Preacher curls for brachialis

While investigating the potential for the elbow flexors to display stretch-mediated hypertrophy, researchers have inadvertently reported some interesting information about how the various components of that muscle group respond to different exercises, especially in respect of the biceps brachii and brachialis.

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#1. Preacher dumbbell curl versus incline dumbbell curl

When comparing the preacher dumbbell curl and the incline dumbbell curl, researchers have shown that the preacher dumbbell curl is better at training the brachialis muscle compared to the incline bench dumbbell curl (while the incline bench dumbbell curl was better at training the biceps brachii).

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Similar research that did not measure individual muscles within the elbow flexor complex still showed different regional hypertrophy as a result of the same exercise comparison.

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#2. Preacher cable curl versus seated cable curl

When comparing the preacher cable curl and the seated cable curl, researchers have shown that the preacher cable curl is probably slightly better at training the brachialis muscle compared to the seated cable curl (while the seated cable curl was probably slightly better at training the biceps brachii).

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#3. Preacher curls only cause muscle pump in the brachialis

When researchers have tested the effects of preacher curls on the amount of intra-workout muscle pump in the elbow flexor muscles, the only muscle that experiences meaningful increases in size is the brachialis. This implies that the brachialis is preferentially activated during the preacher curl.

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#4. Preacher curls only cause delayed muscle swelling in the brachialis

When researchers have tested the effects of preacher curls on the amount of post-workout muscle swelling in the elbow flexor muscles, the only muscle that experiences meaningful amounts of damage and inflammation leading to swelling is the brachialis. This implies that the brachialis is preferentially activated and damaged in the preacher curl.

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#5. Why does this happen?

Which muscle is used the most during an exercise is determined by its internal moment arm length in accordance with the principle of neuromechanical matching (read more).

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The biceps brachii are two-joint muscles and cross the shoulder and the elbow. Thus, when the shoulder is placed into a flexed position (as in the preacher curl), the biceps brachii are shortened, while when the shoulder is placed into an extended position (as in the incline bench curl), the biceps brachii are lengthened. Shortening and lengthening affect the shape of the muscle and therefore they also affect the leverage. Consequently, it is likely that the leverage of the biceps brachii are reduced by shortening such that the brachialis takes over. Indeed, we know that the exact same thing happens to the two-joint hamstrings when we do lying leg curls such that the single-joint biceps femoris takes over.

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Conclusions

When comparing the preacher curl and the incline bench curl, researchers have shown that the preacher curl is better at causing hypertrophy of the brachialis muscle compared to the incline bench curl, while the incline bench curl is better at causing hypertrophy of the biceps brachii. This same result can be observed in multiple studies.​ The biceps brachii are two-joint muscles and cross the shoulder and the elbow. Thus, when the shoulder is placed into a flexed position (as in the preacher curl), the biceps brachii are shortened, while when the shoulder is placed into an extended position (as in the incline bench curl), the biceps brachii are lengthened. In contrast, the length of the brachialis muscle is not affected. Shortening and lengthening affect the shape of the muscle and hence they also affect the internal moment arm length. Thus, it seems likely that the leverage of the biceps brachii is reduced by shortening such that the brachialis takes over. Indeed, the exact same thing happens to the two-joint hamstrings when we perform lying leg curls such that the single-joint biceps femoris short head takes over (the biceps femoris short head grows more after lying leg curl training where the two-joint muscles are shortened than after seated leg curl training where the two-joint muscles are lengthened).​

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