Question

Based on Flanagan (2019). Your athlete is performing a dumbbell biceps curl. During the concentric phase of this exercise, at a constant speed, the athlete realizes that the exercise is easy at the beginning (elbow fully extended), more difficult in the middle (at 90 degrees of elbow flexion) and easy again at the end (at 135 degrees of elbow flexion). Using concepts of biomechanics, explain why the athlete feels differences in external resistance during the concentric phase. If the client performed the same exercise with more speed, how would the ANGULAR MOMENTUM be affected?

Answer 1

There are three things in this bicep curl system.

1. Leverage and moment arm length (external forces - biomechanics)
2. Muscle tension length relationships (internal forces - biomechanics)
3. Strength - Neuromuscular control at various range of motions (neuromusclar system)

Leverage and moment arm

This is a biomechanics concept that explains how external forces affect the movement.

Lever systems vary based on the position of the force applies, the resistance applied and the fulcrum (point of rotation). Each lever system offers a mechanical advantage or a mechanical disadvantage -called leverage which acts like a modifier .

Your leverage is affected by the moment arm length - which is the distance between a force and it’s axis of rotation(fulcrum - or in this case the elbow).

The longer the moment arm of the resistance(the weight in hand) is compared to the moment arm of the force applied (force applied by the muscle) the harder the exercise will be.

In the case of the bicep curl - the farther the weight is from the elbow the harder the muscle will have to work.

Lever of Human Body - here’s a good article to help explain this concept

Muscle tension length relationships (internal forces - biomechanics)

This concept looks at the internal forces created by the muscle.

“The length-tension relationship is the observation that the isometric force exerted by a muscle is dependent upon its length when tested.

The length-tension relationship can likely be explained by interactions between two underlying mechanisms: the active and passive length-tension relationships.

The active length-tension relationship reflects the degree of overlap between the actin and myosin filaments in a sarcomere. Too much or too little overlap leads to sub-optimal tension being developed but where the overlap is “just right” maximal tension is developed.

The passive length-tension relationship reflects the presence of elastic elements within a sarcomere, which stretch and produce force with increasing length.”

In your bicep curl example - the resting length will be at 90 degrees. This is where your biceps will be able to produce the most muscular force - its also happens to be where the leverage is worst

**Angular momentum will increase with increase in speed