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To study intermanual temporal and force differences during contractions of adductor thumb muscles, the force-time history during isometric contraction of both hands was recorded and analysed for temporal phases including initiation of force, moments of maximum acceleration and velocity of force development, and period to peak force. 36 strongly right-handed young participants (18 women, 18 men) were instructed to use their thumbs to push buttons bimanually in response to 20 separate 35-db sound signals in each of four sessions. Sessions 1 and 2 were preceded with a “strong” instruction to “react as strongly as possible” (Strong Reaction task). Sessions 3 and 4 were preceded with a “speed” instruction to “react as quickly as possible” (Speed Reaction task). Lateral differences between magnitudes of forces developed by left and right thumbs and asynchrony between defined temporal parameters in bilateral force-time histories were significantly greater for Strong Reaction tasks. This departure from bilateral correlation observed between identical phases of isometric pushing action may suggest that participants' thumbs operate more independently under instructions that focus attention on force. Thus, specific instructions given to participants may have a profound effect on performance. It was also found that intermanual temporal differences for women were significantly smaller than for men, especially on Strong Reaction tasks. This phenomenon may be partly explained by a lower lateralization in women.

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