Illusion caused by vibration of muscle spindles reveals 3 an involvement of muscle spindle inputs in regulating 4 isometric contraction of masseter muscles 5 Running Head : Calibration between Spindle and PMR Activities 6

performed the experiments and analysed the data. 14 Y.K. designed the study and wrote the manuscript together with T. ABSTRACT 25 Spindle Ia afferents may be differentially involved in voluntary isometric contraction, depending on 26 the pattern of synaptic connections in spindle reflex pathways. We investigated how isometric 27 contraction of masseter muscles is regulated through the activity of their muscle spindles that contain 28 the largest number of intrafusal fibres among skeletal muscle spindles, by examining the effects of 29 vibration of muscle spindles on the voluntary isometric contraction. Subjects were instructed to 30 hold the jaw at resting position by counteracting ramp loads applied on lower molar teeth. In 31 response to the increasing-ramp load, the root mean square (RMS) of masseter EMG activity almost 32 linearly increased under no vibration, while displaying a steep linear increase followed by a slower 33 increase under vibration. The regression line of the Load-RMS relationship was significantly 34 steeper under vibration than under no vibration, suggesting that the subjects overestimated the ramp 35 load and excessively counteracted it as reflected in the emergence of bite pressure. In response to 36 the decreasing-ramp load applied following the increasing one, the RMS hardly decreased under 37 vibration unlike under no vibration, leading to a generation of bite pressure even after the offset of 38 the negative-ramp load until the vibration was ceased. Thus, the subjects overestimated the 39 increasing rate of the load while underestimating the decreasing rate of the load, due to the 40 vibration-induced illusion of jaw opening. These observations suggest that spindle Ia/II inputs play 41 crucial roles both in estimating the load and in controlling the isometric contraction of masseter 42 muscles in the jaw-closed position.