Quantification of Dexterity as the Dynamical Regulation of Instabilities: Comparisons Across Gender, Age, and Disease

Abstract

Dexterous manipulation depends on using the fingertips to stabilize unstable objects. The Strength–Dexterity paradigm consists of asking subjects to compress a slender and compliant spring prone to buckling. The maximal level of compression [requiring low fingertip forces <300 grams force (gf)] quantifies the neural control capability to dynamically regulate fingertip force vectors and motions for a dynamic manipulation task. We found that finger dexterity is significantly affected by age (p = 0.017) and gender (p = 0.021) in 147 healthy individuals (66F, 81M, 20–88 years). We then measured finger dexterity in 42 hands of patients following treatment for osteoarthritis of the base of the thumb (CMC OA, 33F, 65.8 ± 9.7 years), and 31 hands from patients being treated for Parkinson’s disease (PD, 6F, 10M, 67.68 ± 8.5 years). Importantly, we found no differences in finger compression force among patients or controls. However, we did find stronger age-related declines in performance in the patients with PD (slope −2.7 gf/year, p = 0.002) than in those with CMC OA (slope −1.4 gf/year, p = 0.015), than in controls (slope −0.86 gf/year). In addition, the temporal variability of forces during spring compression shows clearly different dynamics in the clinical populations compared to the controls (p < 0.001). Lastly, we compared dexterity across extremities. We found stronger age (p = 0.005) and gender (p = 0.002) effects of leg compression force in 188 healthy subjects who compressed a larger spring with the foot of an isolated leg (73F, 115M, 14–92 years). In 81 subjects who performed the tests with all four limbs separately, we found finger and leg compression force to be significantly correlated (females ρ = 0.529, p = 0.004; males ρ = 0.403, p = 0.003; 28F, 53M, 20–85 years), but surprisingly found no differences between dominant and non-dominant limbs. These results have important clinical implications, and suggest the existence – and compel the investigation – of systemic versus limb-specific mechanisms for dexterity.

Activity in the brain network for dynamic manipulation of unstable objects is robust to acute tactile nerve block: An fMRI study

“…Mosier et al (2011) found that the level of instability achieved showed a strong association with increase in the BOLD signal in basal ganglia and in the cerebellar-parietal network. Lawrence et al (2014) showed that spinal circuits are likely heavily involved in the regulation of instabilities during spring compressions. In the present study these findings are extended: activity was revealed in the basal ganglia, both with and without tactile input.…”

“…According to a hierarchical view of motor control (Kawato et al, 1987;Loeb et al, 1999;Konen and Kastner, 2008) dexterous manipulation capabilities, such as the compression of an unstable spring prone to buckling, are likely not exclusively controlled Q7 by the neo-and somatosensory cortices (Lawrence et al, 2014;Lemon, 1993;Schieber, 2011) but involve also subcortical and spinal structures (Lawrence et al, 2014). Even if we presume a limited role of the spinal cord in movement generation in humans, the spinal cord can certainly shape the motor commands coming from supraspinal structures by gating, inhibiting, or disinhibiting the behavior of spinal circuitry (Pierrot-Deseilligny and Burke, 2005).…”

Sex differences in leg dexterity are not present in elite athletes

“…It was originally developed for fingers[3, 5, 42], and then adapted into a lower extremity dexterity test to quantify the dynamical ability to stabilize unstable foot-ground interactions[19, 23]. We have previously reported sex differences in this leg version in young soccer players[22] and across the lifespan[19] and proposed that may contribute to the disproportionate higher number of non-contact ACL injuries in female athletes compared to males. As such, it is important to understand the influence of athletic ability—likely, in part, a result of long-term exposure to athletic training regimens—on sensorimotor control for leg dexterity.…”

“…Traditional measures of leg function during whole-body activity are confounded by the contribution of the functional domains of strength and/or limb coordination—let alone vestibular function, visual acuity, posture control, risk aversion, etc. However, we have shown that different versions of the Valero-Cuevas dexterity test quantify the functional domain of sensorimotor processing, as distinct from the functional domains of strength and limb coordination, in fingers[3, 5, 19, 41] and legs[18] (Figure 1). This simple, yet reliable, test requires participants to compress a slender spring prone to buckling at low forces to a maximal steady state level[23, 41].…”

“…This simple, yet reliable, test requires participants to compress a slender spring prone to buckling at low forces to a maximal steady state level[23, 41]. The spring becomes increasingly unstable as it is compressed (i.e., undergoes a bifurcation in instability[42]), and one’s ability to compress and hold the spring at the edge of instability is a measure of the person’s sensorimotor control capabilities[3, 5, 19, 41, 42]. Unfortunately, the fact that subjects can reliably control the spring at the edge of instability only for the few seconds produces timeseries that are not well suited for many classical nonlinear dynamical measures (i.e, maximal Lyapunov Exponent, Correlation Dimension) that require longer time series[13, 43].…”