We studied
the finger interactions during maximum voluntary force (MVF) production in
flexion and extension in children and adults. The goal of this study was to investigate the age-related changes and flexion-extension differences of MVF and
finger interaction indices, such as
finger inter-dependency (force enslaving (
FE): unintended
finger forces produced by non-instructed
fingers during force production of an instructed
finger), force sharing (FS; percent contributions of individual
finger forces to the total force at four-finger MVF), and force deficit (FD; force difference between single-finger MVF and the force of the same
finger at four-finger MVF). Twenty-five
right-handed children of 6-10 years of age and 25 adults of 20-24 years of age participated as subjects in this study (five subjects at each age). During the experiments, the subjects had their
forearms secured in armrests. The subjects inserted the
distal phalanges of the
right hand into C-shaped
aluminum thimbles affixed to small force sensors with 200 of
flexion about the
metacarpophalangeal (MCP)
joint. The subjects were instructed to produce their maximum isometric force with a single
finger or all four
fingers in
flexion or extension. In order to examine the effects of muscle-force relationship on MVF and other digit interaction indices, six subjects were
randomly selected from the group of 25 adult subjects and asked to perform the same experimental protocol described above. However, the
MCP joint was at 800 of
flexion. The results from the 20' of
MCP joint flexion showed that (1) MVF increased and
finger inter-dependency decreased with children's age, (2) the increasing and decreasing absolute slopes (N/year) from
regression analysis were steeper in
flexion than extension while the relative slopes (%/year)
with respect to adults' maximum
finger forces were higher in extension than
flexion, (3) the larger MVF,
FE, and FD were found in
flexion than in extension, (4)
the finger FS was very similar in children and adults, (5) the FS pattern of individual
fingers was different for
flexion and extension, and (6) the differences between
flexion and extension found at 20 degrees
MCP joint conditions were also valid at 80 degrees
MCP joint conditions. We conclude that (a)
the finger strength and independency increase from 6 to 10 years of age, and the increasing trends are more evident in
flexion than in extension as indexed by the absolute slopes, (b)
the finger strength and
finger independency is greater in
flexion than in extension, and (c) the sharing pattern in children appears to develop before 6 years of age or it is an inherent property of the
hand neuromusculoskletal system. One noteworthy observation, which requires further investigation, was that
FE was slightly smaller in the 80 degrees condition than in the 20 degrees condition for
flexion, but larger for extension for all subjects. This may be interpreted as a greater
FE when
flexor or
extensor muscles are stretched.
