How to Perform Tricep Rope Extensions
When these three heads are evenly developed they create the appearance of a horseshoe or upside-down U when flexed. Bodybuilders looking to naturally increase their arm size as rapidly as possible should focus on compound and isolation movements targeting the triceps, rather than the biceps. Powerlifters who grind through the midpoint of or fail to lockout their bench press reps should hammer their triceps with compound movements.
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The triceps are primarily built through pushing movements such as presses and extensions. These movements may be performed in a vertical or horizontal plane. Every balanced physique needs a beastly set of triceps.
The triceps rope pushdown is an isolation push exercise targeting the triceps brachii, comprised of the long, lateral, and medial heads. There are no supporting muscles groups assisting the target muscle group during this movement. However the latissimus dorsi, teres major (outer back), posterior or rear deltoid, sternal head of the pectoralis major, pectoralis minor, lower trapezius, rectus abdominis and obliques (abdominals), wrist flexors, hand flexors, hand extensors, and erector spinae (muscle running along both sides of the vertebral column) act as stabilizers during this exercise. 
Stabilizer muscles help maintain a posture or fixate a joint by contracting without significantly moving.  The triceps rope pushdown is most commonly performed using the cable tower apparatus.
MTS Nutrition CEO Marc Lobliner explains how to perform rope cable triceps extensions.
How to Perform the Tricep Rope ExtensionsApproach one side of the cable tower apparatus and move the pin height to the highest possible setting. Attach the rope with two knots or knobs (one at each end) to the cable and select the appropriate working weight.
If this is your first time performing the exercise then pick a conservative weight that you can safely lift for 8 to 12 repetitions. When in doubt, start with a conservative weight and work your way up; this will leave more room for progression and ensure you develop properly form early on.
Once you've selected the appropriate working grasp both sides of the rope above the knobs or knots with a neutral/hammer grip (palms facing each other). You can use a traditional grip (thumb wrapped around the fingers), hook grip (fingers wrapped around the thumb), or a false grip (thumb and fingers on the same side of the handle).
Take a stance in between hip and shoulder width that is relatively close to the cable tower; your feet can be staggered or in-line with each other. Stand tall with an upright posture, shoulders down and away from the ears. Some trainees find a more comfortable starting position with a slight forward lean of the torso and flex of the hips and knees.
However don't turn this slight lean in to a rounding of the back; the back should remain neutral and upright at all times. This is your starting position; your top of your forearms and bottom of your upper arm should be in contact with each other. If your forearms are perpendicular to the ground then your elbows are too far out in-front of you.
After setting your stance and grip, take a deep breath, brace your abdominals for impact and begin pushing the rope downwards. As your push downwards ensure your elbows remain in a fixed position, in-line with the torso and the knots or knobs don't touch.
Continue pushing downwards until the arms and elbows are straight (but not hyperextended). Many trainees find that simultaneously pushing the rope downwards and pulling the ends away from each increase tricep engagement.
If executed correctly this means you may have a pronated grip (knuckles facing you and palms facing the ground) at the bottom of the movement. Hold the rope in the bottom position for 1 to 5 seconds, focusing on flexing the triceps as hard as possible.
Then in a controlled motion slowly allow the rope to return back to the starting position. Complete for the desired number of repetitions. Some lifters choose to exhale while pushing the rope downwards or in between in each repetition. Choose a breathing pattern that feels the most natural and comfortable for you.
Throughout the entire movement your torso angle should remain fixed, posture should remain upright, and your elbow position should remain relatively fixed. If you find yourself swinging your body, raising your shoulders towards your ears, or unable to control the rope as it returns back to the starting position then the weight is too heavy.
This exercise can be performed using straight sets, pre-exhaust sets, drop sets, rest-pause sets, supersets, trisets, giant sets, paused reps, partial reps, forced reps, or slow negatives. As with any exercise, the two most important components are high-quality form and progression. Progression can take a variety of forms (e.g. more weight, sets, or reps, decreased rest period, improved rep quality, etc...) but strive to improve every training session.
Tricep Rope Extensions Form TipsMinimize Excessively Torso Lean & Elbow Movement - Ensure your spine stays neutral, shoulders stay away from the ears, and your elbows remain in a fixed location at your sides. Spinal flexion in to a crunch-like position leads to a form breakdown (drifting elbows and raised shoulders) which minimizes the stimulus of the target muscles.
Hold at the Bottom - If you're looking to increase intensity then experiment with holding the rope attachment at the fully pushed down position for 5 to 10 seconds. Really focus on squeezing the triceps. This will increase time under tension and the burn in the triceps.
Increased time under tension is an excellent variable to adjust for progressive overload and enhanced muscle growth.
Avoid Momentum - The triceps rope pushdown provides maximum benefit when it's performed in a controlled full range-of-motion. Check your ego at the door and don't attempt to start with maximum possible weight on the tower.
Excessively heavy weights on this exercise will lead to body swinging and back rounding in an effort to generate enough momentum to push the rope downwards. This momentum dramatically increases the likelihood of injury and minimizes the stimulus of the target muscles.
References1) "Cable Pushdown (with Rope Attachment)." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.
2) "Kinesiology Glossary." ExRx (Exercise Prescription) on the Internet. N.p., n.d. Web. 5 Nov. 2015.