A standard biceps curl follows a bell-shaped (ascending-descending) strength curve, meaning the exercise feels lightest at the bottom and top of the movement and heaviest somewhere in the middle range. More specifically, the external torque from the dumbbell or barbell peaks when your forearm is roughly parallel to the floor (around 90 degrees of elbow flexion), then drops off as you curl higher or lower from that point. This makes the biceps curl one of the clearest real-world examples of a bell-shaped resistance profile.
Three Types of Strength Curves
Exercises are typically categorized into one of three strength curves based on where in the range of motion the load feels most difficult. An ascending curve means resistance increases as you approach full extension (think of a squat or deadlift lockout). A descending curve means the hardest point is near the bottom or stretched position. A bell-shaped (sometimes called ascending-descending) curve places peak difficulty in the middle of the movement.
The biceps curl falls squarely into that third category. Two separate biomechanical factors converge to create this mid-range peak: the torque the weight creates at your elbow, and the force your muscles can produce at different joint angles.
Why the Middle of a Curl Is the Hardest
When you hold a dumbbell with your arm hanging straight down, gravity pulls the weight straight through your elbow joint. That creates almost no rotational force your biceps needs to fight. As you curl upward, the horizontal distance between the weight and your elbow increases, and so does the torque. That torque hits its maximum when your forearm is horizontal, at roughly 90 degrees of elbow flexion, because the weight is now as far from the elbow’s axis of rotation as it can get. Continue curling past that point and the weight moves back toward the joint, reducing the demand again.
This is why the sticking point of a biceps curl is in the mid-range. You can verify this yourself: hold a moderate dumbbell with your elbow bent to 90 degrees and you’ll feel far more effort than holding the same weight at the very bottom or top of the curl.
How Your Muscle’s Force Output Aligns
Your muscles don’t produce the same force at every length. The length-tension relationship describes how a muscle generates the most contractile force at an intermediate length, not when it’s fully stretched or fully shortened. Research modeling the elbow flexors found that the biceps brachii reaches its optimal force-producing length at about 110 degrees of elbow flexion, while the brachialis (the muscle underneath the biceps) peaks around 100 degrees. These are both mid-range positions.
The moment arm of the elbow flexors, the mechanical leverage they have on the joint, also peaks at an intermediate angle and shortens at both extremes. So your muscles have the most internal leverage in roughly the same zone where the external load demands the most force. These two factors together reinforce the bell-shaped profile: moderate demand at the ends, peak demand in the middle.
What EMG Data Shows
Muscle activation patterns during curls support this profile, though with some nuance depending on how the exercise is performed. In a study published in the Journal of Sports Science and Medicine, researchers compared biceps activation during different dumbbell curl variations. For a standard incline dumbbell curl and a standing dumbbell curl, EMG activity climbed progressively from the start of the concentric (lifting) phase to the final phase, reaching up to 95% of maximum activation. This makes sense: the nervous system ramps up recruitment as the torque demand increases through the mid-range.
Interestingly, when the shoulder was flexed forward (as in a preacher-style position), the activation pattern reversed, peaking early in the lift and declining toward the top. This tells you something important: the bell-shaped curve applies specifically to a standard curl where gravity and forearm angle create that mid-range torque peak. Change the setup and you shift where the curve peaks.
How Different Curl Variations Shift the Curve
One reason this topic matters for training is that you can deliberately choose curl variations that load different portions of the range of motion.
- Barbell preacher curl: Because your upper arm rests on an angled pad in front of your body, the weight creates the most torque when your elbow is closer to full extension. This shifts the peak demand toward the stretched (lengthened) position of the biceps. A study comparing barbell preacher curls to cable preacher curls found that the barbell version applied greater torque when the elbows were extended and the biceps elongated, and it produced a small but significant advantage in building strength at that extended position.
- Cable preacher curl: The cable’s line of pull changes the torque profile so that demand is higher when the elbows are flexed and the biceps are shortened. This effectively moves the peak toward the top of the movement.
- Incline dumbbell curl: Sitting on an incline bench lets your arms hang behind your torso, placing the biceps in a deeper stretch at the bottom. This extends the range over which gravity loads the muscle in its lengthened position, though the peak torque still occurs around the mid-range where the forearm approaches horizontal.
- Concentration curl: With your elbow braced against your inner thigh, this variation keeps the bell-shaped profile but eliminates momentum, forcing the biceps to handle the mid-range peak without help from body English.
Why This Matters for Training
The bell-shaped curve of a standard biceps curl means the weight you can use is limited by the hardest point in the middle. At the top and bottom of the rep, your biceps are working well below their capacity. This is the fundamental reason people use tools like resistance bands or cables to modify the resistance profile. Adding a band to a dumbbell curl increases tension toward the top of the movement, partially flattening the curve so the muscle works harder across a wider range.
It also explains why training with only one curl variation may leave gaps. The barbell preacher curl challenges the biceps most in the stretched position, the cable curl loads the shortened position more heavily, and the standard dumbbell curl peaks in between. Combining variations across training cycles exposes the muscle to high tension at different lengths, which research suggests produces more complete strength and size adaptations.
So when an exam question or textbook asks which strength curve a biceps curl represents, the answer is bell-shaped. The external torque rises from the bottom, peaks near 90 degrees of elbow flexion, and falls again toward the top, creating a symmetrical mid-range difficulty that defines this category of movement.