Lever Systems & Movements

Question 1

Lever systems: what determines the type?

Answer 1

the relationship between the external force, muscle, and load

Question 2

First class lever system:

Answer 2

axis of rotation is in between the load and effort. Here, the moment arm for the muscle is the same as the moment arm for the external force. (ex: cervical atlas and axis).

Question 3

Second class lever system:

Answer 3

load is between the axis of rotation and the effort. This assures that the moment arm will always be longer for the muscle (effort). This is a mechanical advantage! (ex: calf raise)

Question 4

mechanical advantage

Answer 4

when moment arm for muscle is greater than the moment arm for the external force.

Question 5

Third class lever system:

Answer 5

effort is between the external load and the axis of rotation. Here, the force by the muscle is always greater than the external force, but we have to produce a lot of force to do so. This is inefficient. (ex: biceps curl).

Question 6

Why the inefficiency of third class lever systems?

Answer 6

Sacrifice torque producing capabilities in order to get more movement capabilities.

Question 7

Torque

Answer 7

tendency of a force to create a rotation (you don’t have to have a rotation, but it is the tendency).

Question 8

Moment arm

Answer 8

the perpendicular distance from the axis of rotation to the line of applied force

Question 9

Torque = moment

Answer 9

Torque = Force x Moment arm

Question 10

If torques are equal and joint is being held…

Answer 10

no movement, isometric

Question 11

If force generated by muscle is more than the force generated by the external force…

Answer 11

there will be a shortening muscle action, contracting/concentric movement.

Question 12

Inertia

Answer 12

property of all things that tends to keep movement steady (i.e. reduces accelerations, changes in movement). Movements which are performed parallel to the ground or “across gravity” are also resisting inertia and thus require a concentric action.

Question 13

Shortening actions occur when:

Answer 13

A. The movement is in a direction opposite to that of the external force. Examples include movements against gravity and during resistance exercises. Note, the speed of the movement is not a factor.
B. The movement is initiated against inertia. Movements which are performed parallel to the ground or “across gravity” are also resisting inertia and thus require a concentric action. Here again, the speed of the movement is not a factor.
C. The movement is in the direction of the external force and the movement is FASTER than a movement produced by the external force alone. In this situation the muscular forces and external forces are summed to produce the movement. These types of shortening actions typically produce the most rapid movements.

Question 14

Lengthening actions occur when:

Answer 14

the movement is in the same direction as the external force but the movement is SLOWER than the force would produce alone. Lengthening actions are used to slow the effect of an external force.

Question 15

Isometric actions occur when:

Answer 15

the muscle moment exerted is equal to the external moment applied. In this case, both the joint position and muscle length remain fixed. Importantly, the joint position must be held in opposition to an external force when an isometric action is required.

Question 16

Isometric action needed to maintain elbow position in anatomical position?

Answer 16

an isometric action is not needed to maintain the elbow position in anatomical position because the extended elbow is not being held against an external moment (resistance).

Question 17

Axis of rotation:

Answer 17

axis of rotation for a movement is located perpendicular to the plane of the movement through the center of the joint.

Question 18

Identifying muscle control: 6-step formula

Answer 18

1: Identify the joint movement (e.g., flexion, abduction, etc.) or position.
2: Identify the effect of the external force on the joint movement or position.
3: Identify the type of action—based on your answers to #1 and #2:
4: Identify the axis of rotation (it will be perpendicular to the plane of movement). The purpose of this step is to locate which surface of the joint the muscles controlling the movement span.
5: Determine which side of the joint axis are muscles lengthening and on which side are they shortening during the movement?
6: Combine the information from steps 3 and 5 to determine which muscles must be controlling the movement (or position). For example, if a shortening action is required and the muscles on the anterior side of the
joint are shortening, the anterior muscles must be controlling the movement.

Question 19

Uniarticular Muscles

Answer 19

Uniarticular muscles are somewhat rare and span only one joint. Because they control only one articulation, they are usually the most effective muscles at the joints they cross. Ex: deltoid, brachialis, and vastus lateralis muscles.

Question 20

Biarticular Muscles

Answer 20

More common than uniarticular muscles, cross a total of two joints. Because of its dual articular control, a biarticular muscle’s length will be altered by position changes of either one of its articulations, thus altering it’s effectiveness at the other articulation. Thus, the effectiveness of all muscles other than uniarticular muscles is dependent upon the positions of the articulations the muscle crosses. Ex: hamstrings, gracilis, and rectus femoris muscles.

Question 21

Triarticular Muscles

Answer 21

Very rare—cross a total of three joints. Ex: biceps brachii and gastrocnemius muscles.

Question 22

Multiarticular Muscles

Answer 22

Multiarticular muscles are common and include all muscles crossing more than three joints. Multiarticular muscles are typically found in the distal parts of the limbs. Examples of multiarticular muscles include the muscles of the fingers, which attach proximally in the forearm and the muscles of the toes, which attach proximally in the lower leg.

Question 23

agonist

Answer 23

muscle controlling movement or position

Question 24

antagonist

Answer 24

muscle on opposite side of joint as agonist that when activated, will control the opposite movement

Question 25

co-contraction

Answer 25

when both agonists and antagonist are activated. This is energetically wasteful, and increase compressional forces across the joint. They also help stabilize the joint though and prevent buckling.

Question 26

prime movers

Answer 26

most effective agonists for a movement because of their size, leverage, and/or the number of joints they cross.

Question 27

Neutralization

Answer 27

the specific recruitment of agonists that counteract unwanted movements occurring in other planes.

Question 28

Stabilization

Answer 28

similar to neutralization in that it eliminates unwanted movements of the skeleton. Stabilization, however, requires the help of antagonist muscles or external forces to prevent the movement of proximal or distal attachment sites.

Question 29

If joints of importance are coming closer together…

Answer 29

the muscle action is shortening, or concentric.

Question 30

If joints of importance are going further away from each other…

Answer 30

the muscle action is lengthening, or eccentric.