Background The plantaris muscle originates from the side of the femoral condyle medial and superior to the origin of the gastrocnemius lateral head, crossing latero medial the popliteal fossa.¹ This origin can have three variants: 56.5% without contact with neighbouring structures, 30.4% different interdigitating with the head of lateral gastrocnemius (19.6%) and a fibrotic extension to the patella (10.9%) and absent in 13%.² Muscle belly measures 7–13 cm³ and has a cross section between 3 to 7 mm where runs 5.5 cm tendon^{1 4} of 25–35 cm ranging between 1.5 and 5 mm⁵ and runs medially near the tibia between soleus and the medial gastrocnemius until inserted into the medial part of the Achilles tendon between 2–7 cm proximal to the calcaneal,⁶ the tendon is three times higher in the proximal part, while the total length obtained an average of 43.25 cm with a thickness variatons of 1.3 cm in myotendinous junction, 3.8 cm in belly muscle and 0.3 cm in the distal insert.¹ The main goal of this study is to evaluate and analyse if the presence of a muscle belly plantaris with a cross-sectional area (CSA) and grown up thickness is correlated with Achilles tendinitis and/or fasciosis in athletes using ultrasonography.

Methods The number of participants that need to be part of the study sample is calculated with a total volume of subjects 100 participants (50 per group), although a previous pilot study will be conducted when it reaches the volume of 20 subjects (10 per group), to develop a pilot study in order to analyse the behaviour of the variables, in addition to approximate a standard measure of CSA and thickness plantaris muscle belly in order to draw conclusions when this is hypertrophied or normal. Images will be acquired at rest and during a submaximal isometric contraction by one expert examiner with ultrasound machine Esaote Gamma with linear probe SL1543. The reference for asses CSA and thickness is 3 cm proximal to the distal myotendinous Union (UMT), assessed for contraction by plantarflexion and inversion foot in open kinetic chain. Also will be asses state achilles tendon, thickness of the plantar fascia, anthropometric variables, physical activity level, degree of disability achilles tendon and patient satisfaction.

Results It is needed to complete the study.

Conclusions If the results of the study confirm the hypothesis, many chronic Achilles tendinopathy and fasciosis should be reevaluate.