The how-tos and the details of forearm training, at least from what I've seen, seem a bit elusive. I'd like to share my thoughts, based on my own study, because I think the commonplace suggestions of "just do wrist curls" and “just train strapless, brah” are a bit remiss for the attentive bodybuilder.
A cross-section view of the forearm near the elbow shows that the vast majority of its volume is muscle. Forearm size, therefore, is very trainable. Its most significant musculature comprises the following:
- flexor carpi radialis
- flexor carpi ulnaris
- flexor digitorum profundus
- flexor digitorum superficialis (aka sublimis)
- extensor carpi radialis longus
- extensor carpi radialis brevis
- extensor carpi ulnaris
- extensor digitorum (communis)
- brachioradialis
- pronator teres
- supinator
- anconeus (mostly closer to the elbow)
- palmaris longus
Adapted from an original in The Division of General Surgery Manual of Surgical Anatomy (Washington, DC: Medical Departments U.S. Army and Navy, 1918)
What The Muscles Are, And What They Do
Flexors
The flexors' Latin names are mostly self-explanatory: the flexors carpi are primarily wrist flexors and the flexors digitorum are primarily finger flexors that, in crossing the wrist, also contribute strongly to wrist flexion.
There’s also the palmaris longus, which serves only as a wrist flexor. It is absent in approximately 14% of forearms – some people have only a right or left palmaris longus; some people have none.
All of these flexors except one – the flexor digitorum profundus – cross the elbow joint and originate at the humerus and they are also, therefore, minor contributors to elbow flexion. More significantly for training considerations, they can become actively insufficient under a combination of elbow, wrist and finger flexion, and I'll touch on this under Application.
My opinion is that for effective hypertrophy training it is best to treat all flexors primarily as one entity, training them with wrist flexion, and the flexors digitorum separately as an add-on, training them with dynamic grip work, if desired and if the capacity is there.
Application
(n.b. With the elbow passively flexed, propped onto one’s lap or a bench, the common dumbbell/barbell wrist curl can bring the forearm flexors to insufficiency (the muscles become too short to produce effective force) before the joint’s range of motion is fulfilled. You can demonstrate this by then introducing an external force -- your free hand in the case of a dumbbell wrist curl -- to easily passively curl the wrist further still, showing that the target musculature had become ineffective. At the other end – when the hand is hanging down at the beginning of the wrist curl – the flexors are well primed for contraction, but this is where the dumbbell’s gravity provides no resistance. Clearly, there are better options.)
For the wrist flexors, I propose a one-hand cable wrist curl. My preference is to stand with my back to the cable with the active arm down by my side, and the attachment set to a position that pulls the hand backward and slightly downward to allow for a good range of motion without distressing the joint. Flex the elbow for comfort.
A wrist roller, where a weight is attached via a cable and a hook to a bar which is then alternately twisted by the hands to reel the weight up, is also a fine option assuming proper overload is a logistical possibility. Be diligent with full, purposeful range of motion here.
The finger flexors explicitly can be trained very well with hand grippers. These can easily be found online and the two big brands, Heavy Grips and IronMind’s Captains of Crush, are both fine options and I like both. If you have small hands, Heavy Grips might be better as they have a slightly smaller “sweep”. Don’t bother with cheap grippers you can pick up at Wal-Mart; they’re too light for anyone reading this article.
It should be noted here that training isometrically, like merely holding on to a barbell, while great for modality-specific grip strength, is suboptimal for hypertrophy. It is dynamic, overloading training that grows muscles.
Extensors
The extensors carpi and digitorum all extend the wrist. The extensor digitorum indeed also extends the fingers, but it divides into its tendons above the wrist, so it produces more force at the wrist than at the fingers. Thus, the extensors should be trained as one entity – via wrist extension.
Application
Reverse dumbbell and barbell wrist curls are fine options; feel free to also use a cable machine. Muscle insufficiency will not be an issue in this position and it should be easy to properly overload the extensors with the forearm parallel to the ground, resting on one’s lap or over a bench. Strap up if you want; we’re not training grip here.
The aforementioned wrist roller is, again, a fine option if proper overloading is possible. As one wrist is being flexed, the other is being extended.
Brachioradialis
The brachioradialis is an elbow flexor on the radial side of the forearm that is, therefore, best primed for action when the forearm is oriented in the neutral-to-pronated range, where the muscle’s line of pull becomes mechanically optimized relative to that of the biceps and the brachialis. It is a vital component of the all-important Instagram thumbs-up pose.
Application
Anything that involves flexing the elbow when the forearm is turned inward, be it with hammer curls or reverse curls, or neutral- or pronated-grip pulls, will efficiently train this relatively simple monoarticulate muscle that has a lot of growth potential. Rope cable curls and hammer dumbbell curls are both fine isolation options.
The Other Stuff: Pronator Teres, Supinator And Anconeus
The pronator teres serves mainly to pronate the forearm but the larger of its two heads, which is superficial at the mid-forearm, originates at the humerus and is especially active in elbow flexion movements, counteracting the supinating force of the biceps, and that is likely its most effective and repeatable “training” stimulus in healthy people.
