Question

Based on your understanding of muscle force production, describe why the hardest part of a pull up is generally the first part when trying to pull up from the hanging position towards the bar but becomes easier as you get closer to the bar?

Answer 1

The muscle contractions are required for every movement and these contractions are made by the muscle fibers through the actin and myosin cross bridge cycling. This forms tension in muscle fibers that in turn lengthens or shortens the muscle to create movement. There are different types of muscle contractions. They are isotonic and isometric contractions. The isotonic contractions are divided to concentric and eccentric contraction. Pull up exercise is an example to show concentric and eccentric contractions. In pull ups a bar is hanged and a person starts in hanging position and pulls the body up in vertical direction towards the bar, holds the position and then lowers downwards to the starting position. When the person is pulling up vertically towards the bar concentric contractions occurs. In this type of contraction the muscle shortens because cross bridges on myosin filament attaches to active actin filament and muscle shortens about 1% in a single cycle and the contraction cycle repeats to shorten the muscle more. So to pull up towards the bar more contractive force is required. This occurs throughout the muscle length generating force at origin and thus shortening. It uses one molecule of Adenosine triphosphate (ATP) for each cycle, thus requires more energy. While reaching closer to the bar the isometric contractions take charge of muscle function. Here force is generated without changing the length of muscle. So less energy is required. When the person lowers from the hanging position an eccentric contraction occurs. It is occurred when muscle lengthens. So here resistance is greater than the generated force. During these contractions there is decreased rate of cross bridge detachments and thus less energy is required than a concentric contraction. This is the reason why the hardest part of a pull up is generally the first part when trying to pull up from the hanging position towards the bar but becomes easier as you get closer to the bar.