The next step was to work on strengthening them in the gym using weights.

Nick breaks down his swing as follows to show which muscles need to be worked on:

Backswing:
Shoulder, chest, forearm and wrist (flexes and cocks wrist), stomach, hip and buttocks, thighs (pulled as hip rotates) and the lower leg muscles

Start of downswing:
Shoulder, chest (when pulling down), upper back (lats), obliques and abs (when trunk turns), posterior hip muscles (bum!), thighs (pulled and weight shifted from one leg to other) and lower leg muscles (rolls and left foot inward)

Nearing Impact:
Shoulder, chest (pecs), upper back, forearm and wrists (uncocks, rolls and extends)

Impact:
Obliques and abs, trunk, buttocks (hip turn), thighs (weight transfer), lower leg muscles

Followthrough:
Chest, upper back, obliques, trunk, bum and lower leg - movement complete