Hello and welcome! This is Dr. Alexandra Kopelevich and today we will examine a webinar on axial skeleton, muscle and joint interaction. The main function of the muscles of the axial skeleton is to control posture during various perturbation such as walking or whenever we move our head right to left to stabilize the axial skeleton whenever we move our upper extremities or lower extremities, to protect vital organs and neural structures, to generate intrathoracic and intra-abdominal pressure, which is something we will examine today as well, and of course, finally, to produce torque, to generate movement in a potential plane.
Let's remind ourselves about the production of internal torque. Torque potential is equal to the product of muscle force generated parallel to a given plane times the length of the internal moment arm available to the muscle. So let's take a look at some of these examples of the axial muscles and the way they actually move the body. Let's start with flexion and extension.
The axis of rotation is shown to be Right medial to lateral through the intervertebral body. Here were demonstrated two different muscles, longissimus and rectus abdominis. To generate torque, muscle has to be able to create force, but also muscle times the moment arm or the distance. How far does the muscle sit from the axis of rotation?
So what we know for flexion and extension, is that rectus abdominis sits very far from the axis of rotation. That creates a very long level arm and allows rectus abdominis to produce, to generate torque easier than longissimus. Take a look at longissimus sitting very close to the axis of rotation. In order to produce torque, longissimus has to create a higher force, therefore work harder.
Let's take a look at the frontal plane. Here we're looking at lateral flexion. We have two muscles demonstrated in this illustration, obliquus externus abdominis and iliocostalis. Both of these muscles are sitting laterally to the axis of rotation, once again, which is right through the intervertebral body. Because they're sitting laterally, they're able to generate lateral flexion torque.
The lower arm of iliocostalis is shorter than the level arm of obliques. Therefore, iliocostalis will have to generate more force at a shorter distance from the axis of rotation of the joint. Finally, let's take a look at the horizontal plane. Here, we're looking at two muscles, multifidine and external obliques.
Both of these muscles generate force in a horizontal direction, which will create rotation around the axis of rotation of the joint. Once again, pay attention to the lower arm. Multifidus sits pretty close to the axis of rotation of the joint, as opposed to oblique externus abdominis, which sits far from the axis of the joint.
This will allow external oblique to generate more torque. If the force... of external oblique is equal to the force of multifidi throughout this powerpoint we will examine the magnitudes of the force imposed on the lower back during lifting of course we want to make biomechanics as functional as possible therefore we will be examining lifting as one of the functional activities. The research helps clinicians and government agencies develop safety guidelines and limits for lifting, especially in the workplace.
National Institute of Occupational Safety and Health has set guidelines to protect workers from excessive loads on the lumbar region. The recommended upper safe limit is known right now as 3,400 newtons. which roughly equals to 764 pounds of compression force on the L5-S1 junction.
Some variables that we examine during the force imposed on the lower back will be muscle peak force or the torque, ligamentous tension, and compression and shear forces against intervertebral disc and Z-joints. These forces can be measured by video or computer-based modeling. In this webinar, we will examine a simple but slightly less accurate method of estimating forces it is based on the assumption of static equilibrium we will examine an example of compression force on the l2 vertebrae while lifting an object in the sagittal plane it's a two-step process so let's take a look at this demo so we have a model lifting a bucket Let's take a look at some variables we will be examining for calculations of the torque produced. So the first variable, we see this long blue line, which is defined as the muscle force, the muscle force of a rectospiny. As you know, in order to lift an object, you need to use your extensor muscles to bring it up.
Another force we'll be looking at is body weight. We need to account that body weight creates pressure on the L2 segment. And finally, the last force that we will be looking at is the external load, and that is the weight of the bucket.
Remember that in order to calculate torque, you need to calculate the lever arm from the axis of rotation of the joint. Each force will possess its own lever arm. D1 stands for the lever arm of the muscle force. D2 is the lever arm of the body weight. And finally, D3 is the lever arm of the external load, the bucket.
Internal moment arm, D1, is known here to be 5 centimeters. So this is a distance from the axis of rotation of the joint to the point of insertion of the external to the extensor muscle. musculature. Total body weight of this person is 180 pounds, which equals to 800 newtons.
