According to Wikipedia, a pullup is “an upper-body compound pulling exercise. Although it can be performed with any grip, in recent years some have used the term to refer more specifically to a pullup performed with a palms-forward position.”
The exercise begins with you hanging from a fixed bar using an overhand grip. The goal is to then pull yourself up until your elbows are bent, your head higher than your hands and the fixed bar at shoulder height. This pullup action targets many muscle groups of the upper body, requiring them to work together to complete the movement.
The pullup includes movement around two joints – the shoulder and the elbow. To begin with, the hand and forearm are used to grip the bar, before the larger muscles of the upper arm, shoulder and back are utilized to perform the lifting action.
The muscles behind the movement include the trapezius, latissimus dorsi, forearm flexors/extensors, biceps, triceps, deltoids and abdominals.
In case you’re wondering about the difference between a pullup and a chinup; the chinup uses an underhand grip (palms facing towards you), whilst the pullup uses an overhand grip (palms facing away from you). The chinup is commonly regarded as the “easier” exercise (both are super-challenging by the way), so feel free to follow this program doing chinups if you so desire.
Remember, the main aim of the twenty five pullups program is to improve your upper back and lats strength, with a side benefit of working out your core and middle back muscles. The program is all about making progress and constantly challenging yourself. Give the 6-week program a chance and I know you’ll be amazed with your improvement. Good luck!