BACKGROUND: Due to complexity of the plantar intrinsic foot muscles, little is 
known about their muscle architecture in vivo. Chronic plantar fasciitis may be 
accompanied by muscle atrophy of plantar intrinsic foot muscles and tibialis 
posterior compromising the dynamic support of the foot prolonging the injury. 
Magnetic resonance images of the foot may be digitized to quantify muscle 
architecture. The first purpose of this study was to estimate in vivo the volume 
and distribution of healthy plantar intrinsic foot muscles. The second purpose 
was to determine whether chronic plantar fasciitis is accompanied by atrophy of 
plantar intrinsic foot muscles and tibialis posterior.
METHODS: Magnetic resonance images were taken bilaterally in eight subjects with 
unilateral plantar fasciitis. Muscle perimeters were digitally outlined and 
muscle signal intensity thresholds were determined for each image for volume 
computation.
FINDINGS: The mean volume of contractile tissue in healthy plantar intrinsic 
foot muscles was 113.3 cm(3). Forefoot volumes of plantar fasciitis plantar 
intrinsic foot muscles were 5.2% smaller than healthy feet (P=0.03, ES=0.26), 
but rearfoot (P=0.26, ES=0.08) and total foot volumes (P=0.07) were similar. No 
differences were observed in tibialis posterior size.
INTERPRETATIONS: While the total volume of plantar intrinsic foot muscles was 
similar in healthy and plantar fasciitis feet, atrophy of the forefoot plantar 
intrinsic foot muscles may contribute to plantar fasciitis by destabilizing the 
medial longitudinal arch. These results suggest that magnetic resonance imaging 
measures may be useful in understanding the etiology and rehabilitation of 
chronic plantar fasciitis.