How Do Partial Preacher Curls Affect Your Muscle Growth?

Key Points:

  • A group of 24 untrained young adults trained partial preacher curls twice a week for five weeks. One group only trained at long muscle lengths (0°–50° flexion) and the other only trained at short muscle lengths (80°–130°).
  • After five weeks, the group training at long muscle lengths had made greater gains in both strength and muscle size.
  • The difference in muscle growth was especially large in the distal portion of the biceps (the part closest to the elbow), which is in line with emerging research.

Who doesn’t want bigger biceps? Let’s face it, we all do. And in the hunt for bigger guns, you might have stumbled upon the concept of partial rep training. That is, only performing a portion of the full range of motion in a given exercise.

But how does that affect your muscle growth? And is it better to perform the bottom or top range of motion?

A new study investigating how bicep training at short vs. long muscle lengths affects strength and muscle gains sheds some light on this matter.1

Once again, I first read about this study through the newsletter MASS for which we are an affiliate.

Top or Bottom Range of Motion – Which is Better for Bicep Growth?

For this study, the researchers recruited 32 untrained young adults. Eight of the participants were assigned to be a non-training control group, and the other 24 got to train dumbbell preacher curls twice per week, for five weeks.

But here’s the thing: they didn’t train with a full range of motion (ROM).

They were split into two groups, training either at long or short muscle lengths.

  • Long Muscle Length: These 12 participants only trained the bottom part of the exercise, from 0°–50° elbow flexion (where 0° is a straight arm). At this end part of the ROM, the biceps is working at a long muscle length.
  • Short Muscle Length. The other 12 participants only trained the upper part of the exercise, from 80°–130° flexion. At this top top part of the ROM, the biceps is working at a short muscle length.
Dumbbell Preacher Curl
The dumbbell preacher curl with a full range of motion. The bottom turning point is at 0° elbow flexion (= a straight arm), and the top turning point is at about 130° flexion.

The participants did 3 sets x 10 reps at a weight based on their maximal isometric strength around the angles they were training in.

Before and after the study, the researchers measured bicep plus brachialis (a muscle located underneath the biceps) thickness at 50, 60, and 70% percent of their upper arm length. 50% being the midpoint, and 60 and 70% being closer to the elbow. They also used a BioDex-machine to measure isometric, concentric, and eccentric strength.

Greater Gains From Training at Long Muscle Lengths

It turns out that the group training at long muscle lengths (the bottom part of the curl) gained more strength and more muscle volume than the group training at short muscle lengths.

Notably, both groups seemed to gain similar amounts of muscle volume around the midpoint (50% and 60%) of their biceps, but at the most distal measurement site, the group training at long muscle lengths saw clearly better muscle gains.

Muscle Thickness Increase from Partial Preacher Curls

Over the three measurement sites combined, this added up to an increase in muscle thickness of 8.9% vs. 3.4% for the long vs. short muscle length training groups, respectively.

The average gains in strength as measured by maximum voluntary contractions for both groups (long vs. short muscle length training) are noted in the table below.

Strength VariableLong ML TrainingShort ML Training
Isometric15,1%6,8%
Concentric17,1%5,8%
Eccentric15,8%2,2%

Train at Long Muscle Lengths for Full Muscle Growth

This is the fourth study I’m aware of that found that the range of motion you train in seems to affect your regional muscle growth.

  • Pedrosa et al. (2021) found that both full ROM leg extensions and partial leg extensions performed at long muscle lengths yielded similar proximal quad muscle growth but better distal growth than partial leg extensions performed at short muscle lengths.2
  • Bloomquist et al. (2013) found that deep squats led to greater quad muscle growth than quarter squats, and especially in the distal region of the quadriceps.3
  • McMahon et al. (2014) found that leg training through 0°–90° knee flexion led to greater distal quad muscle growth than training only through 0°–50° knee flexion.4

The picture that is emerging seems to be that training at long muscle lengths is necessary for maximizing hypertrophy along the whole length of your muscle. If you only train partial movements at short muscle lengths, your muscles won’t grow as much as otherwise, and especially not in the distal parts.

The simplest course of action is, of course, to just train with a full range of motion in most exercises you do. However, there is at least one instance where partial rep training has yielded larger muscle growth than full ROM training, perhaps because of hypoxia resulting from the constant contraction.5 Likewise, the earlier mentioned study by Pedrosa et al. found some measures of quad growth that improved more in the partial training groups than the full ROM group.

Conclusions and Practical Take-Away

When in doubt: train with a full range of motion to optimize muscle growth and strength.

If you seek variety or want to experiment with partial reps, training at long muscle lengths (i.e. the bottom position of a squat, bench press, or leg extension) seems to be important to maximize muscle hypertrophy along the whole muscle length.

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References

  1. Front Physiol. 2021 Sep 16;12:734509. Elbow Joint Angles in Elbow Flexor Unilateral Resistance Exercise Training Determine Its Effects on Muscle Strength and Thickness of Trained and Non-trained Arms.
  2. Eur J Sport Sci. 2021 May 23;1-11. Partial range of motion training elicits favorable improvements in muscular adaptations when carried out at long muscle lengths.
  3. Eur J Appl Physiol. 2013 Aug;113(8):2133-42. Effect of range of motion in heavy load squatting on muscle and tendon adaptations.
  4. J Strength Cond Res. 2014 Jan;28(1):245-55. Impact of range of motion during ecologically valid resistance training protocols on muscle size, subcutaneous fat, and strength.
  5. J Strength Cond Res. 2019 May;33(5):1286-1294. Partial Range of Motion Exercise Is Effective for Facilitating Muscle Hypertrophy and Function Through Sustained Intramuscular Hypoxia in Young Trained Men.
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Daniel Richter

Daniel has a decade of experience in powerlifting, is a certified personal trainer, and has a Master of Science degree in engineering. Besides competing in powerlifting himself, he coaches both beginners and lifters at the international level. Daniel lives in Lund, Sweden with his wife and three kids. On StrengthLog, Daniel geeks out about all things related to his lifelong passion of muscle and strength.