Hand digit control in children: age-related changes in hand digit force interactions during maximum flexion and extension force production tasks

Jae Kun Shim Æ Marcio A. Oliveira Æ Jeffrey Hsu Æ
Junfeng Huang Æ Jaebum Park Æ Jane E. Clark
Exp Brain Res
DOI 10.1007/s00221-006-0629-x

Abstract:
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 20 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 80 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 MCP joint conditions were
also valid at 80 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 condition than in the 20 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.