Movement variability can be considered normal variations of motor output across multiple repetitions of a task such as reaching and walking. Bernstein described variability rather well in his famous quote “repetition without repetition”. Historically variability has been regarded as noise in the sample, but recently variability has received more attention, and is now regarded as a resource (van Emmerik & van Wegen, 2002).
There are numerous different measures that can be employed to quantify variability. Many of these measures are based upon rather complex non-linear dynamics (Decker, Moraiti, Stergiou, & Georgoulis, 2011). Furthermore, there are different models in regards to the interpretation of variability. One model is based upon a U-shaped relationship and skill. Initially there is a great deal of variability with low performance that decreases with improving performance. As the skilled is further enhanced variability will increase again. Another theory is the optimal movement variability proposed Stergiou and co-workers (Stergiou, Harbourne, & Cavanaugh, 2006). Their model is based upon variability as an inverted u-shape relationship related to complexity and predictability where a healthy system is based upon the largest possible effective complexity. Optimal movement variability is therefore the intermediate region between excessive order (maximum predictability) and excessive disorder (no predictability) (Stergiou et al., 2006).
The analysis of variability is very interesting as it comes to ACL injuries and what we consequently do in terms of training and rehabilitation. ACL deficient knees have been found to be more rigid and have decreased variability. That is a system that is in a state of decreased complexity and high predictability (Decker et al., 2011). Decker and co-workers hypothesized that this could be one reason why osteoarthritis develops over time. An ACL reconstructed knee has been found to have an increased variability, which is associated with a noisy state that has decreased complexity and low predictability (Decker et al., 2011).
Based upon that variability is controlled by the central nervous system (van Emmerik & van Wegen, 2002) one might want to consider this to a greater extent in injury prevention and rehabilitation. The mechanical model, based upon the mechanical constraints provided by the ACL, might be limited since the neurophysiological function of the ACL is ignored. Injury prevention and rehabilitation that has an even greater focus on proprioception might offer even better results than we currently have. Repeating the same strict movement of the knee at the same rate, such as a slow partial single leg squats with knees of the second toe, rather than challenging rate, position and speed to facilitate proprioception, might not be the optimal solution. Regardless of these perspectives, current models of injury prevention have good results.
In the future it seems that there is more to be gained from integrating different fields of study, such as biomechanics and motor control, to get an even better understanding of how to design optimal injury prevention, rehabilitation and training programs.
Jessica, Ali and Ola
Decker, L. M., Moraiti, C., Stergiou, N., & Georgoulis, A. D. (2011). New insights into anterior cruciate ligament deficiency and reconstruction through the assessment of knee kinematic variability in terms of nonlinear dynamics. Knee surgery, sports traumatology, arthroscopy : official journal of the ESSKA, 19(10), 1620-1633. doi: 10.1007/s00167-011-1484-2
Stergiou, N., Harbourne, R., & Cavanaugh, J. (2006). Optimal movement variability: a new theoretical perspective for neurologic physical therapy. J Neurol Phys Ther, 30(3), 120-129.
van Emmerik, R. E., & van Wegen, E. E. (2002). On the functional aspects of variability in postural control. Exercise and sport sciences reviews, 30(4), 177-183.
The more runners I evaluate and treat the more interested I get in the serratus anterior muscle. How can a tight serratus anterior muscle affect running?
As we all know the serratus anterior muscle originates on the surface of the 1st to 8th ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula (Platzer, 2004). The long thoracic nerve, nerve of Bell, a branch of the brachial plexus, innervates the serratus anterior muscle. Damage to this nerve can lead to a winged scapula.
The function of the serratus anterior muscle is to pull the scapula forward around the thorax, which is essential for anterversion of the arm. Thus, serratus anterior is largely responsible for the protraction of the scapula, which occurs when someone for example throws a punch. When the inferior and superior parts act together, they keep the scapula pressed against the thorax together with the rhomboids. The inferior portion of scapula can also pull the inferior part, medial border to inferior angle, of the scapula laterally and forward. This creates upward rotation that makes elevation of the arm possible. This upward rotation is created in sync with the upper and lower fibers of the trapezius (Levangie & Norkin, 2011). Additionally, all three parts can lift the ribs when the shoulder girdle is fixed, and thus assist in respiration (Platzer, 2004).
I have found that it is very common that this muscle is tight, especially in people with a lot of stress that has shallow breathing. If they on top of that do a lot of heavy overhead presses, or bench press, it is even more common.
Now we come back to the question, how can a tight serratus anterior muscle affect running?
When the scapula is tight and is in a protracted position the runner will have decreased shoulder extension when bringing the arm back and the arm /elbow will move more laterally. Furthermore, it will affect same side thoracic rotation. Can that affect the lower extremity? What I have seen is that after stretching a tight serratus anterior muscle the runner is able to bring the arm further back and help to load the same side hip and making it easier for the runner to re-supinate the same side foot for a more effective push off. By stretching the serratus anterior muscle I have been able to help many people with lower extremity pain from the feet, knees and hips.
This was another example of how integrated and linked the human body is. Also keep in mind that this is written from clinical experience and not research based, but a functional and pain free runner is always a good thing.
Jessica, Ali and Ola
Levangie, P. K., & Norkin, C. C. (2011). Joint Structure and function a comprehensive analysis, 5th Ed. Philadelphia, PA: F. A. Davis Company.
Platzer, W. (2004). Color Atlas of Human Anatomy, Vol 1: Locomotor System (5th ed.): Thieme.
In a study performed in 2008 by Van Dillen and co-workers they found that people who participated in rotatory sports with low back pain had decreased hip rotation and more asymmetrical findings that people without low back pain (Van Dillen, Bloom, Gombatto, & Susco, 2008).
Hip rotation was examined in the prone position with the knee flexed to 90 degrees. We all know that there are many ways of measuring hip rotation with different results. Regardless, they found reduction and asymmetry of hip rotation.
This was not a prospective study, so the correlation is not causation. It can be one of those chicken or the egg arguments. Hip rotation could be reduced secondary to pain, or the reduction could have caused low back pain. The mechanisms and relationships between these two variables can be many, and the discussion is open for what system or mechanism that is responsible for this. Regardless this study shows that there is a connection between hip rotatory mobility and low back pain.
The next time you see a patient with low back pain, hip rotation might be something you want to check, especially if they are participating in rotational sports. That is what Van Dillen and co-workers found.
Jessica, Ali and Ola
Van Dillen, L. R., Bloom, N. J., Gombatto, S. P., & Susco, T. M. (2008). Hip rotation range of motion in people with and without low back pain who participate in rotation-related sports. Phys Ther Sport, 9(2), 72-81. doi: 10.1016/j.ptsp.2008.01.002