Fill in the Blanks Complete phrasesOnline version Fill in the blanks of the following sentences using the appropriate terms related to the circulatory system. by Martha Sánchez 1 CP ATP The contraction of skeletal muscles requires energy from , which is broken down to provide the necessary energy for movement . 2 sarcoplasm calmodulin Smooth muscle contraction is triggered by calcium binding to , unlike skeletal muscle which uses troponin . 3 sarcomeres mitochondria In skeletal muscle , myofibrils are organized into repeating units called , which are responsible for contraction . 4 isotonic isometric A contraction occurs when muscle length changes while maintaining constant tension , like lifting an object .