All 3 heads are involved in a tricep extension, and the long head is the prime mover in a shoulder extension and adduction.
What does that mean?
It means that a lying tricep extension will stimulate all the heads of your triceps, but a pushdown would stimulate the long head specifically.
So, if over years of training you have come to the conclusion that the long head of you triceps (this would be the bottom part) is lagging, then a pushdown may help you bring it up.
Note - The lateral and medial heads cannot be seperated under any circumstances.
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Complex problems have simple, easy to understand, wrong answers.