Eccentric muscle contraction
Contraction is a process of becoming smaller and tighter under the influence of force. Muscles generally function by contracting to exert a pulling force, but they never push. Muscle actions can conventionally be defined as the movement that takes place when the muscle contracts, but this is, however, an operational definition that equates contraction with shortening, and relaxation with lengthening. In the context of whole muscles and real movements, this conventional definition does not solely apply. Movements that involve shortening of a muscle are referred to as concentric, and examples include the contraction of biceps brachii and the brachialis. Those in which the active muscle is lengthened are called eccentric, while a muscle that contracts without change in muscle length is termed isometric.
Description of eccentric contraction
Several types of muscle contractions have been identified. They include: reflexive contraction which is generally automatic (i.e., crossed extensor reflex); tonic contraction characterized by muscle tone (tonus) and does not produce movement or active resistance; and phasic (active) contraction. Of phasic contractions there are three types: isometric or static, dynamic concentric and dynamic eccentric contractions. Isometric contractions are generated tension without a change in joint/limb position (e.g., holding a dumbbell at 90 degrees), or more specifically without shortening sarcomere length (e.g., as cardiac tissue develops tension prior to ejection phase). Dynamic contractions have previously been known as isotonic, whereby the muscle generates the same tension across a range of movement. Concentric contractions occur as a result of the muscle actively shortening (e.g., the biceps when lifting a dumbbell), whereas eccentric contractions occur as a result of active lengthening (e.g., the biceps when controlling the drop of a dumbbell). Eccentric movements produce the greatest force, even in situations like the raising and lowering of the same dumbbell, and are typically observed functionally in deceleration of movements. Dynamic contractions are not only present in skeletal muscle but also in cardiac and smooth muscle, and indeed in cardiac tissue isometric and isotonic contractions are intimately related to changes in the concentric contraction.
The relationship between the load and the direction of contraction is best explained by the force-velocity curve. The force of a contraction directly relates to the direction of the contractile movement and the velocity at which the muscle contracts. As described by the force-velocity relationship, the tension developed by concentric contractions reduces as the shortening velocity (in response to reducing load) increases; whereas, tension developed by eccentric contractions increases with increasing load and lengthening velocity. Once the load becomes too great for the muscle to develop eccentric tension, a sudden reduction in tension occurs. The reason eccentric contractions develop greater tension is due largely to changes in the crossbridge cycling, whereby more myosin filaments maintain a “strong-binding state”. As myosin filaments remain bound there is more time for force to develop through neuromodulatory pathways, such as recruitment of additional motor units and increased frequency of the motor units involved. If the force becomes too great the myosin filaments can be torn from the active binding site on the actin filament. The force required to tear the crossbridges from their binding sites is greater than that produced during normal crossbridge cycling, but also poses a great risk for muscle damage. This greater force production during eccentric contractions was first observed by Adolf Fick (1882). Despite the greater force production observed in eccentric contractions, the muscle activation, energy consumption, and oxygen consumption are lower than during concentric contractions at any given force.
In life, movements are comprised of concentric, isometric and eccentric contractions, both in isolation and as stabiliser or antagonistic functions. As illustrated above with the biceps brachii example, many movements consist of an eccentric lengthening phase following a concentric contraction. Similarly, more complex movements such as the gait cycle contain eccentric components when the foot experiences heel strike the knee is seen to briefly change from extension to flexion and back, as the loading on the quadriceps muscle group forces a period of eccentric contraction. This is exacerbated during running and furthermore during downhill running. Thus eccentric contractions are as important as concentric contractions and help in coordinated movements such as running, walking, and sitting down. Furthermore, eccentric contractions are arguably as common in daily life as concentric contractions, as although concentric contractions and eccentric contractions can occur in isolated situations, as the aforementioned examples suggest, they also occur in relation to one another in order to slow, and control movements.