Effects of microgravity on muscle tissues
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- Effects of microgravity on muscle tissues
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Effects of microgravity on muscle tissues
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[post_content] => Practice Passage (Question 1-5)
*This passage is the property of Khan Academy and has been reformatted into an AAMC-style interface in their entirety by MedLife Mastery. MedLife Mastery does not endorse and is not an affiliate of Khan Academy.
A simple model of the effects of microgravity on astronaut’s muscles is given by “unloading,” in which reduced gravity mimics the terrestrial effects of bed rest or suspension, in which the “load” or total gravitational force acting on the muscle is reduced. Based on long-term terrestrial studies, researchers have concluded that the physiological basis of adaptation to microgravity involves a reduction in the diameter of muscle fibers, which bring about corresponding decreases in the maximum tension that can be exerted by the muscle. Due to these effects, astronauts frequently report inflammation and fatigue in their primary muscle groups upon returning to earth.
The effect of unloading on the performance of muscle tissue is shown in Figure 1, which shows examples of force-velocity relationships for hypothetical muscle tissues. The solid line indicates a standard terrestrial muscle response, the dashed line indicates the response of an astronaut who has just returned from space. The graphs were generated by terrestrial experiments in which a patient’s muscles were allowed to shorten in response a force of fixed magnitude, like a hand weight. The rate of shortening is the velocity recorded for that experiment. The experiment is then repeated by varying the load size and determining the new contraction rate, and the results are compiled to generate the force-velocity graph.
Figure 1: Force-velocity relationships for ideal muscle tissues. The solid line corresponds to a standard muscle response, but the dashed line corresponds to the muscle tissue of a recently-returned astronaut. (Adams et al. 2003)
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[question] => What is the load on the muscles due to gravity of an astronaut orbiting at 2Rₑ, where Rₑ is the radius of the earth, relative to the load she experiences on earth (F₀)?
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[answer] => 3
[description] => Reason for the Correct Answer:
An orbiting object is undergoing free-fall.
Free-fall (in the absence of air resistance) is formally equivalent to zero-gravity.
The total gravitational force does not determine the load.
The total load on the astronaut’s muscles is zero during orbit.
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[0] => Array
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[each_answer] => A. F₀
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[each_answer] => B. F₀/4
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[each_answer] => C. 0
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[each_answer] => D. F₀/2
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[question] => What is the total acceleration of an astronaut floating weightlessly in a space capsule orbiting with radius R꜀. Let Mₑ be the mass of the earth, 𝓂 be the mass of the astronaut, and 𝐺 be Newton’s gravitational constant.
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[answer] => 2
[description] => Reason for the Correct Answer:
Because the capsule and astronaut are in orbit, they are both accelerating due to gravity.
The apparent weightlessness experienced by the astronaut does not mean that she is not accelerating.
Gravitational acceleration is given by Newton’s law of gravity:
a = GMₑ/R²꜀
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[0] => Array
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[each_answer] => A. 
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[1] => Array
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[each_answer] => B. 
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[each_answer] => C. 
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[question] => Suppose a scientist fits each data set to a curved line the intercepts both axes. Which of the different attributes of the force-velocity graph for astronauts (relative to that of standard patients) best explains the astronauts’ symptoms?
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[answer] => 4
[description] => Reason for the Correct Answer:
The area under the curve is not obviously different for the two graphs.
The vertical intercept, while physiological in origin, does not clearly explain the astronaut’s symptoms.
The intersection of the graph with the horizontal axis gives the force for which the muscle does not shorten, setting a maximum weight that the muscle can handle.
The change in the intersection with the horizontal axis explains the astronauts’ symptoms: their muscles can handle less weight than usual.
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[each_answer] => A. The area under the curve
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[each_answer] => B. The intercept of the curve with the vertical axis
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[each_answer] => C. The initial slope of the curve when force equals zero
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[each_answer] => D. The intercept of the curve with the horizontal axis.
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[question] => Which of the following variables is most difficult to measure in a laboratory, thus limiting the statistical precision of a terrestrial muscle experiment used to generate Figure 1?
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[answer] => 4
[description] => Reason for the Correct Answer:
The force is the size of weight used for the experiment.
The velocity of the tissue is the variable recorded during each experiment.
The weight, fiber length, and acceleration due to gravity can be measured to arbitrary precision before or after the experiment.
The recorded velocity limits the precision of the experiment.
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[each_answer] => A. The applied force
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[each_answer] => B. The length of the muscle fibers
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[each_answer] => C. The acceleration due to gravity
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[each_answer] => D. The velocity measurement
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[question] => Which of the following determines the rate that the muscle uses energy during the experiment?
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[answer] => 1
[description] => Reason for the Correct Answer:
The rate of energy use is the power generated in the muscle.
The starting and ending conditions of the experiment do not uniquely predict the functional form of the curve.
Power has units of force*velocity.
The area under the curve gives the power used by the muscle.
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[each_answer] => A. The area under the curve.
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[each_answer] => B. The length of the curve.
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[each_answer] => C. The initial slope of the curve.
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[each_answer] => D. The intercept of the graph with the velocity axis
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[post_title] => Effects of microgravity on muscle tissues
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