Question

shevon (58 kg) is performing a bench-step test. she performs the test three times, once on a box that is 41.3 cm (16.25 in.) high, once on a box that is 50 cm (19.7 in.) high, and once on a box that is 21 cm (8.26 in.) high. if she performed the bench step with exactly the same step rate (22 steps ∙ min−1) with each of the different box heights, what would happen to her heart rate response? what is causing or underlying these responses? (look at the equations for work)

Answer 1

Mass of the subject, M=58kg

Weight of the subject, W=58 x 9.81 kg m sec-2=568.98 N (taking g=9.81 m sec-2)

Case A: HA=41.3 cm (16.25 in)

Case B: HB=50 cm (19.7 in)

Case C: HC=21 cm (8.26 in)

Step Rate, R=22 min-1

Given that the experiment is performed over the same amount of time, the following can be expected: the heart rate of the subject will be maximum in Case B, and minimum in Case C. In other words, the heart rate is high if the step height is high.

(Heart Rate)B>(Heart Rate)A>(Heart Rate)C

To understand the underlying cause of the response trend, we will calculate the Power required in each case.

We will use the equations:

Where P is the power, W is the weight of the subject, H is the height traversed in each step, R is the step rate, and t is the time over which the experiment was conducted.

Since t is not mentioned in the problem I will take t=3 min.

Case A:

Case B:

Case C:

We can see that:

and

Since the work done and power in the respective cases show the trend B>A>C the heart rates also vary in the same order.

We can try to think of a mechanical analogy for the human body. Think of the legs and all other muscles associated with each step as a machine. The entire body weight is the load. Now for this "machine" to perform the bench step experiment, the power that needs to be delivered in each case is PA,PB,and PC. Now think of the subject's blood as the fuel that keeps the machine running. And the heart is quite literally the pumping mechanism that is fuelling the machine. Thus, when the required power is more, the heart needs to pump more blood into the muscles. This is reflected in the elevation of heart rate of the subject.