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Hypertrophy and hamstring protection: seated vs prone leg curl

The hamstrings are muscles that make up the posterior compartment of the thigh. It is a polyarticular muscle group composed of the biceps femoris long head (bi-articular), the biceps femoris short head (mono-articular), the semitendinosus (bi-articular) and the semimembranosus (bi-articular) (Fig. 1). Because they pass through the hip and knee, they act as hip extensors (except for the short portion of the biceps femoris) and knee flexors.

In previous articles, we have already discussed the importance of hamstring work in the prevention and rehabilitation of knee injuries, and the differences in activation of these muscles depending on the exercise selected. Strengthening these muscles would also benefit performance during sprints and may reduce their risk of hamstring strain injury.

However, to develop strength and muscle mass, there are many variables to consider. In previous articles, we have discussed volume, frequency, tempo and training to failure. One variable that is often underestimated and implementation is often uncertain is the exercises selection to strengthen a given muscle group. In the case of hamstrings, there are many exercises that can be used to strengthen them. Among them, the leg curl is a common exercise that strengthens the knee flexion in an isolated way using two machines: one allowing the execution of the knee flexion movement, in a prone position and the other allowing the knee flexion in a seated position. Although the exercises can be performed with a full range of motion, the seated version (because of the hip flexion present throughout the exercise) places a constant stretch on the 3 bi-articular muscles of the hamstrings, which implies work on longer muscle lengths, compared to the prone version. And many studies tend to report better hypertrophy results when exercises are performed at longer muscle lengths. However, these results are not unanimous, in particular because of the heterogeneity of the protocols set up and of the level of participants...

Regular training at long muscle lengths would limit the muscle damage induced by eccentric work. Muscle damage and strain injury seem to be triggered by eccentric contractions, and some authors suggest that there is a link between a muscle's susceptibility to muscle damage and the likelihood of strain injury.

So would the seated leg curl decrease eccentric-related muscle damage more than the prone leg curl? And which variation would allow for more hypertrophy?

The Study

To answer these questions, Japanese researchers set up a protocol in 2 phases: the first consisted in comparing the effects of training with seated leg curl and prone leg curl on the hypertrophy of the hamstrings during 12 weeks, and the second phase consisted in carrying out an exclusively eccentric session and analyzing the muscle damage, to see if the preceding program would allow a better protection of the muscle.

For the first 12-week phase, the researchers recruited 20 untrained individuals (7 women and 13 men). For the 20 participants, each thigh was assigned to either seated leg curl (90° hip flexion) or prone leg curl (~30° hip flexion). During the 2 weekly sessions, they performed for each thigh 5 sets of 10 repetitions at 70% of 1RM. To assess muscle hypertrophy, the researchers used magnetic resonance imaging (MRI) to measure the volume of each muscle composing the hamstrings and the overall volume of the hamstrings.

For the second phase, 19 of the 20 participants in phase 1 and 12 control participants (4 females and 8 males) performed a purely eccentric session consisting of 3 sets of 10 repetitions at 90% of 1RM, for each leg, in each condition (seated and prone leg curl). Before, 24, 48 and 72 hours after the session, the 1RM and the T2 relaxation time measured by MRI (which depends on the presence of edema and inflammation) were measured.

Results & Analyzes

The main results of this study showed that 1) hamstring hypertrophy was greater following seated leg curl training for 12 weeks, but 2) there was no evidence that training at greater muscle length (seated leg curl) had a greater protective effect against hamstring damage.

After the first phase of the protocol, total hamstring hypertrophy increased by 14.1% for the seated leg curl and by 9.3% for the prone leg curl. For the the biceps femoris long head, muscle volume increased by 14.4% for the seated leg curl and by 6.5% for the prone leg curl, for the semitendinosus it increased by 23.6% versus 19.3% and for the semimembranosus it increased by 8.2% versus 3.6%. No change was observed for the biceps femoris short head (monoarticular) (+10% vs. +9%). In addition, the biceps femoris increased more in volume proximally (+20.8 vs. +8.7%) and distally (+10.7 vs. +5.4%) with the seated leg curl, and the semitendinosus increased more in volume proximally (+28.2 vs. 21.1%) with the seated leg curl.

Regarding the second phase, the T2 relaxation time measured by MRI was greatest 72 hours after the eccentric session. The increase in T2 reflects the inflammation in the muscle. There was no significant difference between the two types of Leg Curl for any of the hamstring muscles. For the control group, i.e. the participants who did not take part in the first training phase, the T2 values were much higher than for the other participants.

Practical Applications

This study shows once again that a variant of exercise allowing to train a muscle with a long muscle length, here the seated leg curl, would allow to obtain a greater hypertrophy... in beginners. This detail is important because it could explain why the muscle volume increased more during the seated leg curl. The greater passive tension in this variant is probably the main reason for this difference in volume gains. Muscle volume can increase in both series and parallel. And when the muscles work on a longer length, they can undergo more or less passive tension, which induces an increase in volume in series. However, this is mainly observed in people who are not used to training at great muscle length. This phenomenon would tend to be reduced with practice and the progressive increase (but to a certain point) of muscle volume in series. As the researchers did not take this variable into account, further studies with advanced practitioners will be necessary.

Finally, regular training would allow a better protection against muscular damage, whatever the variant of the leg curl, whatever the length at which the hamstrings are trained. That being said, the results are conditional, since the "eccentric" session performed at 90% of 1RM (concentric) does not necessarily represent true eccentric work (>100% of 1RM concentric) and does not necessarily represent the eccentric forces experienced by the hamstrings during high-intensity activities.

References

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