When a muscle is shortened, the force decreases as the filaments slide past one another and lose room to form new cross-bridges. When a muscle is lengthened, there is less filament overlap which leads to fewer cross-bridges. Stretching a muscle before contracting it will not affect the force produced, nor will shortening a muscle before lengthening it. Titin is a protein responsible for some of the elastic properties of muscle, but is not involved in force production.

At rest, the muscle has the potential to form the maximum number of cross-bridges, resulting in the maximum amount of force production. For further review, go over the length-tension curve for a muscle fiber.

The force-tension curve is used to measure the optimal muscle length for maximum muscle contraction. This length corresponds to the optimal overlap of actin and myosin filaments to generate force. The length of actin and myosin filaments determines the minimum and maximum possible overlap. Titin is the protein responsible for the elasticity of the sarcomere after it is stretched past maximum actin-myosin overlap. Titin allows force production to exist at a maximum tension slightly beyond only actin and myosin, thus affecting the force-tension curve.

Myosin and actin are the two major proteins in muscle cells that allow for contraction. Actin is the thin filament; myosin is the thick filament. During muscle contractions the overlap between these two proteins results in a shorter muscle fiber, and a shorter muscle, that pulls on the tendon. The result is movement. The other answers contain other structural elements of muscles but are not the direct cause of muscle contraction.

The A band is the only element that remains constant during a muscle contraction. It represents the segment of the sarcomere that contains the length of the thick filament. The H zone refers to the part of the sarcomere where there are only thick filaments, and no superimposing thin filaments. Conversely, the I band refers to the area where there are only thin filaments and no superimposing thick filaments. As filaments overlap, both the H zone and I band will shorten. The N line does not exist in musculoskeletal physiology.

The only choice that is actually true is that the degree of overlap of myosin and actin plays a role in contractile strength. If there is little to no overlap, contractile strength is low; however, if there is too much overlap then contractile strength is also low. This trend can be represented in a force-tension curve, which demonstrates that maximum force generation occurs when the sarcomere begins at equilibrium.

In a normal sarcomere there is always a small area of overlap of myosin and actin prior to contraction. Myosin appears thicker than actin, and is considered the "thick filament."

To answer this question you need to understand the structural regions of the sarcomere. The I band, A band, and H zone are regions in a sarcomere that constitute of actin (thin) and myosin (thick) filaments. I band is the region of actin filaments that are not superimposed by myosin filaments. The H zone is the region of myosin filaments that are not superimposed by actin filaments. To calculate the length of the I band, you need the length of the myosin filament, the actin filament, and the H zone. Since we don’t have the length of H zone, we can’t solve for the length of I band. Essentially, without knowing the degree of overlap, we cannot determine the length of un-overlapped actin.

The A band is the region of the sarcomere that consists of the entire length of the myosin filament. The question states that the length of the myosin filaments is micrometers; therefore, the length of the A band is micrometers.

Actin and myosin filaments are essential contractile elements found in muscle cells. They are essential because they conduct muscle contraction. A molecule of actin is made up of small microfilaments, which give them a very thin appearance. Myosin is made up of long polypeptide chains that join together to form a thick filament; therefore, actin molecules are classified as thin filaments, whereas myosin molecules are classified as thick filaments.

All muscle cells, regardless of type, contain both actin and myosin filaments. Muscle contraction is not possible without the presence of both contractile elements. Organization of these molecules can vary, as smooth muscle does not contain striations, but the molecules are still responsible for contractile actions. Troponin, calcium, and tropomyosin are all required to initiate the contact between myosin and actin. Calcium binds to troponin, which subsequently removes tropomyosin from actin (thin filaments). None of these interact with myosin, the thick filaments.

A sarcomere is the functional unit of the skeletal or cardiac muscle cell, and is made of interlocking myofibrils. A sarcomere is the smallest unit in the muscle cell to contract and relax.

Note that smooth muscle cells still contract using actin and myosin filaments, but do not organize these filaments into sarcomeres as skeletal and cardiac muscle do. This is why smooth muscle is not striated.

Recall that during sarcomere contraction, the myosin filaments attach to actin filaments and slide along the actin filaments. By this mechanism, the region of overlap between the fibers is increased and the total sarcomere length shortens. Neither actin, nor myosin actually change length; they simply move in relation to one another.

The H zone refers to the region of myosin at the center of the sarcomere that is not overlapped by actin. When the sarcomere shortens, the region of overlap increases and the H zone decreases.

Recall that sarcomeres are functional units of muscles that facilitate muscle contraction. Myosin heads bind to actin filaments and cause the filaments to overlap, shortening the sarcomere and, subsequently, the muscle.

Inside a sarcomere there are several regions. One such region is the I band, which consists of the actin filaments in the region where they are not superimposed by the myosin filaments. This means that the I band consists only of actin filaments; however, actin filaments aren’t exclusive to the I band. They are also found in other regions, such as A band. The A band is the region of the sarcomere that contains the myosin (thick) filaments, regardless of overlap. This means that myosin is exclusive to the A band, but that this region contains both actin and myosin due to overlap.

Sarcomeres are functional units of muscles, but they are only found in skeletal and cardiac muscle cells; smooth muscle cells do not contain sarcomeres. Actin and myosin filaments still cause the contraction seen in smooth muscle, but are not organized into alignment. This means that smooth muscle cells do not contract linearly and can essentially shrink in size during contraction, which can allow for things like constriction around organs and vessels.