Body movements are controlled by muscle forces, Body movements are controlled by muscle forces, initiated by contraction or extension of the muscles. Skeletal initiated by contraction or extension of the muscles. Skeletal muscles control the movements of the body limbs. muscles control the movements of the body limbs.

Most of the muscle forces involve levers!Most of the muscle forces involve levers!

Three examples for lever systems, Three examples for lever systems, WW is the is the applied weight, applied weight, FF is the force supporting the pivot is the force supporting the pivot point of the lever system, and point of the lever system, and MM is the muscles is the muscles force. force.

EXAMPLE:EXAMPLE: THE FOREARM AS LEVER SYSTEMTHE FOREARM AS LEVER SYSTEM

The biceps muscle pulls the arm upwards The biceps muscle pulls the arm upwards by muscle contraction with a force by muscle contraction with a force MM the opposing force is the the opposing force is the weight of the arm weight of the arm HH at its center of gravity at its center of gravity (CG)(CG)! !

Biceps can be strengthened by weight W lifting this adds Biceps can be strengthened by weight W lifting this adds another force which has to be compensated by the muscle force. another force which has to be compensated by the muscle force.

The lower arm can be hold by the biceps muscle at different angles The lower arm can be hold by the biceps muscle at different angles What muscle forces are required for the different arm positions? What muscle forces are required for the different arm positions?

EXAMPLE:EXAMPLE: THE ARM AS LEVER SYSTEM THE ARM AS LEVER SYSTEM

The deltoid muscle pulls the arm upwards by muscle contraction with a The deltoid muscle pulls the arm upwards by muscle contraction with a force force TT at a fixed angle at a fixed angle a a with respect to the arm the opposing force is with respect to the arm the opposing force is the weight of the arm the weight of the arm HH at its center of gravity at its center of gravity (CG)(CG) and the (possible) and the (possible) weight weight WW hold in the hand! hold in the hand!

Consider arm at an angle Consider arm at an angle hold by the deltoid muscle (hold by the deltoid muscle ( 15°) 15°)