If you look it up in the RAE website, involucro comes from the latin "involucrum" but the only definition officially accepted in spanish is botanical referring to flowers and not medical... are tyou 100% positive there could not be en error?

I think this is about Fascia or Muscle Sheath...

Each muscle is surrounded by a connective tissue sheath called the epimysium.

Fascia, connective tissue outside the epimysium, surrounds and separates the muscles.

It is a fibrous membrane, usually quite thin and devoid of fat, surrounding a single muscle, separating it from adjacent muscles and allowing movements between them; related to contraction.

Synonym(s): fascia propria musculi [TA], muscle sheath

https://medical-dictionary.thefreedictionary.com/muscle shea...
*
The muscular sheaths are internally connected, in continuity, in order to form a true fibrous skeleton supported on the bone skeleton. It surrounds all organs in a three-dimensional form and in this way allows them to be kept in a correct position and functioning.

Translated from the portuguese:

Os invólucros musculares encontram-se internamente ligados, em continuidade, de maneira a formar um verdadeiro esqueleto fibroso apoiado sobre o esqueleto ósseo. Rodeia todos os órgãos em forma tridimensional e desta maneira permite mantê-los em uma correta posição e funcionamento.