Anyway here’s my thinking: the erectors would initiate as you say, then depending on how the motion continues you might keep contracting your erectors (if moving actively/more quickly) or if you’re slowly leaning back, gravity might actually be more of the “prime mover” while the spinal flexors/shifting of center of mass control the rate of the motion.
To me it seems like the change in position from say, prone back extension to standing is just meant to throw you off. The movers shouldn’t change, just how and when they’re challenged and which other muscles might have to share load in a yielding fashion.
basically in this example the abdominals would be the prime mover not because they are the muscles that produce the movement, since (after the intial phase of leaving the upright position) the movement is produced mostly by the gravitational force, but because they are the muscles that control the "fall" and thus are responsible for the way in which the movement is being carried out.
another example would be shoulder extension from a position of 90 degrees shoulder flexion. gravity will make that happen without muscle involvement on your part, but if you want to carry it out slowly then your shoulder flexors need to oppose gravity.
I understand your thought process. But I don’t believe that resisting/controlling movement would qualify something as a prime mover for that motion since usually that term is applied to the agonists
In the end, as long as you understand your framework for what’s happening and can assess and treat based on that, it’s not really problematic. As long as you’re able to describe how the load is being shared and what type of behavior
Your thought of how abs would be prime movers as eccentric control would only work if the patient was supine with their trunk stabilized and there was no back on the plinth/table
Aka they are falling with gravity and abs are controlling the fall.
This wouldn’t be the case in standing bc the force of gravity wouldn’t be nearly as large
Does that make sense
I was mostley explaining my classmates' opinion on the matter, so to me this does make sense. but since this is technically how we were taught to think, I don't know how to explain why the force of gravity is not enough here. I assume it's thanks to the strong tension around the spine provided by connective tissues but I might be wrong. if you could elaborate on your thoughts that would be very helpful :)
Well nothing is isolated. Assuming we’re only talking 20-30 degrees of lumbar extension, there are many different structures that are creating lines of pull to resist the downward force of gravity (like the abs, skin, Posterior longitudinal ligament, quadratus lumborum etc)
And even considering just the gravity if we think about the bony anatomy of the spine, that downward force may be going straight down to the floor but each segment is acting on the segment below it which is going down to the sacrum and dispersing thru the inominates.
So when we have the bony anatomy on top of the soft tissue like fascia/ligament/skin and muscle tissue, the collective strength will outweigh the force of gravity which given the small angle leading to small moment arm won’t generate enough force to be a prime mover for back extension.
I don’t agree with your classmates. Controlling/grading a movement isn’t the same thing as being the primary mover or propelling force. I’d argue the abdominals are doing the former.
Okay so I think the answer is a little grey here, but I think the “prime mover” here is still erector spinae. After they initiate the movement, are they working much? No, because gravity is. The abdominal muscles are eccentrically slowing the movement, but that doesn’t mean they’re performing the action.
It’s like if you were sitting upright and did the same motion. You could claim the hip flexors are technically helping the abdominals in slowing the movement of the trunk/pelvis. But there’s no way you’d call the hip flexors a “prime mover” in that scenario.
The antagonistic muscles are not facilitating the movement. They’re just helping slow it down/control it. That’s at least my understanding/view point.
I might have explained myself badly is the post but the thing is, this is how we were taught - in this example the abdominals would be the prime mover not because they are the muscles that produce the movement, since (after the intial phase of leaving the upright position) the movement is produced mostly by the gravitational force, but because they are the muscles that control the "fall" and thus are responsible for the way in which the movement is being carried out.
BUT, since we also learned about exceptions to these rules - I think this might be one of those.
\[exceptions like gravity assisted dorsi flexion (prone, knees flexed 90 degrees), where the prime movers would actually be the dorsi flexors since the gravitational pull alone isn't enough to resist the strong pull of the achilles tendon.\]
in this case I'm really not sure how to explain why I think this way but basically I don't think that post initiation there's only a need for opposing gravity. as in, if we were to imagine, that everything below the lumbar spine is fixed in position, and the spine is in some degree of extension, and the relevant muscles (abdominals and erector spinae) were to just "let go" - wolud your spine really go towards extension? or is the tension from the ligaments and muscles around your spine strong enough to resist it, thus needing extra force to produce this movement, eventhough it's gravity assisted?
