Levers and muscular actions
Updated: Feb 10, 2019
After the summer gyms and sports centres are re-filled with customers, and at this time of year, many athletes ask me the difference between the various muscular exercises that can be done with weights. Thus, this short article is designed for them; I will try to briefly explain the joint levers and the three main types of muscle work.
Usually, when it comes to overloads training, we talk about three different muscular actions: concentric, eccentric and isometric.
Before describing them, it is good to remember that the skeletal muscles act on the bones, exerting on them forces that produce movements through specific points called articulations (or joints). In the biomechanics (we recall here that biomechanics is that area of biology that uses classical mechanics to analyse biology and medicine related to movement) the musculoskeletal system is described as a system of levers.
A lever is a rigid (or semi-rigid) body pivoted on a fixed point (fulcrum) and used to transmit forces, as in raising or moving a weight at one end by pushing down on the other (Flanagan et al. 2013). Therefore, there is a mechanical advantage when the ratio between the arm of the applied force (effort) and the arm of the resistant force (load) is higher than one.
Understanding these aspects is essential because if you do not take into account the relationship between the length between the load and the fulcrum you may injure your self and others in the gym. For instance, by unloading a barbell, if it is not correctly placed on the supports. Based on the arrangement of the fulcrums and the relationship between the load and the effort, the levers can be divided into three classes (Barba et al. 2007).
1. The first class levers are those in which the applied effort and the loading effort act on opposite sides of the fulcrum (Figure 1). An example of this lever appears during a nodding action of the head, in which the fulcrum is the point where the skull connects with the vertebral columns, the load is the weight of the head, and the effort is the neck muscles at the back of our skull.
2. The second class levers are those in which the applied force and the load act on the same side of the fulcrum with the applied effort acting through a longer distance, than the load, from the fulcrum (Figure 2). A simple example in our body is given during the plantarflexion that allows us to lift an extremely high load because the action of the lever is advantageous. By practicing a calf exercise on the standing press, you can understand this example.
3. The levers of the third class are those in which the applied force and the load act on the same side of the fulcrum but the applied effort acts through a shorter arm than the load (Figure 3). An example happens in the forearm flexion during an arm curl movement, in which the lever is known as disadvantageous
Without going into more details of the levers and principles that govern classical mechanics, we can now better understand the muscular actions that, as said at the beginning, are concentric, eccentric and isometric.
The concentric muscle action is a situation in which the muscle is shortened, being the force of muscular contraction superior to the load. Examples of this type of contraction can be found in swimming, cycling or during an arm curl when we carry the weight upwards. To simplify the concept, we can say that concentric muscles action is those that allow moving objects even if this definition is only wholly correct.
The eccentric muscle action is an action in which the muscle stretches since the contractile force is lower than the load. This type of contraction occurs in every braking exercise with overloads. It is often described as that movement in which we can not overcome the resistance so that we can not move the object but only slow down its movement. In some types of advanced training, we use the eccentric action, such as in the assisted squat or bench press, due to its training ability but this action can be found in all the braking movements, including during an arm curl when we move the weight away from the body and try to slow down its fall down.
The isometric muscle action occurs when the muscles length does not change since the contractile force is equal to the load. A typical example of this muscular action can be found in a plank exercise or during an isometric squat. It can be presented in all those situations in which we seek to move an object that opposes an equal and opposite resistance, so the resultant of the forces, equal to zero, will not produce acceleration and therefore displacement (Weineck et al 2009).
Each of these types of muscle actions have different effects on the on the exercise outcomes, keeping in mind the concept of specificity, the knowledge of these three systems and the characteristics of the levers are the foundations of to create a good workout with weights.
S. Flanagan – Biomechanics: A Case-Based Approach. Jones & Bartlett Learning, Ed. 2013.
F. Barba – L’allenamento. Teoria & Metodologia. Idelson Gnocchi ,Ed. 2007.
J. Weineck – L’allenamento ottimale. Calzetti Mariucci, Ed. 2009.