Rehabilitation of soleus muscle injuries in distance runners

There are many causes of calf pain in runners. However, when the most obvious causes are eliminated and pain still persists, therapists may be left scratching their heads. If that is the case, a low-grade soleus strain, which is challenging to diagnose, may be the underlying problem.

Presentation

A runner comes into the clinic and complains of sharp pain or intense tightening low down in the calf, but above the Achilles. The history is typically as follows:

1. Training for an event;
2. Increased distance or added hill work;
3. A little tight for a week or so;
4. With further training, symptoms progress to an intense tightening in the area described above.

In most cases, runners don‘t feel a sudden twinge - just tightening that decreases when they stop running, but resumes when they attempt to run again. The pain subsides after a week of rest and returns again when running is resumed.

Anatomy and physiology

The soleus is an ankle plantar flexor that originates from the posterior surface of the proximal fibula and the soleal line or medial border of the tibial shaft (see figure 1). It joins the calcaneal, or Achilles, tendon along with gastrocnemius muscles to insert into the posterior aspect of the calcaneus. Its plantar flexion action over the ankle is accentuated when the knee is bent and the gastrocnemius muscles are shortened.

Figure 1: Calf anatomy

Important distinctions between the soleus and gastrocnemius muscles are that the soleus is mono-articular and the gastrocnemius muscle is bi-articular. The soleus has a high proportion of slow twitch muscle fibers and gastrocnemius muscle a high proportion of fast twitch⁽¹⁾. For moderate-force contractions like distance running, the soleus is preferentially recruited due to its higher proportion of slow twitch muscle fibers. The gastrocnemius muscle is more preferentially recruited for high-force contractions – eg sprinting and jumping⁽²⁾.

During the early stages of the stance phase, the soleus contracts eccentrically to decelerate ankle dorsiflexion. At mid-phase, the soleus contracts concentrically to plantarflex the ankle and propel the runner. This stretch-shortening cycle results in higher forces produced across the plantar flexors during running - even higher than in jumping⁽³⁾.

Muscle strain considerations

Acute muscle strains are most commonly seen in bi-articular muscles that have a high proportion of fast twitch muscles fibers, usually with internally driven high levels of force like sprinting, or stretching movements like kicking. These instances produce high levels of force across the muscle.  The most common muscles affected include rectus femoris, hamstrings, and gastrocnemius.

However, muscle strains can less commonly occur in muscles with different anatomy and physiology, like the soleus. The reported frequency of soleus strains varies from rare to common. A soleus muscle injury may be underreported due to misdiagnosis as thrombophlebitis or the lumping together of soleus strains with strains of the gastrocnemius⁽⁴⁾.

Muscle strain grading

Historically, muscle strains have been graded as one, two or three, referring to mild, moderate or complete⁽⁵⁾.  There is, however, some ambiguity around this grading system. Alternative grading systems identify different parts of the muscle/tendon complex and more accurately reflect MRI findings of different level injuries⁽⁵⁾. This also includes low-grade muscle injuries that do not have positive radiological findings but have clinical presentations, which can often be the case with soleus injuries.

Grade-one muscle strains, such as those experienced in the soleus, can be classified as mild pain with activity, localized tenderness on palpation, mild spasm and swelling. On MRI investigation, there is the presence of an edematous pattern only, without substantial disruption of muscle fibers or muscle architecture⁽⁶⁾. Also considered here are muscle injuries of a chronic nature where there is no sign of muscle fiber disruption but - due to the buildup of micro-trauma - inflammation can be present within the muscle. This explains the difficulty in diagnosis. The nagging symptoms persist without clear objective findings.

Assessment

The history as described above and the exact location of pain (both reported and on palpation) are key parts of an assessment. Physical assessment is relatively straightforward. The patient completes a double leg calf raise to determine the level of discomfort. Then perform another calf raise with the knees bent. Compare to the level of discomfort when performing with a bent knee versus a straight knee.

Bending the knees takes tension off of the gastrocnemius muscles, and the movement is dominated by soleus⁽⁷⁾. With a soleus strain, the later exercise will elicit more pain, consistent with the type of pain described in the history. However, because soleus strains are typically relatively mild, testing may need to progress to multiple reps or to single leg raises to reproduce the pain.