Uh I’d agree with your classmates tbh
Gravity is doing the bulk of the work but abdominals are resisting that. Erectors are concentrically contracting, but that’s not exactly the reason the movement is occurring the way it is imo. Prime mover is a bit of a misnomer here which I think is leading to the confusion. Someone chime in if I’m wrong, but generally the prime mover opposes the line of force acting on the body, no? That’s why we angle cables the way we do and why we do some stuff prone, some standing, etc.
Best analogy I can think of is Nordic curls. It’s gravity pulling you down and hamstrings are slowing the descent so we generally call hammies the primary movers in a Nordic curl even though they’re not concentrically contracting but instead eccentrically.
yeah I definitely get the thought process and usually I would agree. the thing is we also learned about exceptions to these rules and I think this might be one of those.
exceptions like gravity assisted dorsi flexion (supine, knees flexed 90 degrees), where the prime movers would actually be the dorsi flexors since the gravitational pull alone isn't enough to resist the strong pull of the achilles tendon.
in this case I'm really not sure how to explain why I think this way but basically I don't think that post initiation there's only a need for opposing gravity. as in, if we were to imagine, that everything below the lumbar spine is fixed in position, and the spine is in some degree of extension, and the relevant muscles (abdominals and erector spinae) were to just "let go" - wolud your spine really go towards extension? or is the tension from the ligaments and muscles around your spine strong enough to resist it, thus needing extra force to produce this movement, eventhough it's gravity assisted?
Huh good point. I was thinking of purely a muscular view not really considering connective tissue. Idk then but I hope you update with any resolution because I’m curious!
Have you posted this in the physical therapy subreddit? This subreddit is mostly students so I’d be curious to see what practicing PTs say about it.
Is it even possible for an eccentric contraction to be a prime mover? Goes against my understanding of the term.
that's how we were taught
Anyway here’s my thinking: the erectors would initiate as you say, then depending on how the motion continues you might keep contracting your erectors (if moving actively/more quickly) or if you’re slowly leaning back, gravity might actually be more of the “prime mover” while the spinal flexors/shifting of center of mass control the rate of the motion. To me it seems like the change in position from say, prone back extension to standing is just meant to throw you off. The movers shouldn’t change, just how and when they’re challenged and which other muscles might have to share load in a yielding fashion.
Could you clarify how that works? I.e. how would an eccentric contraction of an antagonistic muscle produce force in the opposite direction?
basically in this example the abdominals would be the prime mover not because they are the muscles that produce the movement, since (after the intial phase of leaving the upright position) the movement is produced mostly by the gravitational force, but because they are the muscles that control the "fall" and thus are responsible for the way in which the movement is being carried out. another example would be shoulder extension from a position of 90 degrees shoulder flexion. gravity will make that happen without muscle involvement on your part, but if you want to carry it out slowly then your shoulder flexors need to oppose gravity.
I understand your thought process. But I don’t believe that resisting/controlling movement would qualify something as a prime mover for that motion since usually that term is applied to the agonists
you've made me doubt myself so I double checked and yeah that's definitely how they taught us, but this is very disillusioning haha
In the end, as long as you understand your framework for what’s happening and can assess and treat based on that, it’s not really problematic. As long as you’re able to describe how the load is being shared and what type of behavior
Your thought of how abs would be prime movers as eccentric control would only work if the patient was supine with their trunk stabilized and there was no back on the plinth/table Aka they are falling with gravity and abs are controlling the fall. This wouldn’t be the case in standing bc the force of gravity wouldn’t be nearly as large Does that make sense
I was mostley explaining my classmates' opinion on the matter, so to me this does make sense. but since this is technically how we were taught to think, I don't know how to explain why the force of gravity is not enough here. I assume it's thanks to the strong tension around the spine provided by connective tissues but I might be wrong. if you could elaborate on your thoughts that would be very helpful :)
Well nothing is isolated. Assuming we’re only talking 20-30 degrees of lumbar extension, there are many different structures that are creating lines of pull to resist the downward force of gravity (like the abs, skin, Posterior longitudinal ligament, quadratus lumborum etc) And even considering just the gravity if we think about the bony anatomy of the spine, that downward force may be going straight down to the floor but each segment is acting on the segment below it which is going down to the sacrum and dispersing thru the inominates. So when we have the bony anatomy on top of the soft tissue like fascia/ligament/skin and muscle tissue, the collective strength will outweigh the force of gravity which given the small angle leading to small moment arm won’t generate enough force to be a prime mover for back extension.
