DEVELOPMENT OF HUMAN PRECISION GRIP .4. TACTILE ADAPTATION OF ISOMETRIC FINGER FORCES TO THE FRICTIONAL CONDITION

The adaptation of the grip forces to the frictional condition between the digits and an object relies on feedforward sensorimotor mechanisms that use tactile afferent input to intermittently update a sensorimot or memory that controls the force coordination, i.e., the ratio betwee n grip force (normal to the grip surface) and load force (tangential t o the grip surface). The present study addressed the development of th ese mechanisms. Eighty-nine children and 15 adults lifted an instrumen ted object with exchangeable grip surfaces measuring the grip and load forces. Particularly in trials with high friction (sandpaper), the yo ungest children used a high grip force to load force ratio. Although t his large safety margin against slips indicated an immature capacity t o adapt to the frictional condition, higher grip forces were produced for more slippery material (silk versus sandpaper). The safety margin decreased during the first 5 years of age, in parallel with a lower va riability in the grip force and a better adaptation to the current fri ctional condition. The youngest children (18 months) could adapt the g rip force to load force ratio to the frictional condition in a series of lifts when the same surface structure was presented in blocks of tr ials, but failed when the surface structure was unpredictably changed between subsequent lifts. The need for repetitive presentation suggest s a poor capacity to form a sensorimotor memory representation of the friction or an immature capacity to control the employed ratio from th is representation. The memory effects, reflected by the influences of the frictional condition in the previous trial, gradually increased wi th age. Older children required a few lifts and adults only one lift t o update their force coordination to a new friction. Hence, the presen t finding suggests that young children use;excessive grip force, a str ategy to avoid frictional slips, to compensate for an immature tactile control of the precision grip.