What we need to calculate is how much body weight is above L2 vertebrae, which is about 65% of the total body weight. If you calculate 65% of the total body weight, that equates to 520 newtons external moment arm is used by body weight so d2 in this case this distance right here is known to be 13 centimeters finally the external load is 25 of the body weight about 45 pounds or 200 newtons so this is the weight of the bucket and finally External moment arm. How far is the bucket located from the axis of rotation of the joint? 29 centimeters. This distance starts from axis of rotation to the point of insertion of the bucket, which is delineated by D3.
For calculation purposes, the centimeters will have to be converted to meters, of course, as a standard unit. So we will be using newtons and meters. Okay, step one in this process is to estimate muscular force.
What is the muscular force of extensor musculature to lift this bucket up? We will assume that the sum of all forces is equal to zero, meaning that the internal torque equals to the external torque produced. The work of our muscles will equal to the work of the external torque, which is the body weight, and the external load, which is the bucket in this case.
So in order to calculate torque, once again, going back to the equation, we know that we have to multiply the force times the lever arm. And we're going to do it for each individual component. We will do it for the muscular force.
Muscular force times D1, so the arachnospiny force times the distance. of the lava arm. This is our internal torque. Our external torque is the body weight times D2, which is the distance from the axis of rotation to the point of application of body weight, plus external load force times the point of application of the external load, which is D3 in this case. When you plug in the numbers given in the slide above, you will get the following equation.
Therefore, muscular force equals to 125.6 newtons times meters over 0.05 meters. Muscular force will equal to 2,512 newtons. which is about 565 pounds. Now, this is not the end, because just now we calculated the muscular force exerted. We didn't calculate the pressure on the L2 segments.
We just calculated the force exerted by erectospinae muscle group. So now let's take a look at the estimate compression reaction force on L2 segments. Compression reaction force implies that L2 vertebrae must push back against the other downward acting forces.
So let's take a look. We have erectospinae pushing down towards the floor. We have the body weight pushing down on L2 towards the floor. And finally, we have the external load being pulled by gravity towards the floor.
All of these forces have to be counteracted by L2. Newton's law number three states that for every action there is equal and opposite reaction and this is exactly what we're calculating here. The reaction force of L2 vertebrae to the forces acting upon it will be assumed we will assume in this equation that all forces equal to zero.
So reaction force will equal to the muscular force plus the body weight plus the external load and of course when i refer to these we are referring to newtons of force now we calculated all the forces here some forces were given some forces the muscular force we had to find so we take the muscular force plus the body weight plus the external load force and this will equal to the reaction force directed in the upward direction delineated by this red arrow up towards l2 this equals about 726 pounds Now, the calculations we examined showed that muscle force is the most influential variable for determining the magnitude of compressive reaction force on the lumbar spine. Proportional reduction in the muscle force has the greatest effect on reduction of the overall compression force on the structures of the lower back, just like we examined the compression force on L2. Important factor here is the disparity in the length of the associated internal and external moment arms. The internal moment arm is assumed to be 5 cm, once again delineated by D2, D1, which puts the extensors at a greater mechanical disadvantage as opposed to the bucket or the body weight. Let's examine some techniques that can help our patients reduce the likelihood of injury.
I've listed for you four factors that can help patients lift the load without injuring their back. You have more techniques listed in your text, so please refer back to your textbook. The first technique that your patient can implement is reducing lifting velocity.
By reducing the rate of lifting, it reduces the lifting velocity and decreases the amount of back extensor muscle force. Second one, which seems to be more as a common sense, is reduction of weight of the external load. Even though it seems as a common sense, in some work scenarios, it is impossible to perform for patients.
Therefore, we have to go to variable number three or variable number four. Variable number three is to reduce the length of the external moment arm. This is likely the most effective and practical method.
Load should be lifted in between the knees. This will minimize the distance between the load and the lumbar region, therefore minimizing the external moment arm. And finally, last strategy that your patient can implement is increasing the internal moment arm. A larger internal moment arm for extension allows a given extension torque to be generated with less muscular force, therefore less compression on L2 segment. Increased lumbar lordosis increases the internal moment arm available to the lumbar erectospine musculature.
