The use of optical flow to characterize muscle contraction

Muscle contraction is usually measured and characterized with force and dis placement transducers. The contraction of muscle fibers, however, evokes in the tissue a two and even three-dimensional displacement field, which is n ot properly quantified by these transducers because they provide just a sin gle scalar quantity. This problem can be circumvented by using optical meas urements and standard tools of computer vision, developed for the analysis of time varying image sequences. By computing the so called optical flow, i .e. the apparent motion of points in a time varying image sequence, it is p ossible to recover a two-dimensional motion field, describing rather precis ely the displacement caused by muscle contraction in a flattened piece of s kin. The obtained two-dimensional optical flow can be further analyzed by c omputing its elementary deformation components, providing a novel and accur ate characterization of the contraction induced by different motoneurons. T his technique is demonstrated analyzing the displacement caused by muscle c ontraction in the skin of the leech, Hirudo medicinalis. The proposed techn ique can be applied to monitor and characterize all contractions in almost flat tissues with enough visual texture. (C) 2001 Elsevier Science B.V. All rights reserved.