Isometric vs concentric vs eccentric 

These three classifications relate to what the muscle is doing when being activated. 

An isometric contraction is when the muscle is stationary under load, concentric is when the muscle is shortening and eccentric when the muscle is lengthening. 

For a bicep curl, a dumbbell held with an arm by one’s side, before lifting the weight upwards towards the chest and then lowering it back down via the elbow.  

Lifting the weight sees the bicep in concentric contraction, lowering the weight sees it in eccentric contraction, while holding the weight parallel to the ground is isometric.