Contraction of myofibrils within a muscle fiber begins when

Answer: calcium is released from the sarcoplasmic reticulum

Explanation:

Muscle contraction is the stimulation of sites that produce stress within muscle fibres.

Calcium ions bind to tropomyosin in inducing muscle fibre contraction, exposing active site for Crossbridge formation on actin. Then ATP binds to myosin, transferring the myosin to its high-energy state, freeing the myosin head from the active actin site. Triggering tension-generating sites within muscle fibres.

Hence, the contraction process is initiated with teh relaease of calcium from sarcoplasmic reticulum.