Blueprint MCAT Full-Length 1: Passage 4 — Physics and Anatomy

Session 186

Feet and physics are on task for passage 4 in Blueprint MCAT full-length 1. Plus, pseudo-discrete questions. What are they and how should you approach them? Find out!

We’re joined by Phil from Blueprint MCAT, formerly Next Step Test Prep.

Listen to this podcast episode with the player above, or keep reading for the highlights and takeaway points.

[03:05] Passage 4 (Questions 18-22)

Ankle sprains account for nearly fifteen percent of sports injuries. The basic mechanism of an ankle sprain is a bend of ligament in a direction approximately perpendicular to the long bones of the leg. Translation of the tallus relative to the tibia beyond what is physiologically allowed by the tissue in ordinary movement is the immediate cause of a sprain or tear. A translation of 6 mm is the upper bound of the ordinary deformation of the body, though this number may be lower in some individuals. The anterior drawer test can be used to measure the degree of translation.

Note: This is physics and biology. It’s like orthopedics and joints and translation and forces and movements. And it’s telling us what’s happening with these different movements that can cause sprains or tears.

Suggested things to highlight: “6 mm translation” and the second sentence

Figure 1 Radiograph of the ankle joint

[05:04] Continuation of Passage 4

The talar tilt test can determine the normal talar stability or range of motion for a given individual. In the test, the patient is seated comfortably, while the examiner gently inverts the heel relative to the tibia. The resultant angle between the dome of the talus and the tibial plafond (see Figure 1) is measured, with a maximum angle of 5” being most typical, though anything from 0” to 23” is possible.

Figure 2 Two tests of lateral ligament, the anterior drawer (A) and (B) talar tilt examinations.

[06:00] Continuation of Passage 4

The standard talar test has a degree of subjectivity and is not as quantifiable as a new approach that uses mechanical isolation of the joint. By applying the force via mechanical stirrup and combining the talar tilt test with the anterior drawer, consistent values have been measured. For an interior force of 98 N and a torque of 13 Nm (Newton meters), the mean talar-tilt translational was 48 degrees and the mean anterior-drawer translation was 5.7 mm.

Note: There’s a good chance there’s going to be torque questions. So knowing our rotational equilibria stuff might be something kind of worth focusing on.

Interior forces of 500 Newtons or higher weighed to abnormal translation and result in injury. This can occur when a person is carrying a larger load, for example, individuals who are overweight or obese. Most more common is when an ankle is subject to stress upon landing from a height or turning at high speed. Injuries may also result from smaller anterior force values if the torque is sufficiently high.

Note: They’re telling you that smaller forces can still cause injury as long as there’s high torque. Torque is that rotational thing. This is why when you’re trying to loosen the bolt, you always grab the end of the wrench not right next to the bolt because that’s hardly going to move at all. That’s why the doorknob is not right next to the hinge.

[07:55] Question 18

During the force test with the mechanical stirrup, at what distance from the ankle joint was the force applied?

(A) 57 mm

(B) 60 mm

(D) 133 mm

Thought Process Behind the Correct Answer:

We need our answer to be in meters. So we have to take the Nm / N, and then the Newton’s canceled out and meters on top, rather than meters on the bottom.

So 13 Nm divided by 98 N, it’s more like 13 divided by 100, which is 0.13 m. Notice that all our answer choices are in millimeters though. That’s your time to buy 1000 to end up with D.

30% to 40% of the actual calculations you see on test day can be solved this way, which is a lot. And a lot of students are really worried about the math on the MCAT and physics and they don’t know the equations.

You can go a long way just by getting the numbers and paying attention to the units and it’ll end up coming together.

Correct Answer: D

[10:57] Question 19

During the anterior drawer test, the joint translation occurs in point 0.2 seconds. What is the average velocity of the talus during translation?

(A) 2.85 x 10-3 m/s

(B) 2.85 x 10-2 m/s

(D) 3.0 m/s

Thought Process Behind the Correct Answer:

Based on the passage, the mean anterior drawer translation is 5.7 mm. And the answers are all in m/s. So we could potentially translate it to m right off the bat. But then we just have many meters in 0.2 s. It’s simple math.

You’re not expected to do this in your head. Just convert that 5.7 millimeters to 5.7 x 10-3 m, and then divide by 0.2. And so 5.7 divided by two is not going to be three. So C and D are out right off the bat.

It’s just going to be A or B at that point. In that case, write it on a piece of paper and keep track of the units, B is the only one that actually ends up being correct.