The supinator, which wraps around the radius, does what its name suggests: it supinates the forearm.
The anconeus is a small but superficial muscle on the posterior forearm near the elbow. In humans, it doesn’t have much specific use, if any, beyond acting like an extension of the medial head of the triceps, and it is thus automatically “trained” with elbow extension, though its potential for growth is small. (In quadrupedal primates, possibly including human babies, the anconeus seems to stabilize the elbow as they walk on their hands.)
Training Considerations
Volume and frequency
The relative size of the forearm musculature means that if we wanted to introduce, say, six weekly sets of wrist flexion work to our current protocol we probably wouldn’t have to then remove six sets of squats to save ourselves from dying. But while these muscles are small they are not insignificant, and no worthwhile training comes for free in terms of energy and recoverability.
Since their actions are so different, you can feel free to train the flexors, extensors and brachioradialis all on the same day(s) or separate them out as you wish. Consider your MRVs in terms of a flexors MRV, an extensors MRV and a brachioradialis MRV.
Flexors and Extensors:
3-6 sets per week of wrist flexion and 5-10 sets per week of wrist extension is probably a fine place to start, and they will likely be able to tolerate much more – probably over three or maybe four sessions per week. If in your mesocycle you’re feeling fine and ready for more accumulation next week, go ahead and add a bit more.
As with any movement, if wrist flexion and extension modalities have not recently been practiced, the first few sessions will probably cause some newbie soreness, so one should be gentle in the very beginning. More on this under When to Train.
Brachioradialis:
The brachioradialis is almost certainly already being trained in your current protocol, but maybe not quite within effective volume ranges for hypertrophy. Even when it’s not in a mechanically advantageous position to pull, say with a regular supinated biceps curl, the muscle does contribute at least a little bit and the extent of its involvement has to be understood in terms of a continuum. Consider its position on the radial side of the forearm: the more the muscle is in line with the joint’s flexion, the more readily it will develop tension.
Assuming you already have some neutral and/or pronated pulling in your protocol and want to really get working on your brachioradialis, you might only need a single weekly session of hammer curls or reverse curls to bring the muscle into its MEV-MRV range. Its MEV has had plenty of time to be bumped up with regular pulls work, especially if you’re diligent with eccentric contractions, but it will likely still be sensitive to volume increases if you’ve never focused on it.
Be mindful of the brachioradialis’s current work volume and err on the tentative side when it comes to introducing more – perhaps starting with a single weekly session of two or three sets. If you’re healing and feeling ready for more, add a second session.
When to train
Flexors:
My experience has been that wrist flexion work has felt better when I have already trained some pull or elbow flexion movement, as these serve to warm up the wrist joints and I’ve been able to then jump straight in and hit the flexors hard.
It may be that this work leaves the forearms sore for biceps work, especially when starting out. Remember: most forearm flexors cross the elbow joint too and while their contribution to elbow flexion is meek they are still under tension while you’re doing those curls, so it might be advisable to not focus on forearm flexors while in a biceps-focused phase.
Remember too that the forearm finger flexors are also wrist flexors. If you do train wrist flexion hard on a given day, you will be fatiguing those flexors digitorum and so grip will probably be compromised for the rest of the session and the next day or two. As always, it is up to you to strap up for pulling movements if grip strength threatens to disturb that work. This cannot be overstated: if the muscle you are supposedly training is not the limiting factor in a work set, your hypertrophy training sucks. Having grip strength undermine pulling work is just as silly as having an unstable surface undermine squat work.
Extensors:
The wrist extensors can be trained on any day at any time. Training wrist extension probably doesn't significantly impact any other work, though you may feel them burn a bit when they’re being passively shortened by subsequent pressing work. It’s likely that you’ll be able to insert at least a couple of weekly wrist extension sessions and then, once the first-timer soreness is out of the way, increase frequency and see how you respond.
Brachioradialis:
It’s probably a good idea to end one’s back day – where you will have probably already made a start on the brachioradialis by pulling things – with some isolation work, as the muscle will likely be able to well tolerate that extra bit of session volume. (Just don’t call it a “finisher”. Please.) Go easy in the beginning in this case – just two or three sets to start.
If you’re really focusing on the muscle you might want some more frequency, so consider scheduling a hammer or reverse curl variant at the beginning of your arms-focused day to give it the best, heaviest training.
Periodization
As with all skeletal muscle, our forearm musculature will become desensitized to the training stimulus after repeated hard training sessions. So how do we deal with this and keep growing? Simple. We train more and more. After a couple of years you should be up to 100 sets per week.
Or maybe not.
If you opt to focus on any of the flexors, the extensors or the brachioradialis specifically, or all of them together, it’s probably a good idea to do so for two or three mesocycles at a time before de-emphasizing them for a mesocycle or two, allowing them to resensitize, before returning to them for another phase.