Mechanism of injury

The potential underlying causes of soleus strain include (but are not limited to):

• Training error – overtraining, rate of increase of training loads too high, introduction of too much hill work over too short a time, poorly designed training program – eg a hard running session after a long run
• Kinetic chain dysfunctions – poor hip extension flexibility and associated poor gluteus maximus recruitment. This is a common problem in triathletes or runners that sit for long periods for work.
• Past injury – to the ankle plantar flexors, the ankle joint or the Achilles or plantar fascia.
• Poor strength-endurance of ankle plantar flexors.
• Footwear – a change leading up to the injury (e.g., change in heel counter, pronation control).
• Running technique - conscious change in running technique to mid-forefoot running or running technique modified by footwear type.

Rehabilitation

There are two considerations during rehabilitation:

1. Rehabilitating the injured muscle so it is physiologically and neurologically capable of returning to the desired capacity for running.
2. Ensuring the identified underlying causes play less of a role in overloading the soleus muscle during future running.

In the acute stage of injury, the goal is to manage the muscle, as with any other type of low-grade strain. Ensure the athlete rests and manages the pain and swelling. After a bit of rest, the athlete may think they have recovered. However, soleus strains are notorious for returning when the athlete ‘feels better’. Often an athlete will resume running and after a few kilometers intense tightening begins. Little specific information with regard to rehabilitation of the soleus muscle exists in the literature. The following guidelines have therefore been developed from a combination of adhering to general guidelines for muscle strain rehabilitation, and the training/rehabilitation principles of overload and specificity. Successfully rehabilitating a soleus muscle involves doing three things well:

1. Developing strength/endurance of ankle plantar flexors

Strengthening exercises for the ankle plantar flexors involve calf raises with both straight knees for gastrocnemius activation and knees bent for the soleus. Begin with both legs on the ground then progress to double legs on the stairs, single leg on the ground and finally single leg on the stairs. Complete two sets each of knee straight and knee bent. Very high repetitions using the athlete’s body weight will focus on strength/endurance, which relates to distance running (as opposed to a sprinter’s requirement of high load for maximal strength). The initial goal is to perform four sets of 20 repetitions on a single leg stair raise in one session with 2-minute rest intervals. The end goal will be dependent on the level of the runner and the training loads intended.

2. Developing power/endurance of the ankle plantar flexors

Athletes should begin hopping exercises after achieving adequate endurance with single leg raises. These should not be maximal jump/hop height exercises, which would preferentially recruit the fast-twitch dominant gastrocnemius muscle. Instead, they should be rather moderate level hopping exercises that mimic the stretch-shortening cycle of soleus, which occurs in running. Hopping should begin on two legs (mini continuous jumps) and progress to single leg. A high level of power endurance can be considered four sets of 100 hops, but you take individual considerations into account. If completing a rehabilitation session, the power exercises should be done before the strength/endurance exercises to ensure dynamic exercises are not performed in a fatigued state, which reduces the chance of re-injury.

3. Graded return to running

Commonly, athletes will ‘feel good’ when beginning running and just keep going until they experience the familiar tightness. This puts their rehabilitation back a couple of weeks. Educate athletes with regard to the importance of following the designated distances and intensity, and to resist doing a little more upon feeling good! As a guide, begin two runs per week: one kilometer on the first run maximum, and add no more than one kilometer at a time to running sessions.

As the rehabilitation progresses, induce overload to produce muscle remodeling. This can result in mild soreness over the strain site and generalized delayed onset muscle soreness (DOMS). Allow enough time for recovery and avoid performing a running session too soon after a rehabilitation session.

During rehabilitation, the soleus muscle will suffer from trigger points and range of movement restrictions. As needed, perform regular dry needling to trigger points, deep tissue massage and stretching. Also, encourage athletes to perform daily self-massage techniques like foam rolling and a home-stretching program⁽⁸⁾.

Although soleus muscle rehabilitation deserves more attention in research, it is hoped that this article has provided both background information on soleus muscle strain in distance runners, and a framework for approaching rehabilitation.

References

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