When we think of a muscle contracting normally, we tend to think of the muscle shortening as it generates force. While it's true that this is a way of muscle contracting, there are many different ways that a muscle can generate force, as seen in Figure 1 below.
|Figure 1: A demonstration of the difference in force responses for between lengthening and non-lengthening active contractions (isometric vs. eccentric), and between active lengthening (eccentric) vs. non-active lengthening (passive stretch).
When a muscle is activated and required to lift a load which is less than the maximum tetanic tension it can generate, the muscle begins to shorten. Contractions that permit the muscle to shorten are referred to as concentric contractions. An example of a concentric contraction in the raising of a weight during a bicep curl.
In concentric contractions, the force generated by the muscle is always less than the muscle's maximum (Po). As the load the muscle is required to lift decreases, contraction velocity increases. This occurs until the muscle finally reaches its maximum contraction velocity, Vmax. By performing a series of constant velocity shortening contractions, a force-velocity relationship can be determined.
During normal activity, muscles are often active while they are lengthening. Classic examples of this are walking, when the quadriceps (knee extensors) are active just after heel strike while the knee flexes, or setting an object down gently (the arm flexors must be active to control the fall of the object).
As the load on the muscle increases, it finally reaches a point where the external force on the muscle is greater than the force that the muscle can generate. Thus even though the muscle may be fully activated, it is forced to lengthen due to the high external load. This is referred to as an eccentric contraction (please remember that contraction in this context does not necessarily imply shortening). There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved are very high relative to the muscle's maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift). Second, the absolute tension is relatively independent of lengthening velocity. This suggests that skeletal muscles are very resistant to lengthening. The basic mechanics of eccentric contractions are still a source of debate since the cross-bridge theory that so nicely describes concentric contractions is not as successful in describing eccentric contractions.
Eccentric contractions are currently a very popular area of study for three main reasons: First, much of a muscle's normal activity occurs while it is actively lengthening, so that eccentric contractions are physiologically common (Goslow et al. 1973; Hoffer et al. 1989) Second, muscle injury and soreness are selectively associated with eccentric contraction (Figure 2, Fridén et al. 1984; Evans et al. 1985; Fridén and Lieber, 1992). Finally, muscle strengthening may be greatest using exercises that involve eccentric contractions. Therefore, there are some very fundamental structure-function questions that can be addressed using the eccentric contraction model and eccentric contractions have very important applications therapeutically to strengthen muscle.
|Figure 2: Plot demonstrating maximal tetanic force prior to and immediately following an exercise bout. While passive stretch causes negligible force decrement, isometric causes a moderate loss and eccentric causes a significant loss of force.
The Virtual Hospital has a more clinical look at this and other forms of muscle injury.
A third type of muscle contraction, isometric contraction, is one in which the muscle is activated, but instead of being allowed to lengthen or shorten, it is held at a constant length. An example of an isometric contraction would be carrying an object in front of you. The weight of the object would be pulling downward, but your hands and arms would be opposing the motion with equal force going upwards. Since your arms are neither raising or lowering, your biceps will be isometrically contracting.
The force generated during an isometric contraction is wholly dependant on the length of the muscle while contracting. Maximal isometric tension (Po) is produced at the muscle's optimum length, where the length of the muscle's sarcomeres are on the plateau of the length-tension curve.
|Figure 3: A series of isometric contractions performed at varying muscle lengths (from -40% (slack) to +40% (stretched). The maximum force is produced at optimum length (Lo). Note that as the muscle is stretched, the baseline of the force record is raised due to passive tension (PT) in the muscle and contributes more to overall force than the active tension (AT).
There is a fourth type of muscle "contraction" known as passive stretch. As the name implies, the muscle is being lengthened while in a passive state (i.e. not being stimulated to contract). An example of this would be the pull one feels in their hamstrings while touching their toes.
The structure(s) responsible for passive tension are outside of the cross-bridge itself since muscle activation is not required. Several recent studies have shed light on what has turned out to be a fascinating and huge protein with skeletal muscle—aptly named, “titin.” A seminal study performed by Magid and Law, demonstrated convincingly that the origin of passive muscle tension is actually within the myofibrils themselves. This is extremely significant because, prior to this study, most had assumed that extracellular connective tissue in striated muscle caused the majority of its passive properties. However, Magid and Law measured passive tension in whole muscle, single fibers and single fibers with membranes removed and showed that each relationship scaled to the size of the specimen. In other words, the source for passive force bearing in muscle was within the normal myofibrillar structure, not extracellular as had previously been supposed.