Effects of Sensory Deficit on Phalanx Force Deviation During Power Grip Post Stroke

Abstract

The effect of sensory deficits on power grip force from individual phalanges was examined. The authors found that stroke survivors with sensory deficits (determined by the Semmes-Weinstein monofilament test) gripped with phalanx force directed more tangential to the object surface, than those without, although both groups had similar motor deficits (Chedoke-McMaster and Fugl-Meyer), grip strength, and skin friction. Altered grip force direction elevates risk of finger slippage against the object thus grip loss/object dropping, hindering activities of daily living. Altered grip force direction was associated with altered muscle activation patterns. In summary, the motor impairment level alone may not describe hand motor control in detail. Information about sensory deficits helps elucidate patients' hand motor control with functional relevance.

Keywords: EMG; hand; power grip; sensory deficit; stroke.

Figure 1.

During power grip, phalanges produce force (F_phalanx) that can be decomposed to normal force (F_normal) and shear forces (F_shear). Stable grip requires that the phalanx force deviation (α = arctangent of F_shear/F_normal) be less than the slip threshold (θ = arctangent of the coefficient of friction between the finger skin and object). If the phalanx force deviation (α) exceeds the slip threshold angle (θ), the phalanx would slip.

Figure 2:

A custom made grip dynamometer measured phalanx forces from each phalanx of a single finger.

Figure 3:

An example trial for a stroke survivor with sensory deficit. On the top, the normal force (solid line) and shear force (segmented line) of the proximal phalanx of the middle finger are shown. On the bottom, the FDS (dotted line), EDC (segmented line) and FDI (solid line) RMS EMGs normalized to the maximum voluntary contraction are shown. The two vertical lines mark the two second window in which the average data were obtained.

Figure 4:

Mean ± SE phalanx force deviation was largest for stroke survivors with sensory deficits, followed in order by stroke survivors without sensory deficits and healthy controls (a, significant Tukey post hoc comparisons are marked with stars). Mean ± SE phalanx force deviation is also shown for each surface (b), effort level (c), phalange (d), and finger (e).

Figure 5:

Mean ± SE phalanx normal force was reduced for both stroke groups compared to healthy controls (a, significant Tukey post hoc comparisons are marked with stars). This reduction was similar for both stroke groups for all surfaces (b), efforts (c), phalanges (d), and fingers (e).

Figure 6:

Mean ± SE EMG for healthy controls, stroke survivors without sensory deficits, and stroke survivors with sensory deficits (a). Mean + SE FDI and EDC EMGs relative to the FDS EMG were reduced for stroke survivors with sensory deficit compared to controls and stroke survivors without sensory deficit (b, significant Tukey post hoc difference for stroke survivors with sensory deficit group compared to other two groups are marked with stars for both relative FDI and EDC EMGs), showing altered muscle activity pattern with particularly reduced FDI and EDC muscle activities for stroke survivors with sensory deficit compared to controls and stroke survivors without sensory deficit. Non-transformed data is shown in the figure.

Figure 7:

Mean ± SE COF between the finger skin and the paper and rubber surfaces was similar for stroke survivors with sensory deficits, stroke survivors without sensory deficits, and healthy controls. The COF for the rubber surfaces was significantly greater than the paper surface (with the star indicating significant difference). Non-transformed data is shown in the figure.