One topic that often confuses students is muscle and joint action during exercise. In textbooks and manuals, the terms concentric and eccentric are typically explained in isolation, without applying this information to a broader context. While this approach may help students memorise definitions for exams, it does not necessarily aid their understanding or ability to apply the concepts to exercise scenarios.

I have found that discussing a few simple examples can significantly improve comprehension. Here, I will keep the explanation straightforward, introducing only a few factors and limiting the nuances of how exercises can be performed. Additional details can always be added later.

Dumbbell biceps curl

Step 1: Break down the phases of the movement

In this step, we identify the muscle actions during the upward and downward phases of the movement.

• Concentric muscle action:

A concentric contraction occurs when a muscle generates force to overcome a load, causing the muscle to shorten. This is often referred to as “positive work” because the muscle actively pulls against resistance.

Gravity is a force that pulls objects down towards the ground. Whenever you lift a weight against gravity, your muscles generate force to overcome it and are therefore working concentrically (shortening). For example, lifting your body off the floor during a press-up, standing up during a deadlift, or pressing a kettlebell overhead are all examples of concentric muscle action. In these cases, the working muscles are shortening to overcome gravity.

If using a resistance machine, observe when the weight stack moves upwards during the exercise (e.g., in a lat pulldown, the weight stack rises when you pull the bar down).

• Eccentric muscle action:

In contrast, an eccentric contraction occurs when a muscle lengthens while still under tension, typically as it controls the descent of a load. This is often referred to as “negative work” because the muscle resists the force rather than actively shortening.

It is important to note that the same muscles are working during both the upward and downward phases. When moving in the same direction as gravity (i.e., downwards), the muscles lengthen under tension to act as a brake, controlling the descent back to the starting position.

For example, lowering your body to the floor during a press-up, lowering a barbell to the floor during a deadlift (rather than dropping it), or lowering a kettlebell to a rack position are all examples of eccentric muscle action. Similarly, when the weight stack lowers on a resistance machine, the working muscles are contracting eccentrically.

For the dumbbell biceps curl:

• The concentric phase occurs when you lift the weight towards your shoulder.
• The eccentric phase occurs when you lower the weight back down, controlling its descent.

Step 2: Analyse joint actions

The next step is to identify the joints involved in the exercise and the joint actions occurring during the concentric phase.

For the biceps curl, the primary joint involved is the elbow joint. During the concentric phase (lifting the dumbbell), the elbow bends, and the angle between the upper arm and forearm decreases. This movement is called flexion, which refers to a decrease in the angle between two bones at a joint. Therefore, during the concentric (upward) phase of a biceps curl, elbow flexion occurs.

Step 3: Determine muscle action

Now that we know the joint action during the concentric phase, we can identify the prime muscles (agonists) involved in the exercise.

In the biceps curl, the biceps brachii is the prime mover (agonist) responsible for elbow flexion.

• During the concentric phase, the biceps brachii contracts concentrically (shortens) to flex the elbow, bringing the dumbbell towards the shoulder.
• During the eccentric phase, the biceps brachii contracts eccentrically (lengthens) to lower the dumbbell back down under control.

Dumbbell triceps kickback

This exercise involves a similar movement at the elbow joint but results in a very different analysis, as explained below.

Step 1: Break down the phases of the movement

In the biceps curl, the elbow is positioned underneath the shoulder. However, in the triceps kickback, the elbow is pulled back behind the shoulder, pointing backwards. One phase involves straightening the arm to lift the dumbbell behind you, while the other phase involves bending the elbow to bring the dumbbell back to the side of your body.

• The concentric phase occurs when you straighten your arm behind you, lifting the dumbbell against gravity.
• The eccentric phase occurs when you bend the elbow, bringing the dumbbell back down to your side.

Step 2: Analyse joint actions

The primary joint involved is again the elbow joint. As you straighten the arm, the angle between the upper arm and forearm increases. This movement is called extension, which refers to an increase in the angle between two bones at a joint. Therefore, during the concentric (upward) phase of the triceps kickback, elbow extension occurs.

Step 3: Determine muscle action

Which muscles are responsible for elbow extension? The triceps brachii is the prime mover (agonist) for this action.

• During the concentric phase, the triceps brachii contracts concentrically (shortens) to extend the elbow, raising the dumbbell behind you.
• During the eccentric phase, the triceps brachii contracts eccentrically (lengthens) to lower the dumbbell back to your side.

This simplified process illustrates a couple of key theoretical concepts that are often misunderstood. While there are additional layers of analysis and nuances to consider, this explanation provides a solid foundation. Hopefully, it has achieved its intended purpose of clarifying the basics of muscle action during exercise.