Below is further discussion.
What About Leverage?
Some suggest that curls that are harder at the start portion (such as preacher curls) will produce greater biceps hypertrophy. This stems from research suggesting that the biceps have relatively greater leverage for flexing the elbow at the initial stages of curling, whereas the brachioradialis has greater leverage for flexing the elbow during the later stages of curling.
Now, I do think leverage relates to muscle activation and hypertrophy under certain circumstances. For example, the side delts have greater leverage for shoulder abduction (the movement that occurs during lateral raises) than the front delts, while the front delts have greater leverage for shoulder flexion (the movement that occurs during a front raise) than the side delts. I think we can pretty much be certain that lateral raises are going to grow the side delts more, while front raises grow the front delts more.
However, in this scenario with the elbow flexor muscles, it’s apparent the research does not support it. Incline curls (which tend to be hardest at the mid to later portions of the curl) appear to produce greater overall biceps hypertrophy than preacher curls (which tend to be hardest at the start of the curl).
Perhaps the research examining the leverage of the various elbow flexors is limited. One possible limitation is that these studies tend to measure leverage when the muscle is relaxed, but leverage values can change when muscles are contracting (as happens during an exercise). Another explanation is that leverage is just one of many other factors that contribute to a muscle’s stimulus, so other factors may explain why the biceps did not grow better from preacher curls.
Why Might Incline Curls Build the Biceps Better Than Preacher Curls?
Some suggest the answer lies in shoulder flexion. During the incline curl, it’s not uncommon for some degree of shoulder flexion (your arm moves a little forward) to occur while you’re curling. With the preacher curl, however, your shoulder generally remains locked in place.
Remember the biceps cross both the elbow and shoulder, and besides their role in elbow flexion, they have some role in shoulder flexion.
It has been suggested that the region of the biceps closer to the shoulder joint (that is, the upper regions of the muscle) will be more involved in shoulder flexion. So the involvement of dynamic shoulder flexion during incline curls enhances the stimulus to the upper biceps region, explaining why it grows more from the incline curl.
The degree to which the biceps are involved in shoulder flexion is relatively minimal, and I’m overall just skeptical this is sufficient to explain the entirety of the greater regional biceps growth seen from incline curls.
There is another possible explanation.
Once again, the biceps cross both the shoulder and elbow. We call this a biarticular muscle (a muscle that crosses two joints).
Another example of a biarticular muscle is the rectus femoris (part of the quadriceps). It crosses both the knee and hip. Leg extensions are an established exercise for developing this muscle.
Leaning back extends the hip joint and will effectively lengthen the rectus femoris more, particularly the regions of the rectus femoris closer to the hip (which are the upper regions of the muscle).
One study found leaning back leg extensions grew the rectus femoris overall more than normal leg extensions, with the largest difference at a more upper region. Another study on competitive bodybuilders recorded muscle activity from the rectus femoris during leaning back versus normal leg extensions, and reported greater rectus femoris activity with the leaning back variation, with the largest difference occurring at the upper region. I should note that the measure of muscle activity used in this study (T2 weighted imaging) has been found to relate to muscle growth in a few studies.
The hamstrings (except for the short head of the biceps femoris) are biarticular, crossing both the knee and hip. We all know leg curls train the hamstrings. Compared to lying leg curls, seated leg curls have our hip joint in a more flexed position and thus lengthens the hamstrings, particularly the regions of the hamstrings closer to the hip (which are the upper regions).
One study found seated leg curls produced greater hamstrings volume increases than lying leg curls. Note, volume is a 3D measure that considers all muscle regions, but fortunately the researchers also examined upper and lower region growth of two of the hamstring heads. Both the upper and lower regions of these heads grew more with the seated leg curls, but the biggest differences tended to occur at the upper region.
This is not too different with the incline versus preacher curl. Incline curls extend our shoulder joint and consequently length the biceps more, particularly the regions of the biceps closer to the shoulder (which is the upper region of the muscle).
Therefore, it could just be that with biarticular muscles, placing the muscle in to a longer length through changing the position of one joint (e.g. putting the hip joint into an extended position with leaning back leg extensions to length the rectus femoris), ends up growing the regions of the muscle closer to the joint that’s been moved (e.g. the regions of the rectus femoris closer to the hip joint).
This is currently my best guess, but undeniably more research is required to have any great degree of confidence. There is also research on the triceps that somewhat goes against this reasoning, but I have thoughts that could explain why that I’ll save for another time.
