heres a list of exercises and which head of the tricep they hit the most this study was done using mri machines just incase any1 lukin to no hope it helps.
exercise/ lateral- long - medial -
French press with EZ bar: 1 2 1
French press with EZ bar on decline bench 2 2 2
Supine triceps extension with dumbbell and neutral grip 2 1 1
Overhead triceps extensions with dumbbell and rotation 2 2 2
Overhead triceps extensions with reverse grip 1 2 1
Standing French press with straight bar 2 1 2
Triceps pushdown with straight bar and narrow grip 2 2 1
Triceps pushdown with rope 2 2 2
Triceps pushdown with angled bar 2 2 2
One-arm triceps pushdown 2 1 2
One-arm triceps pushdown with reverse grip 2 2 2
Bench press with narrow grip 2 1 2
Parallel bar dip with neutral grip 2 2 2
Bench dip 2 2 2
Pullover with EZ bar and narrow grip 1 2 1
Military press with straight bar, behind neck 2 0 2
Standing dumbbell press 1 0 1
Kickback 2 1 2

