The soleus muscle is part of the calf, together with several small muscles and the two gastrocnemius muscles, commonly known as calves, located at the back of the leg, just below the knee.

It is a broad, thick muscle located behind the calves and closely connected to them, which is why it can be considered as one single muscle: the triceps suralis.

It is of vital importance as it is essential for bipedal stance (standing and walking), heel raise and foot flexion, and also jogging.

Challenges in Predicting the Prognosis of Soleus Muscle Injuries

The calf muscles are among the most frequently injured muscles among athletes.¹ Therefore, in an attempt to mitigate possible consequences and to accurately predict their injury prognosis and time of return to play (RTP), the anatomical location of injuries of the calf muscles and the implication of the conjunctive tissue (filling, supporting and connecting tissues) have been studied using different imaging techniques.

Early and accurate diagnosis is also very important for the treatment of these muscle injuries. Ultrasound is the most widely used imaging method for detecting medial gastrocnemius injuries in the calf, with excellent diagnostic results.^{2, 3}

However, due to the anatomical and functional complexity of the soleus muscle, there is no formal consensus on guidelines for determining the prognosis of soleus injuries, despite studies to date.^4-6 In the case of soleus muscle injuries, ultrasound has a very low diagnostic capability and MRI (magnetic resonance imaging) should be used instead.⁷

In this regard, the study Anatomical Variability of the Soleus Muscle: A Key Factor for the Prognosis of Injuries? (Carles Pedret et al. 2022) aimed at providing precise instructions for determining the prognosis of soleus muscle injuries. We will summarize its conclusions below.

Anatomical Variability of the Soleus Muscle

The soleus muscle is not anatomically homogeneous. It has a particular anatomy featuring the proximal connective tissue arch from which the lateral and medial aponeuroses (fibrous membranes consisting mainly of collagen that serve for the insertion of the muscles) and the medial tendon develop.⁸ The most important thing to keep in mind is that this anatomy, although considered standard, is extremely variable. In fact, it can even vary across a single person’s two soleus muscles.

Namely, two main aspects determine the anatomy of the different soleus muscles:

-The presence or absence of the aponeuroses and the central tendon, their length and location, and all their possible combinations.

-The direction and angles of pennation (the orientation of the muscle fibres in relation to the connective tissue/tendon) of the muscle fibres, which are conditioned by this anatomical variability.

In the study led by Dr Pedret, 107 soleus muscles were analysed during 2018 and 2021. Taking into account the great anatomical variability of the soleus muscle, the study applied an individualised approach and performed an MRI scan of the calf region. Thus, different soleus muscle types were classified on the basis of their muscular or connective dominance.

Soleus Muscle Dominance

Muscle dominance is determined by the position of the medial tendon. This divides the soleus muscle into two volume bodies: one from the medial tendon to the medial border and one from the medial tendon to the lateral border. Depending on the position of the central tendon, one muscle volume will be bigger than the other, or the two volumes can be symmetrical (if the central tendon is in the middle).  If there is no central tendon, there is no muscle dominance.

Soleus Connective Dominance

The connective dominance is determined by the length and thickness of the medial and lateral aponeurosis. Depending on the predominance of one or the other, there is medial or lateral connective dominance.

Conclusions and Clinical Relevance of the Study

The study on the 107 muscles analysed in terms of their typology, led to the following conclusions:

