Length-Tension Relationship and Proprioception

Length-Tension Relationship

• states that tension that can be developed by a muscle during a muscle twitch (contraction) is directly related to the length of individual sarcomeres before contraction begins

• sarcomere length is related to how much of the overlapping between the thick and thin filaments

• sliding filament theory predicts: tension that can be generated by a muscle fiber is directly proportional to the number of crossbridges being formed between the thick ad thin filaments during muscle contraction

Length-Tension Graph

• No room for sliding to occur so no force can be generated

• optimal length (2-2.4um) gives optimal actin myosin interaction so maximum tension can be generated

• if overstretching occurs, no crossbridge can be formed, therefore no force can be generated

Active and Passive Force

• active force is at its maximum when the resting length of sarcomere is at its optimum 2-2.4um (it is normally here)

• passive force of a muscle is the tension the muscle measured before the muscle twitch

• muscle gets stiffer as it is extended, increasing amount of force in needed to elongate the muscle cell

• the main contributing factor of this passive force is titin in muscle fibers

Control of Muscle Tension

• action potential is always same size in given neuron or muscle fiber

• force of contraction does vary

• force or tension that a given muscle fiber can generate depends on:

• rate (frequency) of nerve impulses arriving at the neuromuscular junction

• amount of stretch (passive tension) before contraction

• energy substrates and oxygen available

• total number of muscle fibers that are contracting in unison

Refractory Period

• if two consecutive stimuli are applied to a muscle fiber at a very close interval: the 1st stimulus will elicit a response and the 2nd stimulus might not be able to elicit a response because the muscle is still in its refractory period

• absolute refractory period is the time interval after the initial stimulus that the muscle will not contract no matter the strength of stimulus

• relative refractory period is the time interval after the initial stimulus that the muscle will contract weaker when the stimulus applied is stronger than what would normally cause a contraction

Frequency of Stimulation

• if a second stimulus occurs after the refractory period is over and before the muscle fiber has relaxed, the 2nd contraction will be stronger than the 1st (wave summation)

• when the skeletal muscle fiber is stimulated at a rate of 20-30 times per second, the result is a sustained but wavering contraction (unfused or incomplete tetanus)

• when the skeletal muscle fiber is stimulated at a higher rate of 80-1000 times per second, the muscle fiber does not relax at all (fused or complete tetanus)

Motor Units

• consists of one motor neuron and all the muscle fibers it innervates

• a muscle may have many motor units of different types

• the number of muscle fibers in a single motor unit varies

• all muscle fibers in a single unit are of the same fiber type

Motor Unit Recruitment

• group of fibers is innervated only by a single somatic motor neuron

• when the somatic motor neuron fires an action potential, all muscle fibers in this unit contract

• typically, they don’t all contract in unison

• one may be contracting while the other is relaxing

• this is to delay muscle fatigue

• recruitment of specific motor unit can be employed for a specific task

Isometric and Isotonic

• isometric (static): muscle contracts without changing length

• isotonic (movement with muscle length change): muscle contracts but tension stays the same

Slow-Twitch Oxidative Fibers (Type I)

• fibers are smallest in diameter amongst the 3 types and least powerful

• contains large amounts of myoglobin (dark red)

• high density of blood capillaries and mitochondria

• ATP generation mainly by aerobic cellular respiration (oxidative fibers)

• with low glycogen content in muscle fibers

• ATPase in the myosin heads hydrolyzes ATP relatively slowly and therefore contraction cycle is at a slower pace

• fatigue resistant and capable of sustained contraction

• maintaining posture and endurance activities

Fast-Twitch Oxidative-Glycolytic Fibers (Type IIa)

• intermediate fiber diameter with moderate velocity of muscle shortening

• moderate amounts of myoglobin (pink-red)

• good supply of blood capillaries and large amounts of mitochondria

• ATP generated by aerobic glycolysis 

• moderate glycogen content

• ATPase in myosin heads hydrolyzes ATP in a moderate to fast speed and therefore contraction cycle is also at a moderate pace

• moderately to high fatigue-resistant and capacity to resist fatigue increases with endurance training

• walking and sprinting

Fast-Twitch Glycolytic Fibers (Type IIb)

• largest diameter and most myofibrils (fastest velocity of shortening and powerful contraction)

• low level of myoglobin (pale white)

• fewer supply of blood capillaries

• ATP generation by glycolysis

• largest amount of glycogen in muscle fibers

• ATPase in myosin heads hydrolyzes ATP rapidly and therefore contraction cycle is the shortest compared to the other two types

• adapted for intense anaerobic glycolytic means to generate ATP

• high amount of CK

• fatigue quickly

• rapid, intense movement of short duration (weight lifting)