Smith Machine Behind the Back Shrug

How to do it

With the bar at thigh level, load an appropriate weight.
Stand with the bar behind you, taking a shoulder-width, pronated grip on the bar and unhook the weight. You should be standing up straight with your head and chest up and your arms extended. This will be your starting position.
Initiate the movement by shrugging your shoulders straight up. Do not flex the arms or wrist during the movement.
After a brief pause return the weight to the starting position.
Repeat for the desired number of repetitions before engaging the hooks to rack the weight.

What muscles does the Smith Machine Behind the Back Shrug work?: Mainly the traps, with help from the shoulders.
What equipment do I need?: Machine.
Is it good for beginners?: Yes, it is rated beginner level.
How do I track progress on it?: Log each set and beat the weight or reps from last time. Liftrung shows your last set beside the next one and estimates your one-rep max from your sets.

Exercise descriptions: free-exercise-db, public domain.