Human development of grip force modulation relating to cyclic movement-induced inertial loads.

The present study examines the development of grip force modulation relating to self-induced loads during repetitive vertical arm movements at different frequencies with a hand-held object. One hundred and thirty-four 3- to 6-year-old children and 16 adults were asked to move a lightweight object up and down at increasing rates from 0.5 Hz up to individual maximal arm movement rates (> 2.5 Hz). Grip forces were measured by a uni-axial force transducer and the inertial forces (tangential forces) were calculated from the measurements by accelerometers within the object. Generally, the quality of anticipatory grip force control improved from movement frequencies of 1 Hz to 2.5 Hz and decreased above 2.5 Hz. At movement frequencies below 1 Hz, the phase lag between grip and load force cycles was longer in children than in adults (children: median = 17 ms; adults: median = 0 ms); however, there were no significant differences between the age groups above 1.5 Hz. The grip-to-load force ratio and the grip force modulation were not significantly different from the adults up to 2.5 Hz. The fine coupling of grip and load forces (precise temporal and gain control) showed age effects among the 3- to 6-year-old children, mainly between 3- and 4-year-olds, and in adults extremely good coupling at medium arm movement frequencies. However, concerning the pure temporal coordination of grip and inertial forces, there were only differences between the 3-year-old children and the other groups at arm movement frequencies above 2.5 Hz. The results show that, during cyclic movements with hand-held loads, temporal control is well established at the age of 4 years whereas the fine gain control needs a longer time to develop. The anticipatory control is dependent on the arm movement frequency and, therefore, how rapidly the inertial loads change. The high level of anticipatory control during self-induced repetitive actions in children of 3-6 years of age is interpreted as an early developing ability to predict precisely cyclic self-induced inertial loads of hand-held objects when the object properties are known by proprioception. The timing, which is possibly related to cerebellar functions, is, to some extent, developed earlier than the gain control, which may be associated with cortical functions.