Impaired Object Manipulation in Mildly Involved Individuals with Multiple Sclerosis

Abstract

We investigated hand function in mildly involved Multiple Sclerosis (MS) patients (N = 16; EDSS 1−5, 9-hole peg test 14−32 s) during static and dynamic manipulation tasks using an instrumented device. When compared with healthy controls (N = 16), the patients revealed impaired task performance regarding their ability to exert prescribed patterns of load force (L; force acting tangentially at the digits-object surface). Regarding the coordination of grip force (G; normal component) and L, the data only revealed elevated G/L ratio, although both the G and L coupling (maximum correlation coefficients and the time lags between them), and G modulation (gain and offset of G with respect to L) remained comparable in the two groups. Finally, most of the data suggested no MS-specific effects of switching from uni-to bimanual tasks, from available visual feedback to deprived feedback conditions. We conclude that the deterioration in the ability for precise control of external forces and overgripping could precede the decoupling of G and L and decreased G modulation in early phases of the disease. The results also suggest that the applied methodology could be sensitive enough to detect mild levels of impairment of hand function in MS and, possibly, other neurological diseases.Keywords hand function; grip; load; force coordination; coupling; task performanceThe main hand function is to grasp and manipulate objects and external supports. This function is essential for everyday life, and, consequently, an impaired hand function (e.g., associated with neurological diseases or injuries) could be an important obstacle to living an independent life. When an externally fixed (e.g., a handle providing external support) or a free-moving object (a cup of coffee, or a tool) is held with the opposing tips of the thumb and fingers, a load force (L) is applied tangentially to the surfaces in contact to move the object or exert a reaction force. The slippage of the object is countered by friction force originating from grip force (G) acting normally to the surface. A consistent finding throughout the literature has been that the changes in G and L appear to be highly coordinated. This coordination is based not only on an ability of the neural controlling mechanisms to assess the skin-object friction coefficient and, based on it, apply a relatively low safety margin [i.e., G slightly higher than necessary to prevent slippage; (Johansson & Westling, 1984)], but also to modulate G in parallel with L without any considerable time delays between them [i.e., using anticipatory mechanisms (Flanagan & Wing, 1995. The result appears to be a stable and relatively low grip-toload (G/L) ratio (Johansson, 1998), high coupling of G and L (as assessed by both high correlation coefficient and virtually no time lags between them [Blakemore, Goodbody, & Wolpert, 1998;Flanagan & Tresilian, 1994]), and a high level of modulation of G with respect to changes in L [i.e., high gain and low offset assessed through the slope and intercept, respectively, o...

Using Haptic Feedback to Improve Grasp Force Control in Multiple Sclerosis Patients

“…First, the ability to balance the fingertip forces is useful for tasks such as manipulating a glass of water without spilling. The control of grasp and load forces is also a sensitive measure of dexterity and, as reported in [17], deteriorates in mildly impaired patients. Also, because MS patients' hands are often weak, the use of minimal sums of forces is desirable to decrease fatigue during the task.…”

“…In particular, patients with neurological diseases often use larger grasp forces than healthy subjects in tasks that involve grasping and lifting everyday objects [14]- [16]. Patients with mild impairment from MS may show distortions in the coordination of grasp forces when grasping and lifting objects, even when they do not otherwise report difficulty in manipulation [17].…”

Force Coordination in Object Manipulation

“…While holding the object, the smoothness was lower in the patients than in healthy individuals (Figure 3). Note that the obtained findings related to task performance and GF scaling observed in mildly impaired MS patients during both static and dynamic manipulation tasks could be detrimental for controlling object manipulation, and also could generate premature muscle fatigue Krishnan et al 2008). Besides evaluating hand function in individuals with multiple sclerosis, we also investigated other aspects of the control of manipulative activities.…”

“…We also explored GF and LF coordination in individuals with multiple sclerosis (MS) under static and dynamic conditions Krishnan et al 2008). Under the static conditions, participants were asked to exert LF against one or two fixed handles using different profiles and magnitudes of LF.…”

Force coordination in static manipulation: Discerning the contribution of muscle synergies and cutaneous afferents

“…As a result, bending of the skin caused by the change in L direction could cause the consecutive changes in activation of the receptive fields from neighboring sites of the digit pads [14]. Those changes could be interpreted as slips and, consequently, the CNS could tend to prevent slipping by elevating the "safety margin" (i.e., increasing G/L ratio [16,20]), as well as by reducing both the G-L coupling and G modulation. A certain transient time well known to be required to establish highly coordinated motor actions (c.f., of individual fingers [5,33], or postural muscles, [19]) could explain why switches in activity between two opposite sites of sensory afferents could be associated with deterioration of G and L coordination.…”