Italian to English Medical Sports / Fitness / Recreation flexibility exercises

Another correction needed here, this time it's a sentence (highlighted with *) in the following text, which concerns flexibility exercises.

TARGET
Increased flexibility helps bone segments to realign after they have been incorrectly positioned for a long times, as frequently occurs in modern life. In such incorrect positions, muscles and osteoarticular structures often become shortened for prolonged periods; over time, the body interprets this shortened length as the correct and natural length of the muscle in functional and structural terms. One common example is the psoas-iliac muscle, which in most instances stays contracted whilst the subject remains seated. *It then resists being stretched when the subject stands up straight*.

SOURCE (last sentence only)
Possiamo citare ad esempio il muscolo psoas-iliaco, il quale resta in condizioni di accorciamento per tutto il tempo in cui molti soggetti rimangono seduti, opponendo poi resistenza al suo allungamento durante la stazione eretta.

This is apparently a mistranslation of the source context; however, aside from deleting the word 'straight', I am unsure about what the issue is.

Any suggestions appreciated!