• Preliminary results suggest that muscle dominance does not seem to have an impact on the prognosis of soleus muscle injuries, in contrast to connective dominance, which seems to have a worse prognosis in cases where the injury is rooted in the dominant aponeurosis.
• To evaluate the prognosis of soleus muscle injuries, we must consider not only the involvement but also the distribution of connective aponeuroses, which conditions the direction and pennation angles of the muscle fibres.
• Ignoring this aspect is probably one of the main reasons why descriptive epidemiological series on soleus muscle injuries are not reliable and fail to find a reproducible prognostic pattern.^4-6 In fact, classification systems would be more clinically relevant if they reflected the anatomical and functional roles of muscles as organs within a system, and offered a common nomenclature.¹¹
• MRI is a useful tool to determine the exact structure of individual soleus muscles, and to diagnose injuries early and accurately.
• The soleus muscle has an enormous anatomical variability in the distribution of muscle volume and the amount and distribution of connective tissue.
• Using the combination of subtypes based on muscular and connective dominance can be a starting point to better understand the structure of the soleus muscle and reach a consensus on the way we classify every individual soleus.
• Soleus muscle injuries must be individually characterized. General classifications of muscle injuries can be used as a starting point, but the specific subtype of soleus muscle must be considered.
• We need to consider the great anatomical variability of the soleus muscle as a prognostic factor for injuries when planning a player’s return to play. This way, we could plan an individualised and tailored treatment and rehabilitation protocol for each soleus injury and decrease the risk of reoccurrence.

With regard to the variation between sexes, as Dr. Pedret points out, it should be taken into account that in women, injury epidemiology studies are still very scarce and with very few samples, and therefore the data are not yet comparable. However, it seems that there are no major differences in injury susceptibility, since the soleus is not a muscle in which explosive strength predominates (men have more), but its function is practically identical in men and women.

In any case, further research is needed to reach a consensus on the use of the variables and the nomenclature used in the study, as well as to assess their application to return-to-play programs after soleus muscle injuries.

References:

1. Orchard JW, Seward H, Orchard JJ. Results of 2 decades of injury surveillance and public release of data in the Australian Football League. Am J Sports Med. 2013;41(4):734–41. https://doi.org/10.1177/0363546513476270.

2. Traumatology SG of the M and TS from the SS of S, Balius R, Blasi M, et al. A histoarchitectural approach to skeletal muscle injury: searching for a common nomenclature. Orthop J Sport Med. 2020;8(3):2325967120909090. https://doi.org/10.1177/ 2325967120909090.
3. Pedret C, Balius R, Blasi M, et al. Ultrasound classification of medial gastrocnemious injuries. Scand J Med Sci Sport. 2020;30(12):2456–65. https://doi.org/10.1111/sms.13812.
4. Pedret C, Rodas G, Balius R, et al. Return to play after soleus muscle injuries. Orthop J Sport Med. 2015. https://doi.org/10. 1177/2325967115595802.
5. Green B, Pizzari T. Calf muscle strain injuries in sport: a systematic review of risk factors for injury. Br J Sports Med. 2017;51(16):1189–94. https://doi.org/10.1136/bjspo rts-2016-097177.

1. 7. Green B, Lin M, McClelland JA, et al. Return to Play and recur- rence after calf muscle strain injuries in Elite Australian Football Players. Am J Sports Med. 2020;48(13):3306–15. https://doi.org/ 10.1177/0363546520959327.

7. Balius R, Rodas G, Pedret C, Capdevila L, Alomar X, Bong DA. Soleus muscle injury: sensitivity of ultrasound patterns. Skelet Radiol. 2014. https://doi.org/10.1007/s00256-014-1856-z.

1. 8. Balius R, Alomar X, Rodas G, et al. The soleus muscle: MRI, ana- tomic and histologic findings in cadavers with clinical correlation of strain injury distribution. Skelet Radiol. 2013;42(4):521–30. https://doi.org/10.1007/s00256-012-1513-3.
2. 9. Prakash A, Entwisle T, Schneider M, Brukner P, Connell D. Con- nective tissue injury in calf muscle tears and return to play: MRI correlation. Br J Sports Med. 2018;52(14):929–33. https://doi.org/ 10.1136/bjsports-2017-098362.
3. 10. Patel A, Chakraverty J, Pollock N, Chakraverty R, Suokas AK, James SL. British athletics muscle injury classifica- tion: a reliability study for a new grading system. Clin Radiol. 2015;70(12):1414–20. https://doi.org/10.1016/j.crad.2015.08.009.
4. 11. Balius R, Pedret C, Kassarjian A. Muscle madness and making a case for muscle-specific classification systems: a leap from tis- sue injury to organ injury and system dysfunction. Sport Med. 2021;51(2):193–7. https://doi.org/10.1007/s40279-020-01387-5.