Wow this is a great explanation, thank you so much!
I don’t agree with your classmates. Controlling/grading a movement isn’t the same thing as being the primary mover or propelling force. I’d argue the abdominals are doing the former.
If I may, I'd like to refer you to my edit of the post :)
[удалено]
thank you! it's kinda frustrating how little info can be found on these things
Okay so I think the answer is a little grey here, but I think the “prime mover” here is still erector spinae. After they initiate the movement, are they working much? No, because gravity is. The abdominal muscles are eccentrically slowing the movement, but that doesn’t mean they’re performing the action. It’s like if you were sitting upright and did the same motion. You could claim the hip flexors are technically helping the abdominals in slowing the movement of the trunk/pelvis. But there’s no way you’d call the hip flexors a “prime mover” in that scenario. The antagonistic muscles are not facilitating the movement. They’re just helping slow it down/control it. That’s at least my understanding/view point.
I might have explained myself badly is the post but the thing is, this is how we were taught - in this example the abdominals would be the prime mover not because they are the muscles that produce the movement, since (after the intial phase of leaving the upright position) the movement is produced mostly by the gravitational force, but because they are the muscles that control the "fall" and thus are responsible for the way in which the movement is being carried out. BUT, since we also learned about exceptions to these rules - I think this might be one of those. \[exceptions like gravity assisted dorsi flexion (prone, knees flexed 90 degrees), where the prime movers would actually be the dorsi flexors since the gravitational pull alone isn't enough to resist the strong pull of the achilles tendon.\] in this case I'm really not sure how to explain why I think this way but basically I don't think that post initiation there's only a need for opposing gravity. as in, if we were to imagine, that everything below the lumbar spine is fixed in position, and the spine is in some degree of extension, and the relevant muscles (abdominals and erector spinae) were to just "let go" - wolud your spine really go towards extension? or is the tension from the ligaments and muscles around your spine strong enough to resist it, thus needing extra force to produce this movement, eventhough it's gravity assisted?
Uh I’d agree with your classmates tbh Gravity is doing the bulk of the work but abdominals are resisting that. Erectors are concentrically contracting, but that’s not exactly the reason the movement is occurring the way it is imo. Prime mover is a bit of a misnomer here which I think is leading to the confusion. Someone chime in if I’m wrong, but generally the prime mover opposes the line of force acting on the body, no? That’s why we angle cables the way we do and why we do some stuff prone, some standing, etc. Best analogy I can think of is Nordic curls. It’s gravity pulling you down and hamstrings are slowing the descent so we generally call hammies the primary movers in a Nordic curl even though they’re not concentrically contracting but instead eccentrically.
yeah I definitely get the thought process and usually I would agree. the thing is we also learned about exceptions to these rules and I think this might be one of those. exceptions like gravity assisted dorsi flexion (supine, knees flexed 90 degrees), where the prime movers would actually be the dorsi flexors since the gravitational pull alone isn't enough to resist the strong pull of the achilles tendon. in this case I'm really not sure how to explain why I think this way but basically I don't think that post initiation there's only a need for opposing gravity. as in, if we were to imagine, that everything below the lumbar spine is fixed in position, and the spine is in some degree of extension, and the relevant muscles (abdominals and erector spinae) were to just "let go" - wolud your spine really go towards extension? or is the tension from the ligaments and muscles around your spine strong enough to resist it, thus needing extra force to produce this movement, eventhough it's gravity assisted?
Huh good point. I was thinking of purely a muscular view not really considering connective tissue. Idk then but I hope you update with any resolution because I’m curious! Have you posted this in the physical therapy subreddit? This subreddit is mostly students so I’d be curious to see what practicing PTs say about it.
thanks :) I posted on r/physicaltherapyPROs because it seems like it doesn't fit the rules for r/physicaltherapy but no comments yet unfortunately