Summary 25Gait analysis is gaining in popularity for quantification of lameness and two commonly used inertial 26 sensor systems assess trunk movement symmetry: can these be used interchangeably in multi 27 centre studies? 28We compared head and pelvic movement symmetry between two inertial sensor gait analysis 29 systems in 13 horses equipped simultaneously with the two systems. The first system quantified 30 dorso-ventral movement in the local reference frame (System A), the other system global vertical 31 movement (System B). Widths of limits of agreement were calculated employing a well-established 32 regression method dealing with systematically changing differences over the range of measured 33 values. 34Widths of limits of agreement between system A and system B were narrower for pelvic movement 35 than for head movement. For head movement, they ranged from 6.4 to 6.9mm for in-hand trot and 36 from 7.3 to 9.7mm in the lunge and for pelvic movement from 2.5 to 4.4mm in-hand and from 3.6 37 to 5.3mm on the lunge. 38

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