Muscle actions: Isotonic Vs. Isometric

The process of muscle contraction (muscle action) is central to all fitness, strength and sports training, yet there are many aspects of this apparently well-known phenomenon that are not adequately understood even at a more popular level.

We are most interested in muscles when they are in a state of contraction. Muscle contractions all result in an increase in tension or force within the muscle, but some contractions move body parts while others do not.

All muscular contraction can be classified as being either isotonic or isometric. Isotonic contraction can occur in two ways, concentric contraction and eccentric contraction. In other words, there are two components of an isotonic movement: the eccentric phase and the concentric phase.

This article will try to explain the three types of muscular contraction in relation to the biceps brachii muscle (dumbell curl exercise). In our second example we’ll illustrate muscle actions using the barbell squat exercise.

Isotonic contraction

Also called isotonic exercise, dynamic exercise involves a muscle contraction with a change in the length of the muscle. It is a dynamic contraction because the joint will start moving as soon as we produce the tension in the muscle. Dynamic exercises are the most popular type of exercises for increasing muscle strength and seem to be most valuable for developing strength that can be transferred to other forms of physical activity. You can easily perform dynamic exercises with weight machines, free weights, or a person’s own body weight (as in curl-ups or push-ups).

As you can see from the image, there are two kinds of isotonic (dynamic) muscle contractions:

• concentric muscle contraction
• eccentric muscle contraction

Concentric muscle action (concentric contraction)

Concentric contraction occurs when a muscle shortens in length and develops tension. Most of the movements that we evaluate in sport involve contractions of this type. Examples of concentric actions include the upward phase of a biceps curl and the upward phase of a bench press.

• Concentric contraction in the biceps brachii during upward phase of exercise.
• The biceps brachii produces tension and shortens.
• Moving in opposite direction of force.
• It pulls the forearm upwards to cause flexion of the elbow. It causes joint movement.

A concentric muscle action occurs when the contractile force is greater than the resistive force, resulting in shortening of the muscle and visible joint movement.

Eccentric muscle action (eccentric contraction)

Eccentric muscle contractions are the reverse of a concentric actions. This type of contraction involves the development of tension whilst the muscle is being lengthened. When a weight is lowered in a controlled manner (i.e., the resistance is greater than the force that the muscle is producing), the muscles involved are lengthening while producing force. To control the weight as it returns to the starting position, the muscles must lengthen in a controlled manner or the weight will fall.

This contraction is taking place in our biceps as we lower ourselves while performing chin-ups. Plyometric exercises are an example of eccentric contractions.

• Eccentric contraction occurs in the biceps brachii during the downward phase of the exercise.
• The biceps brachii produces tension and lengthens.
• Moving in the same direction as the resistance.
• It slows the lowering of the forearm and controls extension of the elbow. It controls joint movement.

An eccentric muscle action occurs when a muscle develops tension while lengthening. The muscle lengthens because the contractile force is less than the resistive force.

Isometric or static muscle action (contraction)

Isometric contraction occurs when the muscle develops tension but does not change in length. This can occur when a weight is held stationary or the weight is too heavy to lift any higher. In other words, the force produced by the muscle equals the resistance. Some muscles, such as the postural muscles of the spine, primarily perform isometric muscle actions; they act to stabilize the upper body during most lifts.

If we were to examine this contraction in microscopic detail, we would see that in the myofibril, the myosin filaments connect to the actin filaments.

For example, an isometric contraction develops during a biceps curl if you cannot continue the movement
beyond the mid-point – the tension in your biceps equals the resistance of the barbell. Also, activities which involve this type of contraction include arm-wrestling, tug-of-war and a rugby scrum (where there is no noticeable movement).

• Isometric contraction occurs in the biceps brachii when the muscle is holding the weight still.
• The biceps brachii develops tension and stays the same length.
• Dynamically stabilizes force.
• It stops flexion and extension of the elbow. It stops joint movement.

When a muscle is exerting force equal to the force being placed on it leading to no visible change in the muscle length.

Example of Muscle Actions

Another great example to help illustrate muscle actions is the squat exercise. To initiate the squat from a standing position the individual squats down, flexing at the hips, knees, and ankles. This is an example of an eccentric muscle action. The individual is in the “lowering” phase of a resistance exercise. Moreover, as the individual squats downward, the gluteal muscles and quadriceps mechanically lengthen while simultaneously decelerating the force of their body weight and gravity.

The isometric muscle action occurs when the individual pauses at the bottom position and no joint motion is visible.

And lastly, the concentric muscle action occurs when the individual returns to the starting position (lifting phase), contracting the gluteal muscles and quadriceps.

Summing up

The term muscle contraction can be confusing as it suggests the muscle is always shortening BUT in some types of contraction the muscle will stay the same length or even get longer, while producing tension. The use of concentric, eccentric, and isometric muscle actions in strength training will yield somewhat different adaptations. Although isometric muscle actions can improve strength and muscle size to some degree, they provide mainly static strength. Therefore, most strength training programs should focus on concentric and eccentric muscle actions. Greater improvements in strength and muscle mass can be achieved when repetitions include both concentric and eccentric muscle actions.