The straight-arm pulldown isn't seen that often in the gym, but those in the know rely on it to add back width. It is an isolation movement, meaning that only one joint is mobile, and while it works several of the same major muscle groups as the pullup, it removes biceps from the equation. This allows the muscles of the back to be worked intensely, as bicep endurance is often the limiting muscle factor during a back workout.
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To perform the straight-arm pulldown, attach a straight bar to a high pulley machine. Face the machine and hold the bar with an overhand grip. Position one foot slightly forward of the other, flex your knees and bend forward from the waist. Your arms should be extended -- but not locked -- and your hands slightly higher than your head in the starting position. Keep your arms fairly straight as you pull the bar down just short of your thighs. Return slowly to the starting position. You can also use rope handles, with your palms facing each other.
The latissimus dorsi are the large, wing-like muscles that attach to the body along the mid-to-lower back, and on the arm at the upper humerus. They are responsible for retracting the arms -- pulling them into the body. The "Encyclopedia of Bodybuilding" explains that Dorian Yates, the bodybuilder with perhaps the most impressive lats in the history of the sport, found that no exercise hit them like the straight-arm pulldown. Even if you're a champ at pullups, the straight-arm pulldown may make your lats sore the next day.
The teres major sits just above the lats, attaching at the scapula and the upper humerus. Its primary function in weight lifting is to aid the lats in drawing the arm down and back. When built up by exercises such as the straight-arm pulldown, the teres major adds width to the upper back.
The triceps brachii are composed of three heads each: lateral, medial and long. They sit at the backs of the arms, attaching across the elbows, and are primarily responsible for straightening the arm at the elbow. During a straight-arm pulldown, the triceps work isometrically to hold the arms straight. However, the long head of the triceps attaches not to the humerus, but to the scapula. It aids the latissimus dorsi and teres major in retracting the arm, and so is active during straight-arm pulldowns.
Like the triceps pushdown, the straight-arm pulldown requires that you activate your abdominal muscles strongly to prevent the back from hyperextending. This is one of those rare exercises during which, like an abdominal exercise, slightly rounding the back actually relieves the strain on the lower back. The heavier the weight you use, the more intense the crunching action will be in the rectus abdominis and the external obliques.