Stuart Currie DC,
Director of Research, Sole Supports.
INTRODUCTION
A 2008 critical review of lower limb muscle activation examined the evidence for changes in muscle activation patterns while wearing a foot orthotic1. This should be of great interest to clinicians treating lower extremity pathology with foot orthotics.
Questions to consider when considering the clinical implementation of a foot orthotic include:
· How does a device that is placed in the shoe affect muscular function?
· Which muscles are involved?
· In which patients, and when?
The kinematic (movement) and kinetic (force) effects of foot orthotics on the human body are often widely discussed and debated with broad conclusions being difficult to draw. In addition to the forces and motions involved, as chiropractors, we are interested in the resultant changes in muscle activity. The reaction of muscles (whether activated or inhibited) is a core component of treatment making it very important to know what a foot orthotic does (if anything) to the lower limb musculature.
DISCUSSION
Given the wide variety of foot orthotics, the variable construction, and the different biomechanical theories the literature on muscle activity and foot orthotics can be hard to generalize. A systematic review of the literature revealed that peroneus longus and tibialis anterior EMG amplitude and tibialis anterior duration is greater when wearing foot orthoses2. The duration effects may suggest a greater fatigue reduction with a foot orthotic. This review also concluded there is evidence that foot orthotics affect lower back EMG muscle function. This information can be considered with EMG studies that reveal inverter muscles are increased and evertor muscles are decreased in amplitude with a pronated foot posture2. Tibialis posterior, a stabilizer and supporter of the arch, was demonstrated to be selectively activated in adults with pes planus to a level equivalent to those with a normal arch index while wearing a foot orthotic.3
It is important to keep in mind that any increase or decrease in the measured EMG variables could be beneficial or detrimental, depending on patient specifics such as pathology, foot type, activity levels, and overall condition of the musculoskeletal system.
Do not overlook the role of sensory system in the control of muscles. Changes to muscle activation may be altered by sensory input on the plantar aspect of the foot. One investigation that altered only the texture of the shoe insert found alterations in lower limb muscle activity4. In other words, by changing the texture without changing the geometry of the orthotic, the gait pattern and muscle activity was altered by sensory feedback. This underscores the notion that a full-contact or isomorphic orthotic may be of benefit. Isomorphic contact refers to a custom device that is in contact with the bottom of the foot in a corrected posture during the entire gait cycle, in contrast to an extrapolated or low-arched device that only contacts the plantar surface of the foot after it has pronated significantly. The more contact, the more opportunity to affect change. If the rate or temporal firing patterns of populations of sensory afferents are to be influenced, it makes sense to do this as early in the gait cycle as possible. This also concurs with EMG findings that show a molded orthotic has different EMG findings than a posted orthotic alone5.
Shoe wear is another factor that needs to be considered when relating EMG findings to gait. The recent popularity of various minimalist running techniques leads to a discussion of whether the foot orthotic is a brace. This concept is not new to chiropractors who generally do not prescribe back braces indefinitely with the assumption that it may inhibit the body’s own natural muscular bracing, but in the case of a foot orthotic is generally not supported by the EMG literature which shows an increase in muscle activation with foot orthotics in many cases. Masai Barefoot Technology (MBT) shoes are designed to strengthen the lower limb by providing an uneven surface challenging the muscles to be more active. One EMG analysis showed only tibialis anterior activity was increased during standing and no significant differences were seen walking when normalized to control shoes.6
Heel lifts are another orthotic modification that are prescribed regularly and warrant consideration for the muscular effects involved. Heel lifts have been shown to have an earlier onset of muscle activity in erector spinae during gait and a delay in on the onset of gluteus medius activity7.
We must also consider this research in light of the human body as a whole, with antagonist and agonist muscle groups working together to improve efficiency or decrease tissue stress. It is currently unclear to what degree changes in any one isolated muscle affects the system as a whole. In addition, not all orthotics are created equally. There are different theories and different manufacturing processes that result in different final products. Therefore the literature trends must be interpreted with caution considering differences in subjects (injured vs. healthy), the construction of the device (flexible vs. rigid) and the activity measured (walking vs. running).
SUMMARY
In summary, the following are considerations when evaluating both EMG research articles and your patients:
1) Different foot orthotics can affect the EMG activity in different ways.
2) The results may be specific to a specific patient population or foot type
3) The temporal nature of the EMG change must be considered.
4) The EMG results may be related to very different clinical outcomes depending on the pathology involved.
References
1. Hatton A. Physical Therapy Reviews 2011;13(4):280-93.
2. Murley GS. Gait Posture 2009 February;29(2):172-87.
3. Kulig K. Med Sci Sports Exerc 2005 January;37(1):24-9.
4. Nurse MA. J Electromyogr Kinesiol 2005 October;15(5):496-506.
5. Mundermann A. Gait Posture 2006 April;23(3):295-302.
6. Nigg B. Clin Biomech (Bristol , Avon ) 2006 January;21(1):82-8.
7. Bird AR. Gait Posture 2003 October;18(2):81-91.