Mechanical properties of the human hand digits: Age-related differences
Introduction
Age-related changes within the neuromotor system (reviewed in Cole et al., 1999, Grabiner and Enoka, 1995) affect a variety of activities of daily living including prehensile tasks (Francis and Spirduso, 2000, Olafsdottir et al., 2008, Parikh and Cole, 2012, Rantanen et al., 1999, Shim et al., 2004). These behavioral changes may get contributions from changes both within the central nervous system and in mechanical characteristics of the digits. In particular, healthy aging is known to be associated with a significant decrease in the friction coefficient between the digit tips and surfaces of typical grasped objects; this factor has been discussed as a contributor to the higher grip forces typical of older adults (Cole, 1991, Cole et al., 1999, Gorniak et al., 2011).
Changes in mechanical properties of the digits may contribute to safety and stability of prehensile actions. In a first approximation, we consider each digit tip as a point object that can be characterized by such parameters as mass, apparent stiffness, and damping with a clear understanding that estimates of these parameters reflect properties of more proximal portions of the digits and the involved muscles.
We used two newly developed devices in the experiments. The earlier described device (Savescu et al., 2008) was used for estimation of the friction coefficient, which was expected to be lower in older subjects across all five hand digits (Hypothesis 1). The other device involved a handle equipped with spring-loaded force sensors that could be engaged and disengaged during steady-state normal force production leading to a quick, small-amplitude unloading of one of the digits. We used the recorded changes in the digit tip force and trajectory to compute its effective mass, apparent stiffness, and damping. Further, we computed the damping ratio. We expected the ratio to be smaller in older subjects (Hypothesis 2).
Despite the fact that only one digit was unloaded in each trial, we observed nearly instantaneous changes in the forces produced by the other digits involved in the task. These changes were not small and resembled the well-known phenomena of finger enslaving (lack of individuation; Kilbreath and Gandevia, 1994, Zatsiorsky et al., 2000). Older adults have been described as having lower enslaving expressed in percent to the maximal force-generation capability of the fingers (Kapur et al., 2010, Shinohara et al., 2003). Based on those studies, we expected the new phenomenon (we call it “mechanical enslaving”, ME) to follow the same pattern, that is, show proportionally smaller effects in the older group (Hypothesis 3).
Section snippets
Subjects
Ten healthy elderly subjects and ten healthy young subjects (age: mean = 76.1, SD = 5.6 years for the elderly; mean = 26.9, SD = 4.9 years for the young; 5 females in each group) were recruited. All subjects were right-handed determined by the Edinburgh Handedness Inventory (Oldfield 1971). None of the subjects had a previous history of neuropathies or traumas to their upper extremities. The elderly participants were screened with a cognition test (mini-mental status exam ≥ 24 points), a depression test
Mechanical enslaving (ME) and its time delay (TME)
In the trials performed with the handle with yielding sensors, disengaging the rod opposing the sensor (perturbation, see Methods) led to a nearly instantaneous drop in the force of the target digit (Fig. 3). The digit moved into flexion and reached a new steady state after a few tens of ms. Maximal change in the digit force was observed within 30 ms after the beginning of the perturbation. Forces produced by non-target digits also changed after the perturbation with a time delay of about 10 ms.
Discussion
All three hypotheses formulated in the Introduction have been supported by the data. We observed smaller friction coefficients in the older group compared to the younger group in support of Hypothesis 1 (see also earlier reports, Cole, 1991, Cole et al., 1999). Within the second-order linear model, older subjects had larger apparent stiffness and smaller damping values with no significant differences in the inertial parameters resulting in smaller damping ratios as predicted by Hypothesis 2.
Conflict of interest
None.
Acknowledgments
The study was supported by NIH grants NS-035032 and AR-048563.
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