Lifting with increased lordosis is not always possible or desirable either. Lifting a heavy load off the floor, for example, typically requires a flexed lumbar spine, which decreases the extensor muscle moment arm. Okay, let's take a look at another strategy, which will help us decrease the force acting upon the joints. Lifting relatively heavy loads. involves generating a valsalva maneuver.
Valsalva maneuver describes an action of voluntarily increasing intra-abdominal pressure by vigorous contraction of the abdominal musculature against a closed glottis, demonstrated right here. This creates a rigid column of high pressure within the abdomen that pushes upward against the diaphragm demonstrated by this blue arrow, anteriorly against the abdominals, demonstrated by this blue arrow, posteriorly against lumbar spine and posterior abdominal wall, and finally, downwardly against the pelvic floor musculature. It acts as an inflated abdominal balloon, which creates most extension torque on the lumbar extensor muscles.
and lowering the muscular-based compression forces on the lumbar spine. Research supports the idea that generating a Valsalva maneuver does partially unload the lumbar intervertebral disc junction, as well as increase the muscular stiffness and stability of the lumbar region. Exact amount of offsetting forces and muscular biomechanics of Valsalva maneuver are not yet certain.
Some offsetting factors of the Valsalva maneuver is that it increases the activation of all of the abdominal musculature, meaning there is no recruitment of individual musculature. This means that it will increase the activation of flexors. In order to counterbalance that, your body will concurrently activate the extensor musculature.
and that is exactly what we want to avoid with lifting so this is one of the offsetting factors of the valsalva maneuver let's examine the role of transversus abdominis transversus abdominis has horizontally oriented fibers that attach to the thoracolumbar fascia It generates circumferential corset effect that creates stability around the core region. Why is it different from all other abdominal muscles? Because of its fiber orientation. If you think about rectus abdominis and the fibers running from superior to inferior direction, or external oblique where the fibers run in the oblique direction, as well as internal oblique in the opposite oblique direction.
These muscles have greatest potential torque for flexion of the trunk. Therefore, they will require the extensor muscles to generate pressure on the spinal column. Transversus abdominis doesn't require that.
Transversus abdominis increases abdominal pressure without concurrent flexion torque and therefore avoids excessive activation of lumbar spine extensors. Now, let's examine the role of diaphragm. During inhalation, diaphragm pulls the dome inferior, as demonstrated in this image right here.
That increases intra-abdominal pressure. Diaphragm, to be fully effective, requires simultaneous activation with the abdominal and pelvic floor musculature, once again, to create the abdominal canister. Research suggests that people with low back pain do not adequately engage their diaphragm musculature during lifting or resisting trunk movements, leaving the low back vulnerable to injury. Okay, and finally, let's take a look at the posterior ligamentous system. Posterior ligamentous system includes posterior longitudinal ligaments, ligamentum flavum, Z-joint capsule, interspinous ligaments, and thoracolumbar fascia.
This feature, the passive tension generation of the posterior ligamentus system, allows connective tissue to temporarily store a part of force that initially causes the elongation. So, As demonstrated in this example right here, when the patient goes into flexion, we generate force in the ligamentous structures. This force allows us to count the balance, the force created by the extensor musculature. Maximal flexion of the lumbar spine generates about 25% of the total extension torque for lifting. therefore helping us to overcome the force of the lifting an object.
Many researchers who studied the topic generally believe that maximum flexion of the lumbar spine should be avoided during lifting. Lumbar spine should be held in near neutral position. Maintaining neutral position will align the local extensor muscles to be most effective at resisting anterior shear. And finally, let's take a look at the action of thoracolumbar fascia.
Thoracolumbar fascia is thickest and most extensive developed in the lumbar region. We can take a look here. It's arranged in the anterior layer, middle layer, and the posterior All of these layers positioned relatively posterior to the axis of rotation of the joint.
Therefore, once again, being able to passively generate tension. Passive tension generated within the stretched thoracolumbar fascia can produce extension torque. So generation of full tension can be performed by performing two distinct features.
flexion of the lumbar spine, so a person bending forward and we're mechanically tensing the lumbar spine, or active contraction of the muscles that actually attach to the thoracolumbar fascia, such as transversus abdominis, internal obliques, and latissimus dorsi.