Correct Answer: B

[12:52] Question 20

If a 65-kilogram man undergoes a turning acceleration of 5 m/s2 during a running turn, what is the magnitude of force experienced by the foot due to the ground?

(A) 325 N

(B) 650 N

(D) 1075 N

Thought Process Behind the Correct Answer:

They’re asking what is the magnitude of force and all the answer choices are in N. So we’re looking for N here, experienced by the foot due to the ground. So we have acceleration and we have the weight of the man.

Note: There’s not really anything in the question that refers to anything in the passage. So this is going to be a complete pseudo discrete question. And we don’t actually need any of the passage to answer this question so going back is probably not going to help.

If the answer choice is higher than 500 Newtons, then it’s going to result in injury. And it didn’t say that the person was injured. So it has to be lower than 500.

The foundational force equation is:

F = ma

Now, we have a mass and we have an acceleration. In theory, we could just multiply those two to get the force. (5 x 65 = 325)

However, when this is where this question gets tricky. Imagine that he wasn’t running. He was just standing still. His foot is still pushing on the ground. And there’s a force going on in that scenario.

So if he was standing absolutely still, then you have the normal force of him pushing up on his foot and he’s pushing down the ground. That’s just going to be F equals mg. So g is 10. And his mass is 65. So there’s a 650 Newton force pushing straight up perpendicular to the ground. There’s also a 325 Newton force pushing him forward, accelerating and forward. And so those two forces can be combined.

This is like a vector addition where you need to add those together. But it’s not just a straight addition because they’re not in the same direction.

So 650 up and 325 forward. And if we add them, it’s going to be more than 650. And so A and B are out. If they were both in the same direction if it was 650 up and 325 up and we added that, that’d be 975. And so what but they’re not in the same direction. So it’s going to be less than that. And so it’s can’t be D either.

By the process of elimination, you know it’s got to be more than 650 because that’s what it would be if he was just standing still and not when he’s moving forward. So his foots hitting the ground harder than that. But 1075 is just too big.

Correct Answer: C

[19:09] Question 21

Which of the following is the most likely to result in ankle injury?

(A) A 700-N normal force exerted by a flat horizontal surface on the feet

(B) An anterior force of 450 N at the pivot point of the ankle

(D) An anterior force of 400 N at a distance of 10 cm from the pivot point of the ankle

Thought Process Behind the Correct Answer:

A is out right off the bat just because if you’re standing, then there’s going to have to be some sort of force to keep you up, not falling through the ground. And assuming that the more you weighed, the higher the force. Therefore, it doesn’t necessarily mean you’re going to have an ankle injury due to that.

The passage tells us that the anterior forces of 500 N or higher lead to abnormal translation and result in injury. So B is out because 450 is less than 500. And C doesn’t make sense so the answer is D.

That’s like a 40-kg person standing on the end of your foot and you’re trying to hold them up with the end of your foot. That’s going to be a whole lot more difficult than just a 45-kg person standing on your heel.

There’s also that last sentence of the passage that says injuries may also result from smaller interior forces if the torque is high. And so they kind of built this into the passage.

Correct Answer: D

[22:4] Question 22

According to the passage, all the following are true about ankle injuries EXCEPT:

(A) they are more likely to occur in individuals of very low body weight who have been unable to build up ankle strength.

(B) a 13-mm translation of the talus relative to the tibia will cause injury.

(D) torque of the talus increases the chance of ankle injury for a given applied force.

Thought Process Behind the Correct Answer:

Generally, when people hear “except” questions, people tend to focus on “all the following that are true about ankle injuries except” because that’s the question. But it tells you that your answer is in the passage.

So this is the sort of question wherein reasoning and thinking outside could be problematic because there might be something that’s true outside. But we have to make sure our answer is coming from the passage itself. You can almost treat this like a CARS question where the passage or the answer is somewhere in the passage.

So they do say that torque is going to cause injuries. Hence, we eliminate D. They also said that anterior forces of 500 N or higher result in injury and if you added torque in smaller ones, it can cause up at 98. That’s a lot smaller. So it’s probably not going to cause an injury.

Answer choice B 13 mm – that’s likely to cause an injury because they said that the average was around 6 and anything above that is going to cause damage. They do actually mention something in the last paragraph that this can occur when a person is carrying a larger load, for example, individuals who are overweight or obese.

Answer choice A says injuries are going to happen to people who are skinny and that’s the opposite of what the passage said. The passage says that we’re going to see injury more often than people who are carrying a larger load. And so, A is directly counteracted by the B, C, and D which are all in the passage.

Correct Answer: A