this section discuss how gravity affect a human movement gravity can be a good friend to human because it keep our feet on the ground constantly however it can be um a worst enemy because of the gravitational pool um that we basically need to overcome the gravity pool constantly um take uh infant as uh the example if we line the baby on their stomach the gravitational pool kind of um stretch the back muscle that would trigger uh the muscle to contract and that would uh result in movement so when the baby learn how to overcome the gravity they learn how to raise their head they learn how to reach out they learn how to crawl or even stand up and walk so in this sense grav gravity um kind of force the human to evolve to better understand uh the gravity's influence to our human movement there are several Concepts that uh we have to know the first one is the center of gravity uh AKA CG um a person's uh center of gravity is uh at the uh the balance point where the three movement plan intersect to each other and when a person is standing upright in the uh anatomical position we can expect that um the center of gravity actually fall around uh the Bell button level okay and if we uh draw a line that passes the center of gravity uh which we call a line of gravity and then we can use um this uh uh Arrow segment to represent this the strength of the gravitational pool it so that being said the gravity is affected by the person's body mass so let's look at this lady she has pretty strong lower extremity so we can expect that the majority of the her body mass is um um um around the low lower extremity so the center of gravity uh will no longer be at her uh body uh sorry U the Betty button level it can actually go a bit lower or inferiorly and this gentleman he has pretty good build of um his upper body but not so much for his lower body so we can expect that the center of gravity will go higher for this young man because of his p Ure kind of lim back a bit and um so we can expect that the center of gravity um will will fall uh posteriorly and maybe a little bit inferiorly because his body mass is going of fall uh to the posterior of um compared to a person who was standing upright for baby because the majority of their uh mass is in their head so we can expect that the center of gravity is actually very high compared to uh a a regular person uh Cent gravity may change um based on a person's uh position and the movement so when a person standing upright like this uh we can expect that Central gravity is at U the belly uh button level if a person raise their hand like this uh the body going to go move uh superiorly right so the center gravity would actually go higher a bit if this person bend over again because of the change of the body mass distribution we can expect that the center gravity going to move inferiorly and forward and in in this figure it can of fall outside of the uh the body another example um to see the change of the center Gra gravity is when we are trying to reach up let's say to the cboard to get the Mark um we move our weight from both feet to one foot that would change the center of gravity from um a symmetric and then you know move to one side to the we bearing side right here right so and something we can expect that if a person is sitting on the wheelchair because uh their leg is uh move upward and a bit forward so we can expect that the center gravity will move um accordingly right and why is why do we need to concern about this it's similar to a baby whose center of gravity is actually higher and a bit forward because you know when we see a baby walk they're quite unstable because um they per of gravity is quite High we can expect the same thing may happen to a person who's sitting on a wheelchair so to to counter um this change of center of gravity uh the design of the wheelchair really matters and we can uh talk talk more about that um later in the semester so why do we need uh to understand the center of gravity it's related to uh how stable can the person be um so we all know that it's it's very hard to uh do this curb hopping in a wheelchair right so we know that this person's center of gravity is has already go up and forward right and now he has to move you know the wheelchair up and posteriorly that's really really hard so to understand why this is difficult we we also need to understand the base of support a base of support is um an area that's in contact with the Earth or or or uh with the ground so if a person is standing uh normally like this so the um this person's base of support is really the area uh of the Bow's uh feet if a person move one leg forward or anteriorly then then it changed the uh base of support to a different um shape right and if a person stand with the single foot then this single foot is the base of support um same um we can um expand our center of gravity by bending over so now if a person put one hand down but stand with their toes then you can see that the base of support is changing compared to both hands on the ground um the Basel support is actually expanding compared to you know standing normally with uh both feet so combining the center of gravity and the base of support we now can um understand better about how to um remain stable so uh with the uh body mass where the center of gravity is we know uh with a greater uh weight it's hard to move right when the center of gravity goes higher then it's less stable if a person who has a uh wider base of support that means uh it's much more stable so let's put this two Concept in um in one thing when a person when a when a book uh has a good base of support the center of gravity is well within the base of support it's the book is very uh stable in this way when the base of support is reduced it increase the risk of fall of the book right and you know um the fall may actually happen if we um change the base of support if nothing's change U to the center of gravity same thing may happen if we add another book then that we then we change the center of gravity right and with with the same base of support the same B Bas of support may not be stable if we add more weight to it that we can see that when a person standing or walking like this uh there cent center of gravity is well within the base of support so it's quite stable in a way if this person stand with one foot and U the center of gravity kind of tilt to one side so that would make this center of gravity uh out outside of the base of support then it it can become unstable and you know because of the unstable uh a fall may happen so how can uh can a person uh maintain a good balance a person um all all the systems need to um you know play normally or functioning normally the person need to have an adequate um Center uh central nervous system to uh process the information and then to make this good decision um to uh um trigger some movement and to do that they also need to have really good U muscular skeletal uh functioning like a proper Ral motion muscle strength muscle tone or endurance and sensation is really key here because um they need to sense um they need to process uh the sensory um to be able to to make accurate judgment we can expect that if a person with uh a stroke because um because of the stroke they may have some kind of weakness in one side of the body and their uh sensation may be impaired when this person is trying to reach to the cupboard to get a mark then that can increase the uh the risk of fall same thing we can expect if a person experience a lower extremity amputation then the center guard will actually go higher right and then when they're sitting in the wheelchair the likelihood of you know getting a fall increases because of the change of the center of gravity so I hope um this concept can of um help you understand the importance of the gravity and using the base of support versus the center of gravity we as a note we constantly uh use this concept to uh to train our patient U on the safety issue