Muscle Series Part 2: Muscle Movements
In this second episode of a three-part series we’ll be reviewing all the movements produced by muscles in the human body
By Amirali Banani
Actions of Skeletal Muscle
Muscles are arranged in groups with opposing actions: Agonists and Antagonists. A prime mover is the muscle primarily responsible for an action; synergists assist the primer mover. Muscles produce movement by contracting; they pull the insertion bone closer to the origin bone and movement occurs at the joint.
Tonic contraction (few muscle fibres shorten at one time) maintains muscle tone (posture) and heat is produced by muscle fibre contraction to maintain normal body temperature.
Flexion
Bending → brings 2 bones closer together and decreases the angle between these bones
Extension
Straightening → is the opposite of flexion; increases the angle between these bones
Abduction
Moving away → the movement of a limb away from the midline of the body
Adduction
Bringing together → movement of a limb toward the midline of the body
Rotation
The turning of a bone around its long axis
Supination
The movement of the radius around the ulna (opposite of pronation)
Pronation
The movement of the ulna around the radius (opposite of supination)
Dorsiflexion
Lifting the foot upward toward the leg (opposite of plantar flexion)
Plantar Flexion
Pointing the foot toward the ground (opposite of dorsiflexion)
Muscle Contractions
Twitch contractions are single contractions of muscle fibres caused by a single threshold stimulus (e.g. knee reflex). There are 2 types of contraction: Isotonic and Isometric
Isotonic
Contraction of a muscle that produces movement at a joint; muscle shortens → insertion end moves toward point of origin (e.g. walking, lifting)
Isometric
Muscle contractions that do not produce movement; muscle as a whole does not shorten. Tension increases within the muscle (e.g. pushing against an immovable object).
Effects of Exercise on Skeletal Muscle
Improves muscle tone; better posture; more efficient heart and lung functioning; less fatigue
Atrophy: occurs when muscles shrink in mass (usually due to prolonged inactivity)
Hypertrophy: occurs when muscles grow in mass (usually in response to exercise)
When individuals start to engage in weightlifting, the muscles are forced to contract against a heavy resistance. The result is an increase in the number of myofilaments — not the number of myofibrils (muscle fibres). This activity is regulated by fast-twitch fibres.
Endurance training (aerobic training) increases the muscle’s ability to sustain moderate exercise over a long period of time. This type of training allows for more efficient delivery of oxygen and nutrients to the muscles due to the increased blood flow (e.g. running, aerobics, walking …). The muscle fibres that help with this are called slow-twitch.
REVIEW FROM PART 1:
The 2 Fibre Types in Skeletal Muscle
When we are born, we are genetically endowed with certain ratios of 2 muscle fibre types:
Slow-twitch (S-T): Skeletal muscle fibres that have a steadier “tug” or “power” and more endurance despite having a fewer number of muscle units with a smaller number of muscle fibres. These muscle fibres are useful in endurance sports such as running, biking, swimming or jogging. Most of their energy is developed aerobically and usually only tire when they use up their fuel or oxygen supply. S-T are characterized as having many mitochondria and contain the blood pigment myoglobin and are consequently “red” in colour. S-T fibers have a low maximum tension but are highly resistant to fatigue.
Fast-twitch (F-T): Skeletal muscle fibres that tend to be anaerobic and seem designed for strength and power because their motor units contain many muscle fibres. They are used for sports such as weightlifting, sprinting, swinging a golf club or shot put. These fibers are characterized as being “white” in colour because they lack mitochondria and myoglobin. Consequently, these muscles can “tire” easily because they build up lactic acid more quickly. Remember that lactic acid occurs when glucose is burned by the body without oxygen to obtain its energy.
Muscle Innervation
Muscles are stimulated to contract by motor nerve fibres.
These nerve types branch at their axon terminals end in axon bulbs which are in close proximity to the sarcolemma and are separated by a synaptic cleft. The entire region is known as the neuromuscular junction. The neurotransmitter acetylcholine (ACh) is released into the synaptic cleft. The acetylcholine binds to receptor sites on the sarcolemma which generates its own impulse. The impulse spreads over the entire sarcolemma and down the transverse tubules (T-tubules) to the sarcoplasmic reticulum. The sarcoplasmic reticulum then releases its stored calcium ions into the myofibrils within the muscle fibre, ultimately triggering a contraction.
Oxygen Debt
Finally, we’ll be discussing the concept of “oxygen debt” resulting from excess muscle activity.
Muscles are able to operate in the absence of oxygen for short periods of time. However, this “oxygen debt” must be paid back and is the reason why you continue to breathe heavily after heavy exercise. Glucose can also be burned in the absence of oxygen to produce energy for the cells. Instead of producing CO2 and H2O in the presence of O2, lactic acid is produced which is responsible for sore muscles after you do heavy exercise. It takes about 2 days of a high carbohydrate diet to replenish your body’s glycogen stores after extremely heavy exercise.
So, the next time you do a killer workout at the gym or go on a three-hour hike, make sure to eat food rich in carbohydrates to allow your body to recover quickly and efficiently!
Thank you for reading part 2 of this three-part series on muscles! You may check out parts 1 and 3 below:
