How much does training the brachialis affect arm thickness?

When people think about wanting to make their arms bigger, the first things that come to mind for most are the biceps brachii and the triceps brachii.

However, in recent sports science and muscle hypertrophy research, the importance of training the "brachialis" has been highlighted again for creating overwhelming arm volume and three-dimensional definition.

In this article, based on recent papers and evidence, I will explain the impact the brachialis has on arm thickness and scientific approaches to training it.

1. Degree of impact on arm thickness: Why is the brachialis important?

In conclusion, the brachialis functions as a foundation that pushes the width (thickness) of the arm out from the inside, so it is impossible to efficiently increase arm size while ignoring it.

① It accounts for approximately 50% of the volume of the elbow flexor group

According to recent anatomical data and evidence (such as the review by Dr. Milo Wolf et al.), it is known that among the total volume of muscles that bend the elbow (elbow flexor group), not only the biceps brachii, but the brachialis and brachioradialis account for nearly 50%. In other words, standard bicep curls with palms facing up only efficiently stimulate half of the muscles that bend the elbow.

② The "insole" effect of pushing up the biceps

The brachialis is located deep (underneath) the biceps brachii. When this muscle hypertrophies, it physically pushes the biceps brachii above it outward and upward. This dramatically increases the thickness of the arm when viewed from the front or side, and makes the muscle separation stand out.

2. The mechanism of the "brachialis" from recent research and papers

From electromyography (sEMG) studies published between 2023 and 2025, as well as papers on load profiles (research by Zabaleta-Korta et al. and Kassiano et al.), the following important facts have been revealed.

Fact 1: You can reduce "bicep involvement" by changing wrist orientation

The biceps brachii has a strong action not only of "bending the elbow" but also of "turning the palm upward (supination)." Recent electromyography studies (Coratella et al., 2023, etc.) have also shown that turning the palms downward (pronation) or inward (neutral position) significantly reduces biceps brachii activity.

On the other hand, due to the structure of the bone (because the insertion point is on the ulna), the brachialis maintains its function of bending the elbow with 100% force regardless of wrist orientation. As a result, with a hammer grip or reverse grip, the involvement of the biceps brachii decreases, and the ratio of the load on the brachialis increases relatively.

Fact 2: Strong hypertrophy effect in the "stretch position"

According to data on "pre-stretch exercises in the lengthened position," which is a trend in recent muscle hypertrophy research, it is known that local muscle hypertrophy is easily induced when strong mechanical tension is applied while the muscle is stretched. Similarly, for the brachialis, applying a load firmly from the bottom position where the elbow is extended becomes a trigger for strong hypertrophy.

3. 3 steps to scientifically and efficiently target the brachialis

Based on findings from papers and biomechanics, the specific steps to efficiently target the brachialis are as follows.

1. Fix the forearm position to "neutral" or "pronated": By keeping the palms facing each other (hammer grip) or facing down (reverse grip), you intentionally eliminate the biomechanical advantage of the biceps brachii.

2. Focus on the 'stretch' at the bottom position and maintain constant tension: The brachialis exerts particularly strong torque during the initial phase of elbow flexion (from the start of the movement to the midpoint). Carefully absorb the weight from the bottom position where the elbow is fully extended, without using momentum.

3. Control the negative (lowering) phase for 2 to 3 seconds: In recent muscle hypertrophy research, tension during the eccentric (lowering) phase is considered highly effective for stimulating even the deep fibers of the brachialis. Control the weight so that you never let it drop suddenly.

4. Recommended exercises targeting the brachialis

◆ Incline Hammer Curl

This is a hammer curl performed on an incline bench. Because the brachialis is in a strongly stretched state at the start, it is one of the best exercises that aligns perfectly with the modern theory of 'hypertrophy in the lengthened position'.

◆ Preacher Hammer Curl / Reverse Curl

Perform these with your elbows fixed on a pad. By fixing the elbows in front of the body, you prevent shoulder joint compensation (cheating by other muscles) and can concentrate the load purely on the brachialis and brachioradialis.

Summary: To build thick arms even when wearing clothes

If you want to build 'arms that look muscular even when wearing clothes' or 'overwhelming three-dimensional volume on the outside,' incorporating hammer-style or reverse-style exercises for even a few sets per week in addition to regular bicep curls is a scientifically sound strategy to directly target the brachialis.