Biomechanics

Physical models of joints: Put the "arthro" into arthropods and the dogleg into the dog’s leg

By David Alexander, University of Kansas


Type of Resource Class activity 
Topic Skeletal systems 
Taxa Widely applicable 
Organizational Level Systems 
Estimated time to do activity 20-45 min. (depending on class size) 
Background required/level Introductory biology helpful but not essential 
Role of activity in your course To forcefully demonstrate the necessity for antagonistic muscles and the poor mechanical advantage used by muscles in the body. Also to illustrate that putting the muscles on the inside or outside of the skeleton does not significantly change how the joint works. 
What students might learn from this course or activity - Why animals need antagonistic muscles
- Leverage, and how poor the leverage of a muscle is at a joint
- How hinge joints work
- The arrangement of muscles on the inside of an arthropod skeleton and outside of a vertebrate skeleton
- The integration of skeletomuscular systems 
Special tools, equipment or software needed No special tools other than heavy scissors are required if cardboard tubes are used, although a band saw or jigsaw and a drill are convenient. If tubes of harder material are used, hack saws, files and drills may be necessary.

Appropriate cardboard tubes can often be had for the asking (commonly the core around which some sheet-like material is wrapped). Tubes longer and thicker-walled than paper towel tubes are best. Cores from rolls of plastic sheeting are good, and some cores from gift wrapping paper may also work. Check first with departmental shop and store room personnel, they may have appropriate tubes on hand and if not, may have know where to find them. As a last resort, narrow mailing tubes may be purchased from office suply stores.

Harder tubes (PVC pipe, aluminum tubing), as well as cotter pins or nails for the hinge pivot, can be found at hardware stores. PVC pipe is inexpensive, a section long enough for 5 or 6 models should cost under $5.00, plus a similar cost or less for enough string and cotter pins for those models. Thin-walled aluminum tubing may be less common and would cost at least five times more per model. I would recommend against copper or iron plumbing-type pipes: model fabircation would be difficult and the models would be cumbersome for students to handle. 
Safety precautions, possible permissions necessary If using cardboard tubes, no special precautions.
If using harder tubes, cut edges must be filed or ground to remove burs, sharpe edges and sharp corners.

No precautions or permissions necessary for student use. 
Miscellaneous advice - pitfalls to avoid None. 
Frequently asked questions by students None. 
Evaluation I discussed it with students afterwards and asked about it on an exam and on evaluations. Students rate it very highly, and seem to come away with a good grasp of muscle leverage and antagonistic muscles. 
Description I describe a simple, physical model that gives students a concrete, hands-on illustration of joint properties (arthropod, vertebrate, or both). The model clearly demonstrates the poor mechanical advantage under which muscles are forced to operate, as well as the necessity of antagonistic muscle pairs. It also shows the inside-out relationship between arthropod and vertebrate skeletomuscular systems, which nevertheless use joints that function almost identically. The first section gives detailed instructions for making the parts and assembling the model, and explains what the parts represent in a real joint. The second section consists of a sample student exercise that could be used in either a lab or lecture setting. The sample exercise is intended to show all the features that the model demonstrates, so that instructors can use or discard parts relevant to any given course. The exercise concludes with a set of study questions that could serve as a basis for a lab write-up, or quiz or exam questions. 
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