To address this need, the authors have developed a novel engineering paradigm
and technologies that focus on the ability of a person to perform dexterous fingertip
tasks at submaximal force levels.100 The manipulating ability of a multifinger hand is
defined by the mechanical effect each fingertip can produce on an object.101 Effective
and dexterous manipulation arises when the coordinated action of the fingertips can
generate the desired resultant 3-dimensional force/torque vector at the center of mass
of the object being held. The ability with which the magnitude and direction of the
fingertip force vectors are dynamically regulated determines the effectiveness of
dynamic manipulation. The Strength-Dexterity (S-D) system targets the functional
cornerstone of dexterous manipulation in healthy and clinical populations, which is
the ability to simultaneously and dynamically regulate the magnitude and direction
of fingertip force vectors. The S-D system is based on the principle of buckling of
compression springs. When compressed the spring is initially stable, but as the
compression increases to a critical point the combined handspring system becomes
unstable and relies on the passive anatomic structures and active sensorimotor
control of the fingers to dynamically hold that level of compression force. The
compression of this device involves low forces (2–3 N), but requires careful collaborative
control of finger motions and fingertip force vectors to prevent the spring from
buckling. The one parameter that can characterize performance during the production
of dexterous manipulation with the fingertips is the maximal compression a person can
achieve with a spring that is impossible to compress fully. The maximal compression
a person can achieve indicates the limit of sensorimotor integration ability by indicating
how much instability the person can control. The S-D system has the potential
to be used to quantify impairment and compare treatment outcomes in orthopedic
and neurologic afflictions that degrade dynamic manipulation.