The Timing of Intralimb Coordination

The authors examined the manner in which self-selected movement frequencies are impacted upon by repeated engagement in an intralimb coordination task and by alterations in the inertial characteristics of the limb. Twelve healthy adult volunteers rhythmically flexed and extended their elbow and wrist joints at a comfortable self-established frequency in 1 of 2 modes of coordination (in-phase and antiphase), while grasping 1 of 3 weighted dowels (no-weight condition [0.03 kg], light weight condition [0.5 kg], heavy weight condition [1.0 kg]). The movement frequencies adopted by subjects on the 3rd of 3 weekly sessions, following more than 120 experimental trials, were appreciably higher than those obtained during an initial session. The addition of mass to the system had an inconsistent influence upon the preferred frequency of movement. When subjects' limbs were loaded with what was deemed to be a light weight (0.5 kg), the movement frequencies that were adopted were indistinguishable from those selected when there was no (0.03 kg) loading of the limbs. In contrast, when subjects' limbs were loaded with a relatively heavy weight (1 kg), the resulting self-selected movement frequencies were reliably lower than when there was no loading of the limbs. The adopted frequency of movement was also influenced in a reliable fashion by the mode of coordination in which the movements were prepared. Those results are discussed with reference to mechanical and neuromuscular constraints on coordination dynamics.