The Right Stuff at Heart

In a previous post, I’ve talked about some of the challenges facing the Mercury program managers in selecting the first group of astronauts. No one knew with any certainty what the first men in space would be up against. As such, the search for the perfect men was a multi-stage process taking into consideration as many possible scenarios as program managers could envision. One of the more interesting problems in selecting NASA’s Mercury astronauts was their level of fitness – they obviously had to be in physical shape to survive the challenges of spaceflight, but how physically demanding was a mission in a capsule too small to afford the astronaut much movement? (Pictured, Wally Schirra examines his chest x-rays. 1962.)

As possible astronauts, NASA briefly considered acrobats, daredevils, and contortionists – all people who were comfortable and familiar with cramped environments and dangerous jobs. But President Eisenhower had other ideas for the astronaut corps and stipulated that the candidate pool be limited to military test pilots. Applicants form this pool had a perfect mix of security clearance in a time of war and comfort with unknown environments in the air.

A set of stringent requirements narrowed the applicants from test pilots to those deemed fit for spaceflight. Astronaut candidates had to be under 40 years of age, hold a bachelor’s degree in engineering or its equivalent, be a graduate of a test pilot school, and be a qualified jet pilot with at least 1,500 hours of flying time.

Physically, the original guidelines demanded candidates be ‘in excellent physical condition’, under five feet eleven inches, and weigh less than 180 pounds. The height and weigh limits, interestingly, had nothing to do with physical demands associated with spaceflight. The small capsule’s hatch would not shut if the occupant was taller than five feet eleven inches, and the limited lift capacity of the Redstone and Atlas launch vehicles demanded a light astronauts.

From the United States Air Force, the United States Navy, and the United States Marine Corps, only 110 men had the necessary background. The first 69 to volunteer were interviewed, and 32 of those interviewed were selected to proceed to the final round in the testing process – physical and mental fitness testing at the Lovelace Clinic in Albuquerque, New Mexico.

The psychological tests were essential to ensure the astronauts were the most even keeled and balanced men demonstrating clear thinking in dangerous situations. The psychological tests included Rorschach tests, isolation chamber tests, stress tests in hot and cold environments, and a variety of other tests designed to see which candidates would crack under stress and which were able to keep their heads.

The physical testing presented the doctors with a different set of challenges. The Mercury capsule was so small the astronaut was unable to move his legs and had only limited space in which to move his upper body (pictured). This meant that for the moment the astronaut had few physical challenges in space – without moving around or outside the capsule there was little need for specific physical abilities. Nevertheless, overall fitness was a necessity; the Mercury program was the first in NASA’s long term plan and later missions would likely have the same astronauts in much more physically demanding roles. But what about the immediate missions?

Missions planners determined that overcoming stress was likely to be the most challenging aspect of a mission. As such, the one muscle that would really get a workout on a Mercury mission was the astronaut’s heart, a vital muscular organ extremely susceptible to stress.

Mercury astronaut candidates faced a battery of physical tests designed to measure the strength of the heart using two measurements – blood pressure and heart rate.

Blood pressure refers to the amount of blood pumped through the body in a single beat. It is expressed as two numbers, for example ideal blood pressure is 120/80. The first number (120) refers to systolic pressure or the amount of the blood the heart expels through the arteries per beat. The second number (80) refers to diastolic pressure or the amount of blood the heart receives as the muscle relaxes. Heart rate is expressed as beats per minute. Between blood pressure and heart rate, a physician can get a good idea of your cardiac output and overall heart health.

These two very basic measurements were the physicians principle tools in determining the physical fitness of the Mercury astronaut candidates. These measurements were applied to seven tests: the Flack test had the astronauts expel breath to keep a ball on top of a column of Mercury at a certain height as long as possible, the Valsalvaa overshoot test duplicated the Flack test with the addition of blood pressure measurements, the Partial Pressure Suit test put the astronaut through psychological and cardiovascular tests in a simulated high atmosphere environment, and the Tilt Table test measured candidates’ cardiovascular recovery after exercise on an angled platform. (Left, astronaut Scott Carpenter with biosensors attached to his head. 1962.)

