Introduction

The soleus and gastrocnemius are large force producers and have an important contribution to locomotion. Their aponeuroses combine to form the Achilles tendon, yet due to their different anatomy (the biarticular gastrocnemius and the uniarticular soleus), they are subject to different biomechanical loads. Injuries to the calf musculotendon unit are relatively common as they undergo rapid stretch-shortening cycles during forward propulsion. In the later stages of rehab for such injuries, plyometric exercises are often used to build strength and prepare the calf for the rapid stretch-shortening cycles. Guidelines to progress plyometric exercises are poorly developed and therefore this study wanted to compare the output of the musculotendon unit of both the gastrocnemius and soleus – as they theoretically would behave differently during plyometric exercises. These plyometric progressions of the calf can be used to prepare an athlete to return to running.

Methods

In this experimental crossover design study, 14 trained distance runners were included. The runners were experienced and ran on average of 86km per week. They were all familiar with strength training for at least 12 months prior to enrolling in this study and were injury free. Their running was analyzed on a 110m indoor running track when running at 3.89m/s. Next to this, they also performed 4 plyometric exercises: ankle bounce, hurdle jumps, a-skip and bounding.

Three-dimensional and force plate data were collected and computational simulations were used to calculate peak forces, strain, power generation and absorption, and total positive and negative work of the gastrocnemius lateralis and soleus musculotendon unit. Running was compared to the 4 plyometric exercises and plyometric progressions of the calf towards running were established. Muscles were also classified as net energy absorbers or generators.

Results

The analyses revealed that both the gastrocnemius lateralis and soleus running produced the greatest peak power generation. The gastrocnemius lateralis also produces the greatest peak force, while the soleus absorbs most of the energy during running.

When comparing the plyometric exercises to running the following was seen for the gastrocnemius lateralis

• The ankle bounce had similar negative work compared to running 
• The A-skip produced less peak force, peak strain, and peak power absorption compared to running. Total positive and negative work was higher during the A-skip than during running.
• Bounding elicited greater peak strain and total negative work than running, but produced similar peak force, power, and total positive work. 
• The gastrocnemius lateralis behaved as a net energy generator, except during bounding where it acted more as an energy absorber.

When considering the soleus during the 4 plyometric exercises it became clear that:

• The A-skip produces less peak force, peak strain, and power generation and absorption compared to running.
• During bounding, greater peak strain, force, and total positive and negative work were seen compared to running. But peak power generation and absorption were low compared to running. 
• Hurdle jumps created more peak strain and negative work compared to running. 
• The soleus behaved, as the gastrocnemius lateralis, as a net energy generator except during hurdle jumps where the soleus acted more like an energy absorber.

To sum up, plyometric progressions of the calf that can be used to help in your rehabilitation could be the following:

For the lateral gastrocnemius, the a-skip may serve as an excellent exercise for the lateral gastrocnemius before returning to running. The ankle bounce with a similar eccentric load, but less other force outputs can serve as an exercise that may be introduced in plyometric training to rehab gastrocnemius lateralis before running is initiated. Bounding produces more eccentric load, but equal concentric load, and therefore, bounding may be an exercise to do when eccentric overload is desired, however, initially it may be too demanding for injured runners
For the soleus, the a-skip could quite similarly be introduced before running. Hurdles produce high eccentric soleus loads but low gastrocnemius lateralis loads compared to running and therefore this may be suited to improve the energy storing and releasing capacity of the soleus while minimizing the force on the lateral gastrocnemius. Bounding produces high eccentric soleus loads, just as seen for the lateral gastrocnemius here above.

Questions and thoughts

A relevant question mark to this study can be placed on the relatedness of the short indoor running analysis in these runners, trained on outdoor tracks and long distances. As distance running is a continuous activity, the capture of data at such a small distance may be very much different from running outdoors.

Another point of attention to be kept in mind is that these plyometric exercises were performed a few times and compared to running on a short indoor track. Some exercises produced less output compared to running and these exercises were therefore labeled as ideal to include as a preparation for running. However, the cumulative output during outdoor running may be more demanding than estimated here on a short running track. Similarly, a much lower number of plyometric repetitions are typically performed during a single training session compared to the number of running strides an athlete may take per running session. Therefore, the accumulated total loads during outdoor distance running may be much higher than estimated here despite plyometrics generating a greater total work during one exercise cycle.

Talk nerdy to me

Interestingly, this study used a novel approach to quantify the intensity of the plyometric exercises. Previous studies used ground reaction forces and joint moments, where it was impossible to differentiate the actions of individual muscles. Due to the different anatomical properties of the soleus and gastrocnemius, it is likely that this reflects in the loads they are subject to. This study makes use of non-invasive computational simulations to estimate the musculotendon unit output of individual muscles during dynamic tasks. Thus, it is possible to estimate how different plyometric exercises load individual musculotendon units.

A limitation of this study may be that the hurdles were not adjusted to the participants’ height, and this may have been more demanding in certain subjects. This may have influenced the results.

Take home messages

The A-skip may be an exercise to target both the gastrocnemius lateralis and soleus and may be done before running is initiated. Bounding produces large eccentric loads for both calf muscles, while the ankle bounce creates more eccentric output for the lateral gastrocnemius, compared to the soleus which is more eccentrically loaded with hurdles.

Reference

Trowell, D., Fox, A., Saunders, N., Vicenzino, B., & Bonacci, J. (2022). A comparison of plantarflexor musculotendon unit output between plyometric exercises and running. Journal of Science and Medicine in Sport, 25(4), 334-339.