I want to talk about the kinesiology of the muscle force and what is impact to the spine. Typically, if you look at this picture right here, and if you have a mirror, I encourage you to look into the mirror. I wonder how much your posture is like this gentleman.
If you're quite similar to him, you're in danger. You need to listen to this lecture carefully. Okay, I'm going to walk you through this. So if we look at the neck, let's look at the picture first. If this person sits in an upright position, the head is in the middle or neutral position right like this this is like we talked about it's a classic first class lever system okay the weight of the head kind of function in this direction and then the neck extensor kind of counter the weight of the head so if the head is in upright position the liver arm is close to zero right so uh the neck extensor kind of needs kind of minimum muscle force to maintain the head in upright position so this per this lady won't feel too much trouble or um weirdness after using her cell phone like this okay so if she kind of bent her neck to about 45 degrees checking her cell phone which i bet 80 percent of you including me are having this kind of position like this so what it what it costs us is the head is in 45 degrees flexion the moment arm is longer you know when you compare to the upright position right with the same weight but longer moment arm that in that create a larger external torque which means you are the neck extensor we will need to generate a lot more muscle force to counter and remember when we go over the structure of the spine and the muscle the muscle has very little short moment arm.
So, you know, whenever the moment arm can increase when you bend the neck, a lot more muscle force is required to counter and not to mention if you bend the head more. Okay, so what that causes is if we keep in the flexion position for a long time, we add more stress on the C-spine. and the disc is more at risk to develop a herniation or rupture.
So the longer the head that we keep in the flexed position, the greater effect of the gravity that you can expect. And that means the more strength are required from the erector spinae to counter the head. Okay, and what I causes is you know now That will lead to poor a coordinate bilateral activation of this sternocleidomastoid if we keep our head in this position a long time, right and Like we talked about the SCM muscles kind of coupled with a longest collie that that do the tucking chin kind of position right so if one person is keeping the head in flexion and that person may have difficulty in chewing and swallowing because that tucking the chin movement is lacking okay and what should we do we should always encourage people to maintain an upright position right and If you can't, well, try and make frequent breaks.
Okay, and during the breaks, you can do some stretch to the neck, either deflection or extension, or flexor and extensor. Okay, so let's move our focus to the trunk right here. so see this gentleman because he's carrying uh you know big uh boxes or heavy boxes so what i what i mean is um um if the box is too big the center of gravity is really at the center of the box right so it um it the moment arm will be longer compared to a smaller box right here right and if it's heavier if it's heavier again we typically kind of kind of leave it we want to make it closer to the body because the momentum is shorter that the torque is smaller however we tend not to do so when we carry a heavy object because we like to lock our elbow and shoulder.
But instead of using that, we can extend or we can compensate with our spine by leaning backward. So it feels easier, but it kind of increases the stress to the L-spine when we do so. If this person chooses to stand upright, but carrying these big box again you have longer moment arm compared to the smaller box again that will require more threats for require more strength from the erectors by name okay and same thing here when you bend over say drinking water from the fountain again that that ankle will increase the moment arm as opposed to standing upright right now your center of gravity of the upper body is kind of moving from here to here right because you bend over that increase the moment that would get that would require more from the erector spinae and again we'll put more stress on the l-spine especially l4 and l5 because that allows more movement over there okay and that is why people typically do develop kind of pain at the waist or l4 l5 level or the c-spine okay well because we are you know right now we are becoming a zombie using the cell phone or we kind of use a Not so good posture In our daily life.
So a lot of us including me We have pain or chronic pain at the spine like C-spine or L-spine Okay, if you're not convinced try This slide how many of you how many of you have this posture? when you're using your laptop um you kind of do a little bit kyphosis and then your pelvic kind of tilt posteriorly to compensate this kyphosis you have um active cervical lordosis to help you kind of gaze to to operate the computer or you see in this way so you have that pelvic kind of tilt to one side so all these would just add more stress either to the lumbar or to the c-spine okay and in the long run we're gonna hurt ourselves okay and it is our responsibility as an OT to help to correct or make the performance of this occupation better and we will talk more about that next week. So this week is just an introduction of how our posture can affect the spine. Thank you.