In addition to these four tests were three simpler tests: the Harvard Step test, the Cold Pressor test, and the Treadmill test, all of which are easily self-administered. Heart rate is best measured on the radial pulse – the artery that runs down from the thumb on the inside of the wrist. Apply gentle pressure with two fingers and count the rhythmic pulses for 30 seconds. Multiply the result by two to get beats per minute. Blood pressure is best measured with an automatic pressure cuff.

So, do you think your heart has what it takes to go into space according to 1959 astronaut qualification standards?

The Harvard Step Test requires a clock with a second hand, a 19.5-inch high step and a metronome or some other way of measuring pace. Start the clock and begin walking up the step at a rhythmic pace – up the step in one second and down the next and so forth such that 30 steps are completed in one minute. Maintain this pace or as close as possible for five minutes, counting the number of steps completed. At the end of five minutes, restart the clock immediately and begin taking heart rate measurements. Count pulses for thirty seconds from 1 minute to 1 minute 30 seconds, from 2 minutes to 2 minutes 30 seconds, and again from 4 minutes to 4 minutes 30 seconds. (Above, a candidate performs the Harvard Step test. 1959.)

To score the Harvard Step Test, multiple each of the three heart rate measurements by two and add them together. Divide this number by the total number of steps climbed, and multiply this result by 150 to get a final score. The average score of candidates was 52.8 with a standard deviation of 5.3.

Second is the Cold Pressor Test that requires a bucket of ice water chilled to 4 degrees centigrade or 39.2 Fahrenheit and a towel. Sit in a chair and measure blood pressure and heart rate once a minute for three minute. Then, immerse your feet in the bucket of ice water for seven minutes, measuring your blood pressure and heart rate once a minute. Remove your feet – either after the full seven minutes or once the cold become unbearable – and place them on the dry towel. As at the start of the test, measure heart rate and blood pressure once a minute for three minutes.

Scoring the Cold Pressor Test is slightly more complicated. Heart rate, systolic, and diastolic pressure measurements are each taken separately and broken into three categories: resting or pre-test, immersion, and post test. Begin with the heart rate measurements. Average the resting, immersion, and post test pulses. Subtract the average resting pulse from the average immersed pulse to find the value for P1. Next, subtract the average resting heart rate from the average post test pulse to find the value for P2. Add P1 and P2 to get your final pulse score. Do the same thing with the systolic measurements and then the diastolic measurements to get your final systolic and diastolic scores. Add the three values together for your overall Cold Pressor test score; the average score of the candidates was 37 with a standard deviation of 37. (Pictured, a candidate performs the Cold Pressor test. 1959.)

Last is the Treadmill Test. Start by taking three resting heart rate measurements, once a minutes for three minutes. Begin the treadmill in its horizontal position at a speed of 3.4 miles per hour. Every minute for ten minutes, increase the treadmill incline by 0.9 degrees – both the incline and speed are easily controlled by the user on modern treadmills. Throughout the test, measure your heart rate once per minute. After 10 minutes, dismount the treadmill and measure your heart rate once a minutes for three minutes.

Like the Cold Pressor test, the Treadmill test is more complicated to score. The scoring system dismisses all heart rates over 180 beats per minute, an omission that has to do with the age of Mercury applicants. Maximum heart rate is determined by age by simple subtracting age from 220. With a maximum applicant age of 40, the maximum heart rate of some applicants was 180. Discarding heart rate measurements in excess of 180 beats per minutes put all applicants on an even playing field by removing the higher scores only younger applicants could achieve. (Left, an applicant performs the Treadmill test. 1959.)

To determine your final score, divide the number of minutes on the treadmill your heart rate was under 180 beats per minute and divide that by your average heart rate during the test. Multiply that number by 1,000 to get a final score. The Treadmill test yielded an average score among applicants of 75 with a standard deviation of 15.

So, how do you compare with the average fitness level of the Mercury astronaut candidates – is your heart made of the right stuff?

Mercury astronauts: Cooper, Schirra, Shepard, Grissom, Glenn, Slayton, and Carpenter. All roughly the same height and build. 1959.

Suggested Reading/Selected Sources

“Project Mercury Candidate Evaluation Program” Charles L. Wilson ed. Aerospace Medical Laboratory, Arlington Hall Centre, Virginia. 1959.



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