Skeletal System, Muscles, and Levers

• Muscles function by pulling against bones.
• This rotation around joints transmits force through the skin to the environment.
• There is an axial skeleton and an appendicular skeleton.
• Skeletal musculature enables the skeleton to move.
• The origin of this musculature is the proximal attachment, while the insertion is the distal attachment.
• The agonist is the muscle most directly involved in a movement and is also called the prime mover.
• The antagonist is a muscle that can slow down or stop the movement.
• Many muscles in the body function through levers.
• Body movements involved in sport and exercise primarily act through the bony levers of the skeleton.
• A lever is a rigid or semi-rigid body that exerts force on an object impeding its rotation when subjected to a force not passing through its pivot point.
• Levers transmit force tangential to the arc of rotation from one contact point along the object's length to another
• FA = force applied to the lever
• MAF = moment arm of the applied force
• FR = force resisting the lever's rotation
• MRF = moment arm of the resistive force
• The force applied by a lever is equal in magnitude but opposite in direction to FR.
• Mechanical advantage is the ratio of moment arm through which an applied force acts to the moment arm through which a resistive force acts.
• Allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque, if mechanical advantage is greater than 1.0.
• Mechanical advantage of less than 1.0 is a disadvantage.

First-Class Levers

• The muscle force and resistive force act on opposite sides of the fulcrum.
• In elbow extension against resistance (e.g., triceps extension exercise), O is the fulcrum.
• FM is the muscle force, and FR is the resistive force.
• MM is the moment arm of the muscle force, and MR is the moment arm of the resistive force.
• Mechanical advantage equals MM/MR, which is 5 cm/40 cm = 0.125, which being less than 1.0, is a disadvantage
• The muscle and resistive force act on opposite sides of the fulcrum
• Because MM is much smaller than MR, FM must be much greater than FR
• This illustrates a disadvantageous arrangement.

Second-Class Levers

• The muscle force and resistive force act on same side of the fulcrum.
• The muscle force acts through a longer moment arm than the resistive force.
• Due to its mechanical advantage, the required muscle force is smaller than the resistive force.
• In plantar flexion against resistance such as a standing heel raise, FM is the muscle force, FR the resistive force, MM the moment arm of the muscle force, MR the moment arm of the resistive force.
• The ball of the foot, the point about which the foot rotates, is the fulcrum (O) when the body is raised.
• Because MM is greater than MR, FM is less than FR.
• The muscle force and resistive force act on same side of the fulcrum
• The muscle force acts through a longer moment arm than the resistive force.
• Due to its mechanical advantage, the required muscle force is smaller than the resistive force.

Third-Class Levers

• The muscle force and resistive force act on the same side of the fulcrum.
• The muscle force acts through a moment arm shorter than that through which the resistive force acts.
• Provides a mechanical advantage is less than 1.0.
• The muscle force must be greater than the resistive force to produce torque equal to that produced by the resistive force.
• With elbow flexion against resistance such as a biceps curl, FM is the muscle force, FR the resistive force, MM the moment arm of the muscle force, MR the moment arm of the resistive force.
• Because MM is much smaller than MR, FM must be much greater than FR.

Other Considerations

• During elbow flexion with the biceps muscle, the perpendicular distance from the joint axis of rotation to the tendon's line of action varies throughout the range of joint motion.
• There is less mechanical advantage when the moment arm (M) is shorter.
• Most skeletal muscles operate at a considerable mechanical disadvantage.
• Forces in the muscles and tendons are much higher than those exerted by the hands or feet on external objects or the ground during sports and physical activities.

Explore the skeletal system's axial and appendicular components. Learn how skeletal muscles enable movement by pulling against bones, and how levers transmit force, essential for sports and exercise. Understand the roles of agonist and antagonist muscles in movement.