Definition of physical work
Work (joules) = force (newtons) x distance (meters)
Definition of muscle efficiency
Ratio of mechanical work : total metabolic cost (O2 consumption)
Definition of power
Power (watts) = work (joules) / second
What is the average muscle efficiency of a human?
What is the average power someone can put out over an hour of vigorous exercise? 8 hour day of work?
Hour of exercise = 150 watts Days work = 75 watts
Definition of stress
Stress = Force / Area (of muscle)
Why do all muscles produce about the same amount of force?
Because force is a function of area. The bigger the muscle, the more force it will produce.
When you decide to move a muscle, how does that decision result in muscle movement?
White matter in brain -> alpha motor neuron in spinal cord -> generates action potential -> AP releases ACh at synaptic cleft -> Muscle membrane depolarizes -> Muscle AP is produced -> Ca is released -> Contraction
If this system were a whole muscle, what will the different elements be?
Series elastic element: tendon and CT (the tendon must be stretched before muscle can generate force), Parallel elastic element: titin molecule, Viscous element: sliding components of muscle
Where does the other 75% of energy we waste go to?
Heat. It is dissipated due to the viscous force during muscle contraction
What do the muscle ends do in an isometric contraction?
Nothing, they don't move
What slows relaxation in muscle after an isometric contraction?
The small stimulus causes the surrounding fibers to contract and stretch out the tendon (series elastic element). When the stimulus is gone the muscle wants to go back to normal but is slowed in its return by the viscous elements (sliding components of the muscle).
Why does force top out at a particular stimulus?
It reaches the super maximal stimulus where every fiber in the muscle is contracting and cannot possible produce any more force.
What is a muscle twitch?
Tension produced in response to a single muscle action potential. Some muscles respond more quicker than others to an action potential.
What happens in an unfused isometric tetanus?
An AP happens before the muscle totally relaxes…but it relaxes little so you would be shaky.
What happens in a fused tenatus isometric contraction?
Minimal relaxation occurs between action potentials. Intracellular Ca isn't completely removed from the cytoplasm and cross bridges crawl over each other until the muscle is maximally shortened.
Why is a tetanus not the best way to make large increases in muscle force?
They only produce force increases 3-5 times that of a twitch.
Why do we care about passive muscle stretch?
When you flex your bicep your triceps stretch. Initial force is generated at the tendon and then the muscle stretches a little so force decreases a little bit. Comparing the tension in the muscle to its length gives a linear line as a spring would.
What happens to force in a muscle with passive stretch that is coupled with a muscle being stimulated?
Total tension is made up of passive and active tension. Passive tension is generated by the simple stretching of the muscle. Active tension is generated by cross-bridge sliding. Adding the tensions together gives you total tension.
What generates force in a sarcomere? What determines the magnitude of force?
Making and breaking cross-bridges. The magnitude of force generated is proportional to the number of cross-bridges formed.
What is happening in the sarcomere at the different points of this diagram?
Just after point 1: a few cross-briges have formed at the end of the A band and tension increases. Points 2-4 (shown below): no more tension is generated because all myosin heads are cross-bridged to actin, but force remains constant. Point 5 (shown below) produces less force because thin filaments get stuck to each other and you lose myosin cross-bridges in areas with thin filament overlap. At point 6 (shown below) tension falls to 0 just before thin filaments contact the Z-line.
How is all of the tension being generated at the point indicated by the arrow?
Super stretching of the muscle. There is no active tension going on, only passive
What range does skeletal muscle work it?
Definition of an isotonic contraction.
Contraction against a constant force. Passive stretch sets the initial muscle length, super maximal stimulus is applied and the ends of the muscle are allowed to move.
How is length established in an isotonic contraction?
The length when no force is applied and the end is swinging freely.
What happens when a preload is placed on a muscle in an isotonic contraction?
The muscle stretches and creates a preload force.
What happens when a stimulus is applied to a muscle in an isotonic contraction when the after load has been applied?
When resting, the after load is not noticeable because the preload is supporting it on the platform. Contraction does not occur until the muscle generates enough force to lift the object. When contraction occurs the muscle feels the force of the after load. The muscle shortens and lifts the after load.
What happens when you but a heavier after load on a muscle that isotonically contracts?
Upon stimulation, it will take longer to generate the force to lift the load, longer to lift the load and you can't move the load as far.
What tension is generated by the preload in an isotonic system?
How does the heart generate enough force to contract and expel blood out of vessels?
Both ends of the muscle contract isotonically
What tension is generated by the after load in an isotonic system?
Active tension. When this happens the muscle will shorten. The amount of muscle shortening is proportional to the distance you can move the object.
What is indicated by the yellow dot?
The maximum amount the muscle can shorten and thus move the object.
Why can't isotonic contractions move heavier loads as far as they can move lighter loads?
The sarcomere shortens until it can overcome the force of the heavier object. Once it is overcome, then the muscle can shorten. However, the initial increase in force needed by the muscle takes up cross-bridges and you run out of cross-bridges when the muscle is shortening…thus you can't move the load as far.
What causes the valve leading to the aorta to open in contraction of the heart?
The blood-filled ventricle (preload) is squeezed by the surrounding myocardium while all valves are closed. This causes an increase in pressure against the aortic valve because of the pressure of blood on the other side in the aorta (after load). Once the heart squeezes the blood in the ventricle enough so that the pressure in the ventricle exceeds that in the aorta, blood is pushed out.
What causes the aortic valve to close?
The isovolumetric pressure inside the ventricle decreases as the heart relaxes and the valve shuts because it is now less than the aortic pressure.
Why aren't length changes a source of regulation in skeletal muscle?
The length of muscle is restricted by bones and tendons
Why are length changes a source of regulation in cardiac muscle?
Cardiac contractions and relaxations cycle all the way from 2-6 where there is maximal and minimal tension.
What determines the force generated by contraction in skeletal muscle?
# of fibers contracting (primary source) and the frequency of firing (secondary source)
What is the difference between motor units in the eye vs. motor units in the postural numbers?
1 motor neuron innervated many more fibers in the postural muscles because you don't need as much fine control. A place that needs fine control will have lots of motor neurons only innervating a few fibers. Also, the motor unit innervating fewer fibers will have a smaller soma and axon (diameter).
What are the 3 types of muscle fibers?
Slow-oxidative (fatigue-resistant, red, postural muscles), Fast-oxidative (mix between the two others) and Fast-glycolytic fibers (fatigue quickly, short-duration/high forces, white)
A grocery store tells you that they only sell cage-free, range chickens. How can you confirm this by looking at the meat before you cook it?
Cage free chickens should fly more and have red, slow-oxidative breast meat from flying and white, fast-glycolytic meat in the thighs from less walking. Usually its the reverse because farm chickens rarely fly and mostly walk.
How can training affect the type of athletic activities you participate in?
Humans have all 3 types of muscle fibers and you can push the major type of fiber in a muscle toward one type or another based on the type of exercise you are doing.
How does motor unit size/type distribution look in human muscle?
Mostly small motor units that tend to be slow oxidative fibers. Intermediate motor units (fast oxidative) are recruited if small motor units cannot do the job. Large motor units (fast glycolytic) are recruited only at the highest forces (panic).
When does a motor unit fall out after it has been recruited?
When the contraction stops or the force needed falls below the recruitment level.
How is a fused tetanus contraction produced without making the same muscle fire all the time?
Asynchronous firing of different motor units. Gaps in tension are filled by the other motor units.
When a single motor unit is recruited in lifting an object, how much force does it contribute to lifting the load?
About 4% of the total load is generated because it recruits larger motor units.