Lecture 8: Skeletal Muscle Mechanics Flashcards
Isometric contraction
Developing tension at a constant muscle length; represented by tension vs. length curve
Isotonic contraction
Shortening/lengthening of muscle at constant tension; represented by change in length vs. time
Muscle tension
Force exerted by the contracting muscle on a load
Load
Force exerted by an object on the muscle
Tension vs load
Moving a load requires total muscle tension to be just greater than the load
Concentric contraction
When tension > load, the muscle shortens during contraction
Eccentric contraction
When load > tension, the muscle lengthens during contraction (lengthening only bc of external forces)
Muscle twitch
Mechanical response of muscle fiber to 1 action potential
Latent period
Delay between the action potential and corresponding increase in muscle tension
In isometric contraction, how can the muscle develop tension without changing length?
In isometric contraction, the filaments don’t slide. Instead, power stroke rotation is absorbed by the elastic elements of the fiber. Cycling cross-bridges repeatedly bind to the same actin molecule.
Contraction time
Defined as the time from beginning of tension to peak tension
Muscle fiber types
Fast twitch: contraction time ~10 ms
Slow twitch: contraction time 100 ms or longer
What makes different fibers fast vs. slow?
Contraction time depends on 1) how long cytosolic Ca++ stays elevated and therefore amount of SERCA activity and 2) speed of cross-bridge cycle (myosin ATPase rate)
Differences between isometric and isotonic twitches
Isotonic twitches have longer latent periods and are quicker mechanical events, because they need to first build enough tension to actually move the load
Effect of heavier load on an isotonic twitch
Heavier loads means:
-Longer latent period
-Slower velocity of shortening
-Less distance shortened
Maximum shortening velocity and maximum isometric tension
Max shortening velocity occurs with 0 load, max isometric tension means 0 velocity. Velocity then increases in lengthening with heavier loads until the muscle tears
Why do heavier loads slow shortening velocity?
More load slows the forward movement of each power stroke
What determines max shortening velocity?
Rate of myosin ATPase, which determines the speed of an individual cross-bridge cycle
Muscle summation
Muscle twitches sum if a new stimulus is applied before the muscle completely relaxes. Higher frequency stimuli create greater peak tension, and eventually a smooth continuation of tension
Tetanus
Maintained muscle contraction due to repetitive stimulation
Fused vs unfused tetanus
In unfused tetanus, tension oscillates due to low frequency stimulation. Fused tetanus has no oscillations.
Max tetanic tension vs isometric twitch tension
In tetanus, [Ca++] stays high so more cross-bridges are recruited and there is time to stretch out elastic elements. Max tetanic tension is much higher than 1 twitch, and is good for sustained work like maintaining posture.
Passive muscle tension
Relaxed muscle fibers have passive tension due to titin. Passive tension increases with length of fiber
Optimal length L0
Active tension depends on resting fiber length and is highest at some intermediate length L0, the length at which the fiber develops the greatest isometric active tension
Why does active tension decrease above L0?
As the fiber lengthens, there is less thin/thick-f overlap and so fewer cross-bridge interactions available
Why does active tension decrease below L0?
At shorter lengths, thin-f’s from other sarcomeres overlap and interfere with X-bridges. Rigid thick-f’s can also collide with Z-lines, creating internal resistance to further shortening
Resting length of all muscle
Passive elasticity keeps most muscle fibers near L0 when relaxed
How do muscles maintain high ATP levels?
- Phosphorylation of ADP by creatine phosphate (rapid)
- Oxidative phosphorylation in mitochondria (slower)
- Glycolysis ATP production in cytosol (slower)
What supplies ATP for moderate muscle activity?
At moderate intensity, most ATP is from oxidative phosphorylation (aerobic)
Where does glucose for ATP come from for oxidative phosphorylation in muscle?
First 5-10 min: muscle glycogen stores
Next 30 min: blood glucose and fatty acids
After: More fatty acids, glucose use decreases
What supplies ATP for intense muscle activity?
At intensities >70% max ATP consumption, oxidative phosphorylation maxes out and glycolysis contributes more as intensity continues to increase. Uses blood glucose and produces lactic acid as a byproduct. Anaerobic.
Oxygen debt
Increased oxygen demand continues post-exercise to restore creatine-Pi/glycogen and metabolize accumulated lactate
Muscle fatigue
A decline in muscle tension due to previous contractions. Also causes slower shortening velocity/rate of relaxation
Metabolic changes in active muscle
[ATP] decreases
[ADP], [Pi], [Mg++], [H+], O2 free radicals increase
Effects of metabolic changes in active muscle
-Decreased Ca++ release, reuptake, and storage by SR
-Decreased thin-f sensitivity to Ca++ activation
-Direct inhibition of myosin X-bridge cycle
Why does low intensity, long duration exercise demand disproportionately long recovery times?
RyRs become leaky -> constant high [Ca++] -> increase in protease activity on contractile proteins, requiring longer recovery for protein synthesis
-Dehydration/glycogen depletion also mediate fatigue
Central command fatigue
Phenomenon where cerebral cortex regions fail to send excitatory signals; possible feedforward response anticipating glucose depletion
Types of skeletal muscle fibers (twitch speed)
Type I: slow twitch (slow myosin ATPase velocity)
Type II: fast twitch (fast myosin ATPase velocity)
Types of skeletal muscle fibers (energy)
Oxidative: many mitochondria for oxid. phos. and lots of myoglobin for O2 diffusion/storage; AKA red muscle fibers
Glycolytic: few mitochondria, less vascularization/myoglobin, lots of glycolytic enzymes/glycogen; AKA white muscle fibers
3 primary muscle fiber types
Type I (slow oxidative)
Type IIa (fast oxidative glycolytic)
Type IIx (fast glycolytic)
Isometric tension differences between muscle fiber types
From least to most: I, IIa, IIx
Due to different fiber diameters (filament density is equal, so bigger means more X-bridges), also variation in proportion of attached X-bridges and force per X-bridge
Fatigue differences between muscle fiber types
Type I fatigue slowest, Type IIx fatigue fastest
Mainly due to energy metabolism profiles
Properties of sarcomeres in series
Sarcomeres in myofibrils are in series to enable faster/greater shortening at the cost of extra energy
How much force does a myofibril generate based on the number of sarcomeres it has?
A myofibril generates the same amount of force as 1 sarcomere, because the sarcomeres are in series
Max tetanic tension without elasticity
Without elastic elements, max tetanic tension would equal twitch tension
Total muscle tension
Total tension = active + passive; these forces are in parallel
Preload
Passive tension in the muscle before activation
Afterload
Load lifted by the muscle (active + passive)
How much will a muscle shorten for a given load?
The muscle will shorten down to where the total muscle force capability is equal to the load.
