Masaj yaptırdıkça kaslarım güçleniyor.

The suffix -dıkça follows a verb stem to form an adverbial clause expressing a proportional relationship: as one action happens, another action happens to that extent. In masaj yaptırdıkça kaslarım güçleniyor, it literally means as I get massages done, my muscles get stronger.
Additional notes:
• After stems ending in consonants, use -dıkça; after vowel stems, use -dikçe.
• It works with any verb: çalıştıkça (as one works), okudukça (as one reads).

yapmak = to do something oneself
yaptırmak = causative form of yapmak, meaning to have someone else do something or to get something done
Therefore, masaj yaptırmak means to get a massage, whereas masaj yapmak would mean to give a massage to someone.

kaslarım is built from:
• kas = muscle
• -lar = plural suffix (equivalent to -s in English)
• -ım = 1st person singular possessive suffix (my)
So kas-lar-ım = my muscles. In Turkish, the plural suffix comes before the possessive suffix.

The suffix -iyor creates the present continuous/progressive tense, which in Turkish also covers habitual actions and general truths. In güçleniyor (is getting stronger), it emphasizes an ongoing or repeated process each time a massage is received.
You could use the simple present (güçlenir) for a general statement, but it doesn’t highlight the ongoing progression. Past (güçlendi) or future (güçlenecek) would shift the focus to completed or anticipated actions.