Elbow Flex

We are living in uncertain times and as students and families are stuck at home during this pandemic we thought it would be fun to demonstrate some basic biomechanical principles in action using commonplace items that are found around the home,  to provide an understanding of the diversity of body form and how it may relate to function.

Materials needed for Activity 2:

• Nothing, just you and a friend/family member.
• However, the result will be more evident if you have a small weight to hold (~1 kg).

What to do:

• One person (the ‘subject’) holds their upper arm still and in a normal position. If you have a weight, the subject hold it in the hand of their test arm.
• The other person (‘the experimenter’) then flexes the elbow of the subject by lifting the forearm upwards.
• Test 1: first lift the forearm at the wrist.
Test 2: then lift the forearm close to the elbow.

• Now swap and try it on the other person.

Elbow Flex

What does this illustrate?

• This time the pivot point (or ‘fulcrum’) is the elbow joint, and unlike activity 1 we’re not varying the distance that the load is from the joint.
• Rather, we’re now varying the distance from the joint that effort is applied to raise the forearm.
• In the first test the effort was applied a large distance from the elbow, requiring relatively little muscular effort to flex the elbow.
• In the second test the effort was applied at a smaller distance from the elbow, yet the moment required to flex the forearm upwards hadn’t changed – in turn requiring more effort to raise the elbow.
• In real life it is muscles like your biceps that acts to flex your forearm. It is a very strong muscle, and it leaves a distinctive bump or tuberosity on the forearm bone where it attaches (radius).
• This activity also illustrates the concept of ‘mechanical advantage.’ Collectively, activity 1 and activity 2 illustrate how mechanical advantage is determined both by the relative distances of the load and the applied effort.Mechanical Advantage