The sliding filament model of contraction states that

In the sliding filament model of muscle contraction, the myofilaments slide over each other, resulting in the overlapping of actin and myosin. Hope this helps. Have a nice day. Feel free to ask more questions. Thank you.

The independent variable is the type of music.the dependent variable is the pulse rate.the intervening variable is the age of the subject.if the mother does not like rock, her pulse is likely to not increase.the same might be true for the sister.the friend may like rap and gets excited.to establish cause/effect relationship, all of the subjects must be consistent.hope i .