Whether you use a pullup bar at the gym or the high bar on a kids' play structure at the park, pullups are a challenging body-weight exercise. Incorporating this exercise into your workout routine helps develop your upper-body strength. While a set of pullups requires a significant amount of strength in a number of your muscles, the activity targets only one muscle group.
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The target muscle of a set of pullups is your latissimus dorsi, more commonly known as your lats. These muscles are found roughly on the sides of your torso and the middle of your back. Your lats play an integral role in frequent movement -- including the adduction, internal rotation and extension of your shoulder -- and also assist with scapular movement.
Your lats are the target muscle of a set of pullups, but you can't execute the movement without the help of several other muscles, known as synergists. Synergist muscles during the execution of pullups include muscles in your arms, such as the brachialis, brachioradialis and biceps. Other synergists are the rhoboids in your back, the lower and middle trapezius in your back, the posterior deltoids in your shoulders and the pectoralis minor in your chest.
In addition to using the target and synergist muscles, performing pullups requires the use of the long head of your triceps muscles, which are found in the rear of your upper arm. These muscles help in the extension of your elbow and extension and adduction of your shoulder. During pullups, your triceps serve as the dynamic stabilizer, which means they provide assistance as you accomplish the motion.
If you're new to strength training, you might be unable to immediately perform a single pullup, much less regularly incorporate several reps of the exercise into your workout. You can increase the ease of this exercise by recruiting a partner to lift your lower body as you perform the exercise. Doing so allows you to work your muscles and eventually gain enough strength to perform pullups without assistance.