that studies in which subjects wore orthoses for long

periods of time might show more correction of their pronation, which

some experts believe can cause injuries to the medial ligaments of

the foot.

The study found that posting and custom molding of the orthoses

increased the global EMG intensity of most muscles of the lower

extremity for the stance phase of running. In addition, EMG

intensities in the peroneus longus and gastrocnemius medialis muscles

were significantly greater for the posted orthoses than the other two

test conditions during the latter phases of stance. Orthoses-related

increases in EMG intensity were greater in the high- than in the low-

frequency bands.

In the posted condition, the highest global EMG intensity increases

pre- and post-heel strike occurred in the peroneus longus and biceps

femoris muscles, but only small changes occurred in the tibialis

anterior muscle. In the molded condition, the highest increases in

global EMG intensity pre- and post-heel strike occurred in the

peroneus longus, gastrocnemius medialis, and biceps femoris muscles.

Intensity increases

Mundermann and Nigg speculate that muscle activation for muscle

tuning may have caused increases in EMG intensities immediately after

heel strike. Their 2003 study in Clinical Biomechanics found that

vertical ground reaction forces and loading rates were higher for the

posted than for the molded or the posted-and-molded conditions. They

hypothesize that the calf muscles may be working harder in the posted

condition to damp soft tissue vibrations.

" The increased EMG activity of the shank muscles in the posted

condition may have been required to dampen soft tissue vibrations as

the greater loading rate may have moved the frequency of the ground

reaction force closer to the natural frequency of the muscle packages

of the lower extremity, " they wrote.

A study of 12 recreational runners with lower extremity pain and

structural malalignment of the foot, published in the May 1999 issue

of the Archives of Physical Medicine & Rehabilitation, found

significant increases in EMG for the tibialis anterior and hamstring

muscles when orthoses were worn.

The increases in tibialis anterior and hamstring activity may be due

partly to the interface of the firmer material of the orthosis with

the ground and the need to contain the impact, said that study's lead

author, Deborah Nawoczenski, PT, PhD, a professor of physical therapy

at Ithaca College. The orthoses may also be helpful in slowing or

preventing tibial rotation, which may be linked to excessive

pronation and may affect the kinematics of the knee. But slowing

tibial rotation may also increase the risk of tibial stress

fractures, she said.

Nawoczenski cautioned that EMG doesn't necessarily correlate directly

to muscle force. Researchers often assume that increased EMG means

more force is coming out of a muscle, but that's not always the case.

Pronation effect

The peroneus longus muscle probably has a greater effect on pronation-

and also on tibial rotation-than the tibialis anterior muscle,

Mundermann said. So it may be possible to control pronation without

increasing stress on the tibia. Also, other variables, such as

loading rate, may have an important effect on tibial stress, and her

studies found that foot orthoses reduced loading rate.

A study led by Hertel, published in the January issue of the Archives

of PM & R, also found significant increases in EMG intensity with off-

the-shelf orthoses, regardless of posting or foot type. The orthoses

enhanced vastus medialis and gluteus medius activity during slow,

controlled exercises such as the single-leg squat and lateral step-

down. But the orthoses didn't cause a similar EMG increase for a more

explosive exercise such as the maximal vertical jump.

" We just used a very short-term orthotic intervention. We don't know

if muscle activity will still be changed a month after they've been

wearing it, " Hertel said.

The EMG activity may be more affected by the different sensory

information the runner receives from the plantar cutaneous surface of

the foot, which leads to a change in behavior, Hertel said. Thus,

wearing an orthosis may cause only the same changes as wearing a pair

of hard-soled shoes. There's not enough evidence yet to show the

orthoses are actually changing motion enough to influence muscle

activity, he cautioned.

To back up this view, that orthoses prompt a change in proprioception

rather than a correction of movement path, Hertel has research in

progress showing that patients achieved comparable improvements in

balance simply wearing beach sandals with hard struts that press

against the soles of the feet (in place of softer shoes) as they

experienced wearing orthoses.

Researchers need to explore the impact of foot orthoses on pronating

runners in more detail, Mundermann said, before they can draw any

conclusions. But she pointed out that there's no conclusive evidence

linking pronation with an increased risk of overuse injuries.

(Charlie Kupperman is a freelance writer based in San Francisco. )

Table 2. Foot orthoses study information

Participants : 9 men, 12 women

Pronation : 13 degrees at 4 m/s

Ave weekly running distance : 15 to 40 km

Running sandal : Bryce Canyon (Rockport), original inserts removed

Source: Mundermann A, Wakeling JM, Nigg BM. Foot orthoses affect

frequency components of muscle activity in the lower extremity. Gait

Posture 2005, June 7 (Epub ahead of print)