Why Might Preacher Curls Be Better Than Incline Curls for the Brachialis and Brachioradialis?
One suggestion might be that since the biceps seem to grow overall less from preacher curls, the brachialis and brachioradialis ultimately receive the bulk of the stimulus. In other words, the slightly lower biceps involvement is compensated for by an increase in brachialis and brachioradialis involvement.
However, there is evidence from other muscle groups going against this logic. As we noted above, the rectus femoris seems to grow overall more from leaning back leg extensions. Some might think the vastus heads of the quads then must end up growing more with normal leg extensions, but this does not appear to be the case (vastus lateralis growth was similar between leaning back and normal leg extensions in the paper). Likewise, in the study comparing seated to lying leg curls, we know the growth of the biarticular hamstring muscles was greater from seated leg curls, but growth of the biceps femoris short head (which is not trained at a longer length with seated curls, rather the same length) was similar between both exercises.
Of course, things could be different across muscle groups, so I can’t rule out this idea completely.
Another suggestion relates to the resistance challenge of each exercise.
Of course, both the brachialis and brachioradialis cross the elbow joint, but neither crosses the shoulder joint. Therefore, the brachialis and brachioradialis move through the same muscle lengths between the incline and preacher curl (assuming we move the elbow joint through the same range of motion).
However, the resistance challenge of both exercises differs. Preacher curls (specifically the dumbbell and barbell variation, which we’re talking about here) are hardest in the earlier part of the curl, where the brachialis and brachioradialis are at a longer length. Incline curls tend to be hardest in the middle to later portion of the curl, where the brachialis and brachioradialis are at a relatively shorter length.
Thus, it’s possible the greater tension at longer muscle lengths for the brachialis and brachioradialis explains why they grew more from preacher curls.
This argument could further be strengthened by the regional differences observed in the Kobayashi abstract. Remember it specifically noted it was the 70-80% regions of the brachialis that grew more with preacher versus incline curls, and it was specifically the 60% region of the brachioradialis that grew more with preacher versus incline curls. These regions (60-80%) can generally be described at the distal regions of the muscle. Note that distal refers to the region further away from the origin, while proximal refers to the region closer to the origin.
When examining the range of motion literature, we tend to see that compared to a shortened partial (such as performing just the top part of the curl), a lengthened partial (such as performing just the bottom part of the curl) or full range of motion tends to result in preferentially greater distal region growth.
To get a clear answer, what we’d need to do is compare a cable preacher curl (with the cable positioned to provide the greatest challenge in the later portion of the curl, where the brachialis and brachioradialis are at a shorter length) to free weight preacher curls (which we know are harder in the earlier part of the curl). If brachialis and brachioradialis growth is better, particularly at those distal regions, then this might serve as evidence that resistance challenge explains why free weight preacher curls elicit overall greater brachialis and brachioradialis growth.
Discussion on the Long and Short Heads of the Biceps
It has been suggested that incline curls grow the long biceps head more, while preacher curls grow the short biceps head more. This is commonly suggested because people believe that placing the shoulder into extension (which happens in an incline curl) lengths only the long head of the biceps, and consequently puts it into a position to more effectively generate force. Conversely, placing the shoulder into flexion (which happens in the preacher curl) only really shortens the long head of the biceps, thereby making it less effective for producing force such that the short head takes over.
However, it is important to point out that both the long and short heads cross the shoulder, so both are lengthened more with the shoulders extended. Although, the relative lengthening does appear to be greater in the long head with shoulder extension.
Therefore, one could still suggest the relatively greater lengthening of the long head with incline curls, and consequently the relatively greater shortening of the long head with preacher curls, means that incline curls build the long head a little more, while preacher curls build the short head a little more.
This could be true, but there are also reasons to be skeptical. Even though the long head may be relatively lengthened more than the short head during an incline curl, I’m not sure this has to mean the short head of the biceps grows any less. For one, the short head is still at a longer length in incline curls compared to preacher curls. Additionally, as mentioned earlier in this article, leaning back leg extensions lengthen the rectus femoris more, but do not result in any worse vastus lateralis hypertrophy. Seated leg curls lengthen much of the hamstring muscles more than lying leg curls, but are not any worse for the head it does not lengthen more (the biceps femoris short head).
None of the current studies comparing incline to preacher curls separated out the long and short heads of the biceps, so I can’t conclude anything with a large degree of certainty. But until more research comes out, I wouldn’t really worry about this.