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If two people row a boat, they often call to each other to synchronize their strokes. It is anticipated that such a call promotes periodic joint action. The present study thus examined the effects of speech on both complementary and synchronous strategies in joint action using the same task as we used previously (Masumoto and Inui in J Neurophysiol 109:1307-1314, ). Ten pairs of participants produced periodic isometric forces such that the sum of the forces they produced was the target force cycling between 5 and 10 % of maximum voluntary contraction with an interval of 1,000 ms with the right hand. There were three speech conditions crossed with the presence or absence of visual information. Whereas two participants synchronized an utterance/ba/with the peak and valley forces in the 'Both' condition, one synchronized it with both forces in the 'One-side' condition, and nobody uttered it in the 'None' condition. When the total force was visible, the One-side and Both conditions exhibited lower correlations than the None condition, although the correlation between forces produced by two participants was negative in all conditions. When the total force was invisible, although the coherence between force and time series produced by two participants was low under the None condition, it was high at 1 and 3 Hz under the One-side and Both conditions. Thus, although periodically uttering a syllable worsened complementary force production when the target was visible, it promoted synchronization of their performance to each other's timing when the target was invisible. [ABSTRACT FROM AUTHOR]