Shorts: The high & low gear propulsion model
Bojsen-Mollor (1979) identified two axes across the forefoot (specifically across the met heads):
1. The transverse axis (across first and second metatarsal heads)
2. The oblique axis (across mets 2–5).
Progression through the transverse axis is termed “high gear” propulsion and through the oblique axis is “low gear”.
During non-pathological gait, the centre of pressure (COP) progresses anteriorly through the foot during stance as shown in the images below. Crucially, towards the end of stance COP excursion moves medially and there is progression predominantly through the hallux, utilising the larger joint and initiating the windlass mechanism, which acts to turn the foot into a rigid lever in combination with the action of peroneus longus. This progression (through Bojsen-Mollor’s transverse axis, high gear) enables efficient power generation by the plantar flexors. It also means that we exit the foot with our COM heading in an antero-medial direction, eg. forwards and towards the contralateral limb.
Functional hallux limitus can significantly reduce the amount of dorsiflexion available at the first MTPJ when weightbearing and during gait. This can mean that an individual’s ability to progress through this joint is negatively affected, therefore encouraging progression through the lesser toes, that sees the COP excursion follow a more lateral (less medial) path. This is low gear propulsion, through Bojsen-Mollor’s oblique axis.
Low gear propulsion may be undesirable for a number of reasons:
1. There is increased loading through the lesser toes, which can result in metatarsalgia (strictly transfer metatarsalgia) and related pathology.
2. The lever arm (from the axis to the insertion of the Achilles tendon) is shorter than in high gear propulsion. This means the action of the Achilles complex produces a smaller moment at terminal stance/pre swing. This makes gait less efficient.
3. There may be less of a windlass effect, because of reduced dorsiflexion at MTPJ1. The foot becomes less rigid, also reducing the effectiveness of power generation of the plantar flexors.
4. When leaving the stance limb, the individual is moving in a more antero-lateral (not medial) direction, actually away from the contra lateral limb. This places additional demand on more proximal structures which have to compensate for this movement, since we need the body’s COM to move towards the contralateral limb to enable progression into swing phase.
Clinical signs of low gear propulsion
Some signs may be present due to increased pressure (force) through the lesser metatarsals:
Callous pattern under some or all of the lesser met heads.
Pain across the plantar aspect of the forefoot.
Pain across the dorsum of the foot, due to dorsal interosseous compression.
Non specific pain in the midfoot region.
Wear pattern on the forefoot of the sole of the shoe.
It is also possible to spot low gear propulsion during a gait exam. Sometimes a “wobble” during terminal stance can be seen, and sometimes a patient visibly exits stance through the lesser toes/via oblique axis, sometimes with a notable medial whip at terminal stance.
Intervention
In order to intervene successfully, it is essential to determine why low gear propulsion is occurring.
Is there a limitation in range of motion that means it is easier to progress through the oblique axis than the transverse axis? This could be a structural or function limitation, such as functional hallux limitus as outlined above. Appropriate foot orthoses can resist pronatory moments at the subtalar joint, reducing functional hallux limitus. This can be achieved via use of medial rearfoot posting, with or without a kirby skive. For patients who have a plantarflexed first ray, this will need to be accommodated to reduce plantar pressure under the first MTPJ either through the use of a reverse morton’s extension, or first ray cut out.
Weakness at the external rotators of the hip may result in reduced foot progression angle, which may encourage progression through the lateral aspect of the foot (in-toed gait). Spasticity or tightness in muscles that internally rotate at the hip may have a similar effect.
Is the patient walking in this way as a compensatory movement? Perhaps as a means to avoid other, painful movements? Arthritic joint changes and resulting pain can be a huge motivator when it comes to changing the way someone walks!
References and further reading
Bojsen-Møller F. (1979). Calcaneocuboid joint and stability of the longitudinal arch of the foot at high and low gear push off. Journal of anatomy, 129(Pt 1), 165–176.
Perry, J. and Burnfield, J., 2010. Gait Analysis. Thorofare, N.J.: SLACK.
