Muscle

Question 1

Describe how cross-sectional area of a muscle influences force production.

Answer 1

A larger cross-sectional area of a muscle typically correlates to a larger potential for force production in a muscle.

Question 2

Describe how pennation angle influences force production.

Answer 2

A higher pennation angle is typically correlated to a greater potential for force production. Higher pennation angles allow for more fibers to be placed in parallel which allows for a greater maximal force to be achieved.

Question 3

Describe each of the components in Hill’s model of skeletal muscle

Answer 3

The components in Hill’s model of muscle are the contractile element, the parallel elastic element, and the series elastic element. The contractile element is responsible for the generation of the force, these would be the myofibrils. The series elastic element and the parallel elastic element are representative of the elasticity of connective tissues around the contractile element. The SEE is the tendons as they are directly attached to the CE, and the PEE is the sarcolemma, endomysium, and perimysium, the sheath surrounding the myofibril. The CE generates force through contraction resulting in the contraction of the spring structures of the PEE and SEE.

Question 4

What structures within the muscle fiber are included in the SE component of the Hill model of skeletal muscle?

Answer 4

The tendons

Question 5

Describe in detail why the curves appear as they do for the active (open circles) and passive (filled circles) components.

Answer 5

At shorter lengths only the active components were responsible for force and only at longer lengths were the passive components helping. The passive components were only able to be active once the muscle was lengthened enough for them to be under enough stress to provide elastic energy to assist in the force generation. This assistance causes an initial decrease in the rate force is actively generated in proportion to the length of the fiber.

Question 6

Describe the force-velocity relationship for skeletal muscle. Describe in detail the relation and why this relation between force and velocity occurs

Answer 6

As the force placed on a skeletal muscle decreases the velocity it contracts at decreases. This is due to their being cross-bridge binding sites moving past each other rapidly at higher velocities and as such not being able to bind as well and being missed. Less cross-bridges being formed during contraction means that there is less force that can be generated. In this curve the negative velocity occurs as a result of a maximum number of binding sites being reached but the force being too great causing them to pull apart and forcibly lengthen. The section around a velocity of zero including such a wide range of force percent is a result of a maximum number of sites being reached but no further sites to be able to bind with.

Question 7

Describe why the torque produced about a joint could vary through a specific range of motion if 1) the muscle force does not change; or 2) the moment arm does not change.

Answer 7

Torque is the product of the force generated and the length of the moment arm, so if the muscle force is not changing but torque is then the moment arm will varying in length throughout the movement. If on the other hand the moment arm is not changing but there is a varied torque then the force produced through the motion must be.

Question 8

g) What is Lombard’s paradox? Use the graphic as an example to explain.

Answer 8

Lombards paradox is when the muscles responsible for flexion and extension at a specific joint contract simultaneously but still result in either flexion or extension. In this graphic the rectus femoris is responsible for hip flexion, while the gluteus maximus and hamstrings are responsible for the hip extension. Although they contract simultaneously a greater extension torques is a result of a greater extensor torque being generated than the flexor torque. At the knee however the hamstrings act as flexors while the rectus femoris and vasti act as extensors. The vasti and rectus femoris generate a greater extensor torque than the hamstrings due resulting in an extensor torque at the knee.

Question 9

What is short-range stiffness in a muscle fiber? Use this graphic to aid your explanation

Answer 9

Short-range stiffness occurs at the beginning of a forced muscle lengthening. As is seen in the graphic when the muscle first begins to lengthen the force has a large change while the length does not. This shows a high initial stiffness that is referred to as short-range stiffness. When the muscle is forcibly lengthened the cross-bridges are stretch and it results in an increased average force generated from the cross-bridges.

Question 10

What is the stretch-shorten cycle? Use this graphic to aid your explanation

Answer 10

The stretch-shorten cycle is when the active muscle is stretched prior to shortening. This allows the active muscle to perform a greater amount of work than it could’ve from shortening at resting length. In this graphic a squat jump and counter movement jump were performed. As can be seen in the hip and knee (two joints that play a large role in the jumping movement) a greater torque was able to be generated by first stretching the muscles acting on those joints, than in the squat jump.

Question 11

What are the four mechanisms that contribute to the stretch-shorten cycle?

Answer 11

1. Time to develop force
   - Greater force at beginning of concentric phase
2. Elastic energy
   - “stretch power”
3. Force potentiation
   - cross-bridges are “boosted” (stretched cross-bridges)
4. Reflexes
   - stretch reflex