IB SEHS
Chapter Four
Student Driven Questions for the Exam
1. What is circumduction? What joint does it require? Give an example
of an exercise that uses circumduction? (MS)
a. Circumduction is a combination of hyperextension, abduction,
extension, and adduction. It requires a biaxial joint. Arm circles
are an example of circumduction. Movement is 360 degrees.
2. What are the CNS and PNS? (TL)
a. CNS – central nervous system consists of the brain and the spinal
cord. Most sensory and control takes place.
b. PNS – peripheral nervous system arrangement of nerves extending
from the spinal column to limb and other parts
3. What are the two ways that motor units are recruited and describe
these two ways? (CT)
a. Size principle – smaller motor units recruited first and only when
larger muscles are required are large motor units stimulated. Not
effective when activities require very large forces.
b. Frequency or rate of coding of motor unit recruitment – refers
to the variable rate of motor unit innervation. A higher rate of
motor unit recruitment refers to the activation of additional motor
units to accomplish an increase in contractile strength in a
muscle.
4. What is the center of mass and why is it important to know the
position of the center of mass? (VK)
a. The center of mass is the mathematical point around which, the
mass of a body or object is evenly distributed. Knowing the center
of mass helps determine the stability of static positions, helping
with activities involving balance. It is also the axis for all free
airborne rotations of the body or object such as a somersault or
diving. Finally, it acts as the reference point when observing whole
body or object translations such as during the long jump in track.
5. Describe the process by which an electrical impulse causes a muscle
to contract. (JC)
An electrical impulse is first generated either voluntarily or involuntarily by
the brain. The impulse then travels along a motor neuron and reaches the
synapse at the muscle motor unit. Here, the synapse reaches the chemical
acetylcholine, changing the electrical state of the muscle. The electrical
impulse then travels along the muscle as a so-called “action potential”,
stimulating the release of calcium ions and causing the motion of actin and
myosin molecules – creating a contraction.