Lab Report 365 – 02/02/16


🎓 📝 LAB REPORT-365 📝 🎓
#33 Mechanical Analysis of the Bench Press. Which technique do you use?

According to physics, the shortest distance between two places is a straight line. The reasoning behind many coaches emphasising a straight bar path when lifting, is to reduce to amount of work performed during a lift. This is often referred to as “efficient lifting”. This poses the question, why do Powerlifters have so many techniques for bench pressing? In specifics: a straight press vs. a press back. This comes back to the point of reducing total work or increasing efficiency.

Firstly we need to understand what is work. The foundation formula is: Work = Force x Distance

Let’s apply this to a 100kg bench press. We need to find out what the Force out put is. To find out this number we apply another formula: Force = Gravity x Mass

A 100kg Bench Press is 9.8m/s x 100kg = 980 Newton’s

If we compare a Press Back and a Straight Press the total distance between the start position and the finish position is the same.

If we now know that the bar has travelled 0.25m. This means the Work completed is 980 Newton’s x 0.25m = 245 Joules

So far this doesn’t demonstrate any advantage to either technique. However, we now need to consider two things: Newton’s First law of Motion An object in motion tends to remain in motion, and an object at rest tends to remain at rest, and that there are horizontal forces (Vectors) acting on the bar. Think about Bent Over Rows vs a Pendulum Rows. A pendulum row seems easier to move due to being able to “swing” the bar towards you. A similar principle applies to the straight press and press back techniques.

However, if we add other techniques such as:

• Arching
• Elbow Tuck/Flare
• Scapula Retraction/Depression

We can shorten the distance travelled. However, if we lengthened the distance travelled through a horizontal plane of motion i.e. the press back technique/creating a longer bar path, could we create a mechanical advantage? If conventional knowledge is applied (shortest distance between to places is a straight line), the answer is no. However, if use a press back technique, you move the bar in both a horizontal and vertical plane, with gravity only acting in the vertical plane. This means you are essentially using the fourth basic machine, the inclined plane. See the photo below. The press back creates a longer lever and actually reduces the force required to lift the bar. Think of it like using the back of a hammer to pull a nail out. It is easier to pull the nail out by pushing on the end of the handle rather than closer to the head of the hammer.

So if we apply the formula of the fourth basic machine, we can find out the required force. 100kg x 9.8m/s Sin(45) -0 = 692.5 Newton’s

This means the Work is reduced as well:

692.5 Newton’s x 0.31m = 215 Joules (Using trigonometry we can figure out the distance travelled i.e. 0.31m)

If we perform a straight press with a shortened distance travelled/Range of motion, the amount of work is the same.

980 Newton’s x 0.22m = 215 Joules

But, the force output required to move the bar is different. A Straight Press needs 980 Newton’s but the Press Back needs 692.5 Newton’s.

Now these formulas are based on estimates, however the point of this section is to make you understand that there is more than one way to bench and each bench press technique must be individualized through specialized coaching and experts. To maximize bench press weight must take into account correct physics, individual anatomy, and biomechanics of the shoulder, not dogma from a powerlifting/fitness community aka “Bro-science”.

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