Biomechanical Rationale for Production of Muscle Activity

Muscle Activity

Muscle activity is required to overcome a resistance.

In biomechanical terms, muscle activity is required to overcome a moment or torque that is described as external, resisting, or demand moment or torque. A muscle produces an internal, muscle, or muscle-tendon unit moment or torque in response to the imposed external, resisting, or demand moment or torque. Other structures, such as bones, ligaments, tendons, etc. cause internal moments in the same way.

Moment or torque: is the tendency of a force to produce movement around a pivot point. This could also be viewed as the tendency of a force to produce a turning effect around an axis. Note that a tendency does not imply that joint movement occurs. In clinical practice, moment and torque are used synonymously. In engineering, moment and torque are not synonymous terms. An internal moment or torque occurs in a direction that is opposite that produced by the external, resisting, or demand moment or torque.

A moment/torque is produced by multiplying a force times the distance through which the force acts, which is called a moment arm. A moment arm is the perpendicular distance from the joint center of motion to the line of action of a force.

Mechanical Advantage is the advantage that a force with a long moment arm has in producing a moment or torque. A force with a longer moment arm has the mechanical advantage over a force with a shorter moment arm. This fact only allows the force to produce a greater moment if the product of the force x the moment arm is greater than that produced by another force.

Muscle Activation

Muscles can be activated in one of three ways:

• Concentric Muscle Action- muscle activity occurring while the muscle is shortening.
  • The muscle is acting to produce motion to overcome the external, resisting, or demand moment or torque.
  • The joint motion occurs in the same direction as the muscle is pulling.
  • Example: Quadriceps femoris muscle activity while standing up after sitting in a chair.
• Isometric Muscle Action- muscle activity occurring while the muscle stays the same length.
  • The muscle is acting to maintain a posture to match the external, resisting, or demand moment or torque.
  • No joint motion occurs.
  • Example: Quadriceps femoris muscle activity while maintaining a partial squat.
• Eccentric Muscle Action- muscle activity occurring while the muscle is lengthening.
  • The muscle is acting to control motion occurring in a direction produced by the external, resisting, or demand moment or torque.
  • The joint motion occurs in a direction opposite the pull of the muscle.
  • Example: Quadriceps femoris muscle activity during slowly sitting onto a chair.