20. length tension diagram: working range and power of a muscle, heat production, muscle fatigue

Question 1

Length tension diagram

Answer 1

• Length-tension curve is obtained when one stimulates muscles with maximal single impulses. The muscles are passively stretched with varying loads.
• As a result of isotonic, isometric, preload or afterload experiments we can construct the range where the muscles execute normal physical work.
• Length x Tension (load) = Work

1. If we passively stretch the muscle to A, B, C distances above the resting length (L0) and in these positions we stimulate the muscle with maximal stimuli, we can obtain the isotonic maximum curve for that muscle:
2. If no shortening is possible, we can measure the extent of the tension and get the isometric maximum curve.
3. We can also conduct the experiment under preload conditions too. The result will be the preload-maximum curve.
4. The experiment can be conducted under afterload experiments to get the afterload-maximum curve.

• Asasummaryoftheaboveexperiments,wecanconstructtheareaofthephysiological working range of that muscle. Normally, muscles work in this range. Animals adjust their muscle length in a way that, during work, these conditions will be kept. E.g. a cat that collects its limbs before jumping. In skeletal muscle, length measured under maximalpower is identical to the normal working range of the muscle. In cardiac muscle, however, normal working range is much below the length, which would ensure maximal tension: i.e., cardiac muscle has a reserve!

Question 2

Work

Answer 2

• Total work consists of two components: outer work and inner work.
• We can estimate the total work of the muscle from its oxygen consumption. Efficiency of the mechanical work can be calculated if outer work is divided by total work.

Question 3

Heat production

Answer 3

Muscle produces a considerable amount of heat during work. This is partly caused by ATP breakdown during contraction and partly by synthetic processes after contraction.

• There are 3 main phases of heat production:
  
  • Resting: Heat is produced even when muscle is in resting state. A considerable ratio of basal metabolic rate (BMR) comes from heat production of muscles.
  • Initial: At the beginning of contraction, initial heat is produced first.
    
    • Activation: The first phase of initial heat is called activation heat, which is heat production of electromechanical coupling.
    • Contraction: Most of the initial heat can be explained by the heat production of contraction. The role of sliding filaments and calcium pumps is important here.
  • Restitution: Fast muscles generate contractile energy by utilization of their energy stores. After contraction, these stores must be filled up again: synthesis results in energy investment and heat production. This is called restitution heat.
• Heat production in fast, glycolytic, white fibers:
  
  • Initial heat is much larger than restitution heat.
  • High power for a short period is possible.
  • Muscle “pays back” its oxygen debt during re-synthesis of energy reserves.
• Heat production in slow, oxidative, red fibers:
  
  • After short initial glycolytic phase of heat production, a long lasting oxidative period begins.
• Muscle is not exhausted and there is no oxygen debt.

Question 4

Power

Answer 4

• By drawing the velocity-tension diagram instead of the length-tension diagram, we could have an idea of the power of muscle.
• Small tension is paired with high velocity
• High tension (heavy load) is paired with small velocity during contraction
• Optimal position: under intermediate load can the muscle contraction be optimally fast.
• Power values of skeletal muscle:
  
  • Maximal tension: 3-6 kg/cm2
  • Maximal speed: 7m/s
  • Total force: 200N
  • Efficiency: 20%
  • Maximum power:
    
    • Short term: 3000-5000 W
    • Long term: 1200 W

Question 5

Fatigue

Answer 5

• Muscles are capable of long lasting activity. After a certain period, muscle become fatigued.
• Fatigue depends on the ratio of glycolytic and oxidative fibers of the muscle.
• Under physiological conditions, fatigue is not caused by the lack of transmitters or oxygen. It is rather the increasing concentration of metabolic by-products that cause the inability of concentration in muscles.
• Signs of fatigue (both observable in mechanogram):
  
  • Decrease of twitch amplitude
  • Increase of twitch duration
• In vitro fatigue:
  
  • After fatigue, restitution takes place faster if the environment is abundant in oxygen. In nitrogen-rich environment, muscle reaches an unrecoverable fatigue state.
• In vivo fatigue:
  
  • Peripheral fatigue, a consequence of:
    
    • Decreasingenergystores
    • Increasing by-product concentration
    • Direct effect of lactic acid
  • Central fatigue, after long term tension:
    
    • Exhaustion of motor unit
    • Exhaustion of myoneural junction.
• Fatigue is developing earlier in fast, glycolytic, phasic fibers than in tonic, oxidative fibers.
• Subjective feelings of fatigue:
  
  • Increased heat production
  • Decrease of pH
  • Direct effect of lactic acid
  • Dehydration
  • General hypoglycaemia