Eccentric muscle contraction

Mechanism

Let’s describe the mechanism using the biceps brachii muscle (forearm flexor) as an example. During concentric contraction, the biceps shortens and pulls the weight towards the shoulder joint. 

Two situations can lead to an eccentric movement from this point;

• The biceps is loaded with a force greater than the one it produced during concentric contraction (e.g. more weight added to the dumbbell).
• You intentionally start relaxing your biceps.

In both situations, the force produced by the muscle is insufficient to hold the biceps brachii in a fully contracted state. This will cause the muscle fibers to forcefully lengthen, which is called eccentric contraction.

The word “contraction” might be confusing because the biceps brachii lengthens, so what’s actually happening? During an eccentric contraction the muscle is trying to shorten by generating tension, but it’s in fact, lengthening. That’s because the external force applied on the muscle is overpowering the force produced by the concentric contraction. Eccentric contraction isn’t a simple passive stretching of the muscle, but rather stretching under tension intended to decelerate and smooth out the repositioning of the heavy load.

Why is eccentric contraction stronger than concentric?

Let’s quickly recall how skeletal muscle contracts. The contractile apparatus of the muscle is the sarcomere, a distinct feature of striated musculature. A sarcomere consists of two types of myofibrils called actin and myosin, which are the key elements of the contractile apparatus. Several accessory proteins support the function and integrity of the sarcomere including titin, tropomodulin, alpha-actinin, myomesin, dystrophin, nebulin and desmin, among others.

The concentric muscle contraction, or concentric phase, is described by the cross-bridge (sliding filament) theory. According to this theory, the myosin heads cyclically attach to specific sites on actin filaments, forming cross-bridges. Each cross-bridging cycle consumes one molecule of adenosine triphosphate, or ATP (energy). Once attached, the myosin heads pull the actin filaments and cause the myofibrils to slide past each other. This process shortens the sarcomeres and hence, the entire muscle. 

The number of cross bridges is directly proportional to the number of actin filaments and myofibrils that shorten. Therefore, more cross bridges will cause the muscle to produce a greater force. As you can see, a strong and efficient concentric muscle contraction will require a great deal of energy investment.

In contrast, an eccentric action (i.e. eccentric phase) produces greater forces at lower costs, meaning that muscles are stronger while using less energy. Even after decades of research, the mechanism for this property is still not completely understood. Several explanations are currently accepted and studied: 

• The accessory protein titin is responsible for the greater force. Titin is an elastic molecule found in sarcomeres which attaches to the actin filament and coils like a spring as the sarcomere shortens in the concentric phase. When the sarcomeres elongate in the eccentric phase, titin resists and opposes the stretch like a spring uncoiling. This resistance produces extra force without additional ATP (energy) consumption  because potential energy has been stored as elastic recoil during the concentric phase. 
• More cross bridges are formed during eccentric than concentric contractions , increasing the generated force.
• Instead of using one ATP per each cross-bridging cycle, eccentric contraction utilizes one ATP molecule for several cycles. As a result, the overall energy consumption is lower. 

To recall the details about the sarcomere, myofibrils and everything related to the cross-bridging theory, check out our resources:

Test your knowledge on skeletal muscle with this quiz.

Examples

Put simply, eccentric contraction happens whenever you activate a muscle to resist, smooth out and control a movement happening in the opposite direction. For example, when you use your flexors to control an extension, and vice versa. Here are some examples of everyday eccentric contraction:

• Tibialis anterior muscle is a major anterior leg muscle that dorsiflexes the foot. It eccentrically contracts during walking and running in order to smooth out the foot drop after a heel strike.
• Quadriceps femoris muscle is the thigh muscle that extends the leg by acting on the knee joint. However, it eccentrically contracts while walking down a hill or stairs to smooth out the flexion and prevent the knee joint from collapsing.

Learn about the functions and contractions of the tibialis anterior and other muscles of the lower limb with our 3D video tutorials.

Eccentric exercises are common in athletes. One popular example is the biceps curl; lifting the dumbbell is due to concentric contraction of the biceps brachii muscle, while lowering the dumbbell is the result of eccentric contraction. In alpine skiing, eccentric contraction of quadriceps femoris is important for a turning cycle. In tennis, the superficial and deep forearm extensors exhibit eccentric contraction in every racquet swing to protect the wrist joint from excessive palmar flexion.