The Journal of Bone and Joint Surgery, Vol 77, Issue 7 1050-1057, Copyright © 1995 by Journal of Bone and Joint Surgery, Inc
Strain and loading of the second metatarsal during heel-lift
NA Sharkey, L Ferris, TS Smith and DK Matthews
Orthopaedic Research Laboratories, University of California, Davis, Sacramento 95817, USA.
Metatarsal stress fractures occur in military recruits after long marches
and in athletes after episodes of overtraining involving running or
jumping. It has been demonstrated that contraction of the plantar flexors
of the toes helps to counteract the moments placed on the metatarsals by
body weight. It is possible that physiological fatigue due to strenuous or
repetitive exercise reduces the rate and force of contraction of the
plantar flexors, thereby increasing metatarsal strain per cycle, and that
this mechanism is the primary cause of stress fractures of these bones. To
test the hypothesis that fatigue of the plantar flexors causes increased
metatarsal loading, thereby predisposing these bones to stress fracture, we
measured metatarsal strains in nine fresh cadaveric feet with use of an
apparatus that simulated physiological loading due to body weight as well
as contraction of the plantar flexors. Each foot was loaded to 750 newtons
of ground-reaction force by simulated contraction of the triceps surae, and
strains were recorded in the mid-part of the shaft of the second
metatarsal. Tests were repeated with use of simulated activity of different
combinations of the flexor digitorum longus, flexor hallucis longus,
peroneus brevis, peroneus longus, and tibialis posterior muscles. In situ
bending moments and axial loads subsequently were derived for each
configuration. Dorsal strain was significantly reduced by simulated
contraction of the flexor hallucis longus. Plantar-dorsal bending was
significantly reduced by simulated contraction of the flexor digitorum
longus.
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