Activity patterns and hypothesized functions of the epaxial muscles in tetrapods during locomotion [modified from ]. Data for the epaxial muscle activity were assembled from: spotted salamander, Ambystoma maculatum, m. dorsalis trunci, 8th external trunk segment, mean and standard error ; desert iguana, Dipsosaurus dorsalis, m. longissimus dorsi, 14th trunk vertebra, mean and standard deviation (S. Moritz, unpubl. data); dog, Canis familiaris, m. longissimus thoracis et lumborum, 6th lumbar vertebra, median and upper and lower quartiles . The x-axis represents the stride cycle beginning with the touch down of the ipsilateral hindlimb. The footfall patterns of the both hindlimbs are illustrated on the bottom of each graph (walk, trot: black: ipsilateral limb (iHL), gray: contralateral limb (cHL); gallop: black: trailing limb (tHL), gray: leading limb (lHL). Note that for the galloping dog, the EMG trace associated with the trailing hindlimb is black, the one associated with the leading hindlimb is gray. Bending traces above the electromyograms indicate the unimodal lateral flexion and extension on the body side ipsilateral to the recorded muscle activity (salamander, lizard) and the bimodal flexion and extension in the sagittal plane (mammal). Body planes in which moments and/or movements are suggested to occur are illustrated in the right top corner of each graph (for details see Figure 1). Note that the unilateral and monophasic epaxial activity in the walking salamander and lizard associated with the ipsilateral stance phase corresponds to the main activity observed in mammals. In mammals, the increased need for sagittal stability is met by bilateral activity resulting from a second burst during ipsilateral swing phase.