Contraction of myofibrils within a muscle fiber begins when

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.

a researcher conducts an experiment on the secretion of a particular hormone in mice. scientists inject mice with a substance that stimulates the production of the hormone. the scientists then test the levels of hormones produced by the mice. the tool used to measure the hormones consistently detects the levels at 10 points lower than the actual hormone levels in the mice. this tool makes measurements, but the measurements aren't .

explanation:

no u

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.