In college anatomy and physiology courses (e.g. BIOL-203 at Howard Community College), students learn about the stimulation, contraction, relaxation and fatigue of muscle tissue. Specifically, students learn the physiological relationship between muscle fiber length and tensile force. The relationship can be demonstrated experimentally by measuring biomechanical forces at various muscle lengths. In particular, the hand's grip force can be measured at different lengths of the forearm muscles, showing that grip strength is maximized at a specific optimal muscle length. Biology faculty could demonstrate this phenomenon with the aid of a simple grip strength measurement device, to be used by students during an in-class activity.

Project Goal: Develop a grip strength measurement device for use by college faculty and students in a physiology course.

• Physiology of muscle contraction
• Data on grip strength

The following products exist for measuring grip strength:

1. Hydraulic hand dynamometer used by physicians and physical therapists.
   • Not designed for educational use.
   • Most expensive option, at $210.
   • Features a "standard" grip diameter of 2.5 inches.
   • Force cannot be observed by person holding the device (until after the test is complete).
2. Electronic Hand Grip Dynamometer used primarily by athletes to measure and record maximum grip strength.
   • Low cost ($25) compared to some other options
   • Digital display of grip force (quantitative output)
   • Ergonomics unclear for different wrist orientations
   • Display not visible for certain wrist orientations
3. Squeeze Bulb Dynamometer, measuring grip strength using air pressure (psi)
   • Moderate cost of $60
   • Quantitative output, but in psi, not a unit of force
4. Smedley Hand Dynamometer, purely mechanical operation (springs, etc)
   • Costly option at $160
   • Measures highest max force (220 lb) of all options

The existing products typically measure grip force up to approximately 200 lb. This maximum should not be necessary for the educational product, as it will not be used by professional athletes. Of the products reviewed, options 2 and 3 are feasible solutions in terms of cost. It is unclear how/if option 2 would work at different wrist orientations. All of these devices feature quantitative output, instead of the preferred qualitative output. Most appear easy to use, but some do require calibration, adjustment for different hand sizes, and/or understanding of the electronic user interface (buttons, screen, etc).

The grip strength measurement device must:

• Cost less than $50.
• Fit all hand sizes (from 10th percentile female to 90th percentile male)
• Be simple to use, requiring minimal verbal instructions (no written instructions).
• Be self-contained, requiring no wired or wireless connections for power, data, etc.
• Measure grip forces ranging from 0 to 100 lbs.
• Display the grip force (qualitative display preferred) with sufficient accuracy to show differences at different hand orientations.
• Display the grip force, such that it is visible at any hand orientation.
• Be durable enough to survive a drop from table/desk height (4 ft).

Enclosure design here, including prototype iterations. Sensor only rated up to 100 N (22.4 lbf), so bending resistance of half-cylinder should be increased such that at least 10 lbf is required to make contact with sensor.

Circuit and wiring diagram here

Arduino code here

• Enclosure/housing subassembly, $5 (estimated cost of PLA filament)
• Force Sensitive Resistor, $8
• Pro Micro Microcontroller, $20
• 24-LED bargraph with I2C backpack kit, $10
• 9V battery, $1
• Snap connector for 9V battery, $1
• Assorted wires and heatshrink tubing

Discuss considerations of long-term reliability, battery usage and replacement, etc.

• video of prototype device in use

• Add on/off switch to eliminate frequent opening of housing.
• Add mechanical feature (e.g. snap-fit cover) to access battery for replacement.
• Modify housing to allow assembly of all components without clearance/fit problems.
• Modify code to show green/yellow/red zones in bargraph, instead of all red.
• Adjust sensitivity of LED display, if needed.
• Incorporate feedback from students and/or faculty.