Task Hand Strength Demands and Capacities
T. Armstrong
U of Michigan, C4E
http://www-personal.umich.edu/~tja/
11-30-2023
T. Armstrong
U of Michigan, C4E
http://www-personal.umich.edu/~tja/
11-30-2023
2.1 Worker ratings
2.1.1 Verbal ratings
2.1.2 Visual analog scales
2.1.3 Borg Scale
2.1.4 Force matching
2.2 Observer ratings
2.3 Instrumentation
2.4 Biomechanics
2.5 Extrapolation
3. Hand strength
3.1 Strength factors
3.2 Strength measurement
3.3 Published strength data
3.4 Biomechanics
The tool box with tools weighs 50 pounds(222 Newtons)
> (a) (b) (c)
Percentile 1% 2.5% 5% 10% 25% 50% 75% 90% 95% 97.5% 99% z -2.326 -1.96 -1.645 -1.282 -0.675 0 0.675 1.282 1.645 1.96 2.36
Example: Estimate the percent of workers with sufficient strength to lift and carry the tool box shown in Figure 1.
The tool box with tools weighs 50 pounds | Grip strength capacities for a sample of 30-34 year old females and males were found to be 121.8∓22.4 pounds and 78.7∓19.2 pounds respectively. (Mathiowetz et al. 1985) | Standard normal values for z=(x-average)/ std dev |
(a) | (b) | (c) |
%MVC = / × 100% =
Example: Determine the percent maximum voluntary contraction, %MVC, for someone with average female grip strength to hold the toolbox as shown in Figure 2a:
Figure 3a: Simple pivot action (fiskars.com) |
Figure 3b: Extended handles with compounded linkage to amplify hand force (fiskars.com) |
2.1.2 Visual analogue scale (anchor points ⚓)
Marshall, M., T. Armstrong and M. Ebersole (2004). "Verbal Estimation of Peak Exertion Intensity." Human Factors and Ergonomics 46(4): 697-710
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(a) | (b) |
The force on handle is recorded as the platform is lowerd and the worker's weight is transferred to the handle. Break away forces expressed as Newtons and rations of body weight and grip force. a Instrumentation for determining the effects of friction on break away force (Young et al. 2012) |
Electrical activity of finger flexor muscles in forearm are displayed and recorded with images of hand and work objects Force profiles based on EMG recordings as worker bones two turkey thighs b Use of EMG (electromyography) for indirect measurement of hand force (Armstrong et al. 1982) |
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Keyboard mounted on force transducers record force exerted on keyboard; Surface EMG electrodes record forearm finger flexotr muscle activity
Finger forces exterted to type the word "driven"
Average force profiles for 3 different keyboards
Force amplitude probability distribution for multiple key strokes |
c Instrumented flange and EMG combined with motion tracking to measure complex hand movements and forces for installing flexible hoses (Griesbaber et al. 2007) |
e Force mapping used to determine force distribution on hand to insert hose on flange. (Seo 2007; Tekscan |
2.4 Biomechanical analysis of task hand force demands
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Grip force = Object weight (neglecting friction) | Grip force is related to clamp or clip opening | Grip force a function of tool chuck force + possible over exertion force | Pinch force related to weight of object and friction (Frederic and Armstrong 1995; Buchholz et al. 1988; Seo et al. 2009) | Finger and thumb forces related to the weight of object and finger & thumb locations. |
Fgrip = Weight | Fgrip = k × Opening | Fgrip> = FChuck × r1 / r2 | Fpinch ≥ w / (2 × CoF) CoF = Coefficient of Friction |
Ffingers = (W × a) / b Fthumb = W + Ffigners |
(a) | (b) | (c) | (d) | (e) |
2.5 Extrapolate task hand force demands from previous studies
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3.1 Factors affecting hand strength capacities
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<(a) | <(b) |
(Note: strength tends to be bounded on the lower end. and skewed towards the higher end. The use of a normal approximation can lead to significant errors for estimates of extreme high or low percentiles.)
Hand strength exercise
3.3 Published hand strength data
 
(a)>