The effects of partial repetitions in the initial range of motion on muscle hypertrophy Original paper

In this randomized controlled trial, training in the initial range of motion of a biceps curl was more effective than training in the end range of motion for increasing muscle size and strength.

The effects of training with different partial ranges of motion in the seated dumbbell preacher curl exercise on (i) changes in 1-repetition maximum strength in the full range of motion (0°–135°) and (ii) biceps brachii cross-sectional area (CSA) at 50% and 70% of humerus length (measured using B-mode ultrasound).

19 untrained women (average age of 23).

Using a within-participant design, each participant had one arm randomly assigned to train in the initial range of motion (iROM, 0°–68°, 0° indicating fully extended elbow) and the other arm trained in the final range of motion (fROM; 68°–135°) for 8 weeks. Half of the participants had their dominant limb assigned to iROM, and the other half had their dominant limb assigned to fROM. The participants performed three resistance exercise sessions per week, which consisted of 4 sets of curls performed until volitional failure.

Training protocols

A1 and A2 show the starting and finishing points in the iROM protocol. B1 and B2 show the starting and finishing points in the fROM protocol.

Compared to fROM, iROM increased CSA at 70% of humerus length to a large degree, whereas changes in CSA at 50% of humerus length were similar between conditions. Also, compared to fROM, iROM increased 1-repetition maximum strength to a large degree, despite the use of heavier absolute loads in the fROM condition.

At the beginning of a concentric muscle action (i.e., the lifting portion of an exercise, as opposed to the lowering portion), muscle length is longer than at the final angles of the action. Muscular contractions at longer (compared to shorter) muscle lengths have been reported to induce greater metabolic stress and IGF-1 release,^[1][2][3][4] both of which have been associated with muscle hypertrophy.^[5] As a result, it’s hypothesized that the iROM is the most important portion of an exercise for muscle growth.

The results of the summarized study support this hypothesis and add to a growing body of evidence demonstrating that training in the iROM promotes a greater hypertrophic response than training in the fROM.

Similar to the summarized study, a 2021 study that had untrained participants perform a 5-week resistance exercise intervention consisting of seated dumbbell preacher curls performed either in the iROM or fROM reported greater overall muscle growth in the iROM group, which was primarily driven by differences in the distal region of the muscle. Muscle growth was defined as the changes in the biceps brachii and brachialis combined.^[6]

In other studies that compared the hypertrophic response between iROM and fROM training, iROM produced greater hypertrophy in the vastus lateralis,^[7][1] rectus femoris,^[7] and medial and lateral heads of the gastrocnemius.^[8] The main limitation of this body of evidence is that the participant populations were untrained, so it remains unclear whether the results are applicable to well-trained individuals.

As it stands, studies consistently report that training in the iROM is superior to the fROM for muscle growth, mainly due to greater gains in the distal portion of the muscle. However, it’s important to compare partial range of motion exercises to full ROM exercises. Some studies have reported greater muscle growth with full ROM training compared to partial ROM.^[9][10] However, in these studies, the partial ROM group trained in the fROM, as opposed to the iROM. A proper comparison involves taking a look at some studies that compared training in the iROM to full ROM.

In a 12-week study that had untrained women perform knee extensions, there was a greater relative increase of muscle size at 50%, 60%, and 70% (but not at 40%) of femur length in iROM compared to full ROM.^[7]

In comparison, a 10-week study that had resistance-trained participants perform unilateral leg press reported similar vastus lateralis hypertrophy between iROM and full ROM. A limitation of this study is that hypertrophy was only measured at a single site along the length of the vastus lateralis.^[11]

A study that had untrained women perform calf raises for 8 weeks found there was a greater increase in medial gastrocnemius size after iROM training compared to full ROM, but there was no difference between groups for lateral gastrocnemius size.^[8]

In an 8-week study that had resistance-trained men perform lying barbell elbow extensions, partial ROM increased triceps brachii size more than full ROM.^[12] Interestingly, the partial ROM group in this study did not limit ROM to the iROM. Instead, elbow flexion during the exercise ranged from 45° to 90°, whereas in the full ROM group, elbow flexion ranged from 0° (full elbow extension) to 120° (full elbow flexion). Essentially, the partial ROM group avoided both the iROM and fROM in this study.

Despite the evidence demonstrating the benefits of training in the iROM for muscle growth, it’s unknown whether training in the iROM would have been as effective or more effective than the partial ROM used in this study, as the results from other studies were in different muscle groups, and thus they may not be generalizable to the triceps.

In conclusion, the available evidence indicates that training in the iROM promotes a greater hypertrophic response than training in the fROM. Furthermore, training in the iROM produces a superior or similar hypertrophic response to training with a full ROM. However, these findings have only been reported in the quadriceps, biceps, and gastrocnemius muscles. Further research is needed to determine whether other muscle groups (e.g., the hamstrings and pectoralis major) respond in a similar fashion.

The participants in this study were untrained, so the results may not be generalizable to well-trained participants.