Determination o f upper limb function & grip strength

6.2.4.1. The hand **torque measuring apparatus** and dynamometer**

• The hand dynamometer

The hand dynamometer used was the Smedley’s hand dynamometer (made by Ya g a m i

In t e r n a t io n a l, see Figure 72). It consisted o f an adjustable handle (for hand size) around which the four fingers hooked (at the joint between the proximal and middle phalanges), with the thumb resting on the outside frame to provide leverage. The reading dial was an analogue needle reading with values displayed in Kilograms (range

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of O-lOOKg). The reading is directly, and linearly dependent on the amount of force applied onto the handle.

Handle for 4 fingers hook

S w iv e ls around for hand size

adjustm ents and attaches (at the back) to

spring attachm ent Dial

Stabiliser (to keep handle from s w iv e llin g around during grip attem pts)

Figure 72. Schematic representation of a hand dynamometer. At the back on the instrument, the handle is attached to a spring, which moves both the white and red arrows. The white arrow moves while the spring is being stretched and goes back to resting position (0 reading) once the applied force is removed. The red arrow will stay in the maximum position that the white arrow reached, and will only return to zero when manually moved back.

• The hand torque measuring apparatus

The hand torque-measuring tool included various attachments resembling everyday tools such as: a tap, various key types, a screwdriver, a doorknob, a bottle top and a jar top attachments. O f these, the latter two (bottle and jar top attachments) were used for the functional torque measurements, and the T-handle for calibrations.

As shown in Figure 73 the subject applies a moment about the axis of a handle attachment (here showing the T-handle). The force signals were amplified and then stored into a computer.

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Signal to am plifier = Moment = (Fxd)

Figure 73: Schematic representation of the outside view of the hand torque measuring apparatus with T-handle attachment. The force applied on the transducer F=(Load (Kg) xd)/D. Where D=109.5mm.

Figure 74 shows inside views of the funetional torque apparatus. The subject can press on the strain gauge with either left of right hand side levers o f the main axle, the length of the levels either side of the two secondary axles is the same, and the force is directly transferred onto one of the two sensor surfaces.

These are buttons that will compress when force is applied, thus transferring the forces onto the force transducer (figure 74A). Figure 74B illustrate an example of what happens when the subject applies anti-clockwise torque (big red arrow). The left arm (on the picture) of the main axle is moved upward, hence pressing down onto the left (on the picture) sensor surface.

The specifications of the manufacturer of the force transducer unit stated that the maximum load that each of the two sensor surfaces could take was 20Kg. Which mean for instance that if a load is placed at 46mm from the main axle, it should weight no more than ~43.48Kg i.e. a maximum torque measuring capacity of ~19.6Nm at that distance (43.48Kg x 9.81 = 426.54N. Thus 426.54N x 46mm = 19620Nmm =

19.62Nm).

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A)

On top of the strain gauge (Force transducer unit)

Main axle

(External handles attachm ent site)

D = 109.5

©

2 1 ='90 5

Secondary axles

Bottle top handle

Subject applies moment (M)

dies

ir

Secondary axle

Figure 74: Schematic representation of the interplay of the different levers in the hand torque apparatus. A) Cross-sectional view from the front B) Cross-sectional view from the front with perspective and anticlockwise subject torque applied.

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Calibration o f the hand torque apparatus

To calibrate this apparatus, an attachment in the form of a T-handle was used, as it was easier to hang weight on. In the first calibration set, a known weight was hung at various distances from the main axle. In the second instance, various weights were applied at the same distance.

A ) 1 . 2 1 y = 0.123X - 0.000 = 1.000 10 0.8 0 6 a- 0 4 y = 0.123X + 0.011 = 0.999 02 00 6 8 10 0 2 4 T orque (Nm)

O S a m e weight (2 0192Kg) ♦ S a m e lever arm (46mm)

B J y = 0.130x + 0.004 R^= 1.000 0.8 - 8- O 0.4 - y = 0.1244X + 0.0058 R^= 1 0.0 0 2 4 6 8 10 Weight (N) Right A Left

Figure 75. Hand torque-measuring apparatus calibration curves. A) The method of calibration did not matter (whether changing lever arm or changing weights), and B) both force transducers (left and right) read forces in the same manner.

Both methods of calibrations gave very similar relationship between torque and output voltage. Both force transducers (for clockwise and anticlockwise torque) read forces with almost the same sensitivity (figure 75B).

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