Skeleton Muscle LabIntroduction: The contraction of a muscle is a complex process, which requires several molecules including ATP and Cl-, and some regulatory mechanisms [1]. Myosin is a motor protein that converts the chemical bond energy of ATP into mechanical energy of movement [1]. Muscle contraction is also regulated by the amount of action potentials the muscle receives [2]. A greater number of action potentials are required to induce more muscle fibers to contract, thus increasing the force of contraction [2]. Studies indicate that larger motor units, recruited with higher threshold forces, tended to have shorter contraction times than smaller units [3]. The purpose of the experiment was to reinforce the concept that many chemicals are necessary for skeletal muscle contraction to occur using rabbit (Lepus curpaeums) muscle [2]. Furthermore, the experiment was an opportunity to measure the force of contraction and to observe the number of motor units that must be recruited to maintain a constant force when the muscles begin to tire [2]. Hypothetically, the rabbit's muscle fiber should contract more with ATP and saline; and the amount of motor units involved would increase as the level of applied force decreased until the fatigue phase was reached. Materials and methods: The experiment that tested the contractile level of the muscle in various solutions used a muscle fiber from rabbit muscle. A fiber was detached, placed under the microscope and immersed first in a solution of ATP and salt (KCl and MgCl2), then in a solution of only ATP and finally in a solution of only salt [2]. The level of fiber contraction was measured in micrometers. Muscle contractile force and number of motor units employed at various levels of force...... middle of paper ......gth of contraction: the muscle can be fine or coarse depending on the quantity of motor units connected to the fibers muscles. For example, fine muscles contract faster due to the greater number of motor units. The length of the muscle in terms of sarcomeres has an effect on the duration of the contraction. Action potentials must go farther and reach more sarcomeres to make them contract (Q3) [1], [2]. Reference: [1] Freeman, S. 2013 Biological Science, 5th ed. Pp 973-976[2] Zelick, R. 2014. Exercise in the muscle laboratory. Bi253 Laboratory Manual. Portland State University, OR, pp. 1-5[3] HS Milner-Brown, RB Stein and R. Yemm. “The orderly recruitment of human motor units during voluntary isometric contractions.” The Physiological Society 230th ser. (1973): 359-70. Network. May 22, 2014.[4] Nicholas, M. 2014. Muscle Lab PowerPoint. Bi253 laboratory. Portland State University, OR