Coordination is governed by a coalition of constraints. The extent to which particular constraints are expressed depends in large measure upon the task context. When simple movements of the index finger about the metacarpalphalangeal joint are executed at relatively low frequencies, the flexion and extension phases of the movement cycle appear equivalent.

In these circumstances, strategic or higher order planning elements of the task (e.g. synchronise vs. syncopate) are prominent (Kelso, 1998). At higher frequencies, patterns of coordination requiring flexion on-the-beat are more stable than patterns requiring extension on-the-beat (Carson, 1996). In this context, musculo-skeletal constraints are expressed more prominently than strategic constraints. In the present study, we sought to explore these interactions, by examining frequency induced changes in neuromuscular dynamics, an endeavour anticipated by the classic work of Wachholder and Altenberger (1926). Thirteen participants performed rhythmic flexion and extension movements of the index finger, while being paced by an auditory metronome, in one of four modes of coordination: flex-on-the-beat, extend-on-the-beat, flex-between-the-beats or extend-between-the-beats. Ten trials of 66 s duration were performed in each condition. In each trial, the metronome frequency was increased from either 0.75 or 1.25 Hz, in 8 steps of 0.25 Hz. Electromyographic activity was recorded, using fine-wire electrodes, from Extensor Digitorum Communis, Flexor Digitorum Superficialis and the First Dorsal Interosseous. During many trials, the participants exhibited transitions from the prescribed movement pattern, to either flex-on-the beat, extend-on-the-beat, or to phase wandering. The times at which these transitions occurred were influenced by the mode of coordination in which the movements were prepared (c²(3) = 28.3, p<.01). The alacrity of transitions from the extend-between-the-beats pattern (t = 30.5 s) was greater than from the flex-on-the-beat (t = 58.27 s), and extend-on-the-beat (t = 57.62 s) patterns (p<.05). In addition, transitions occurred more rapidly from the flex-between-the-beats pattern (t = 41.43 s), than from the flex-on-the-beat pattern (p<.05). In order to further assess the impact of musculo-skeletal and strategic constraints, in the period prior to transitions, the stability of coordination was evaluated at both the behavioural (movement kinematics) and the neuromuscular level (EMG).