By Patrick JohnsonPosted on August 5, 2019October 30, 2019Posted in Biomechanics, Sit-to-StandTagged biomechanics, measures, sit to stand1 Comment on The Physics of Sit-to-Stand

This essay by Tim Cacciatore and also Patrick Johnson on the biomechanics of sit-to-stand also was originally published in STAT News, Fall 2016.

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Sit-to-stand is among the essential movements that Alexander Technique teachers use to teach. We generally leave the mechanical details of the movement aside and also focus on teaching inhibition and direction. But understanding the basic physics of sit-to-stand also offers insights right into standard elements of the movement that have the right to aid in expertise the student’s postural behavior. It might additionally inspire brand-new means of interacting with more technically minded students as well as physicians and physical therapists. Additionally many type of teachers already use words and also principles from physics and engineering, such as gravity, momentum, stcapability, effectiveness, mechanical benefit, stiffness, and also mobility, in their description of Alexander Technique.

This article introduces the physics of sit-to-stand with respect to teaching and finding out Alexander Technique. It additionally demonstprices exactly how various executions of sit-to-stand also can obstacle students in physically various ways. Six “games” that you have the right to play while analysis are had to give you a feel for what is being questioned. We’ve tried to save the level of technological information controlled for most readers. If the text gets overwhelming at times, feel complimentary to skip ahead to the following game and also rerevolve later to the text.

Many kind of of the values of sit-to-stand deserve to be applied directly to the reverse activity (stand-to-sit) though tright here are some essential distinctions. To store the write-up concise, we will emphasis on sit-to-stand also in our explacountries and also note a few of the vital differences via stand-to-sit.

Some general body physics

At the heart of all actions, according to Newtonian mechanics, are forces. A pressure is a press or a pull. As you move via the civilization, every change in your state of movement is linked through a collection of pressures that cause the adjust. These forces can be divided into outside pressures which act on the body from external and also inner forces which act inside the body.

Two kinds of external pressures influence you, gravity and contact forces. Gravity pulls eexceptionally part of you down towards the ground. Most of us know this. Less well known is that the ground likewise pushes on you through a call pressure. If you are sitting in a chair right currently both the chair and also floor are pushing up on your bum and your feet. Another call pressure is friction which avoids your feet from sliding as you stand up. As the name means, call pressures only occupational when the objects are in call through each various other. By comparison gravity is constantly pulling you down, even if you are jumping in the air.

You can visualize the effect of the exterior pressures by imagining what would certainly take place if they were not there. If gravity were not tbelow, the slightest motion would certainly sfinish your body floating up off the ground. If the contact pressures were not there you would autumn down via the floor as if it had actually no substance, or in the instance of friction you would certainly not have the ability to protect against sliding any type of which method alengthy the floor.

Internal forces are produced inside the body by bones, muscles, tendons, ligaments, and various other tproblem. Some of these forces consume energy, such as the force from muscles. Others consume no power, such as the forces from bones, ligaments, and also tendons. Without the interior forces from the body, it would certainly collapse because of the outside pressures gravity and also call pressures.

The balance of all of these pressures, both interior and also exterior, determines whether and also how activity will certainly occur. Balanced pressures are connected via a state of either rest or consistent motion. Imbalanced pressures are linked with change in the state of movement. To discuss exactly how this functions we should first define momentum.

The momentum of a things defines both its motion and its resistance to change of motion. The even more mass and/or rate a things has, the more momentum. A auto passing a cyclist has actually even more momentum both because it has more mass and also bereason it is travelling quicker. The automobile will therefore be much harder to sluggish down than the bike. Momentum also has direction. A car heading southern and also an the same vehicle heading north at the very same speed have oppowebsite momenta. The humale body is a little bit even more facility than the auto because it deserve to fold, bend, and also twist in so many kind of methods. In movement, the various parts of the body deserve to be relocating in different directions via different speeds and also masses. Hence eincredibly component of the body have the right to have its own momentum. For example at the start of the tipping phase of sit to stand also the head has actually a forward momentum while the feet have actually hardly any type of momentum

Forces and also momentum are intimately related. A change in momentum calls for a net pressure. If a relocating auto tries to suddenly schild to a speak, an external force—the frictional force from the ground rubbing on the tires—is needed to slow it dvery own. The tendency for objects to resist transforms in momentum is called inertia. In the instance of the auto trying to speak, if the road is exceptionally icy and slippery the car proceeds forward because the frictional pressure is gone and inertia maintains its movement. In sit to stand also, a combination of forces, both inner and also outside, are necessary to rate up, slow-moving down, or readjust the direction of the assorted components in motion, while inertia can be offered to lug a activity forward.

The last terms to know around are torques and rotational activity. Much of the movement of the body entails rotation of objects around joints. In these cases, it can be less complicated to talk about torques rather than pressures and also rotational momentum/inertia fairly than direct momentum/inertia. Precisely specifying these terms goes past the scope of this post. But for our purposes it is sufficient to remember that torque and rotational momentum/inertia are to rotating objects what pressures and momentum are to objects moving in a right line.

Slow, smooth sit to stand

We begin our conversation of sit-to-stand by considering incredibly slow consistent movement. Picture a Tai Chi master moving incredibly progressively. Not only is the motion sluggish but there are likewise no jerks or sudden shifts in the time of the motion. In this instance the momentum and alters in momentum are nearly zero. With bit momentum, all pressures should remain around well balanced throughout the activity. This implies that as the motion slowly flows forward, the mover deserve to pausage at any minute via no adjustments and also continue to be in balance. This lack of momentum and also succeeding balance of forces makes the physics simpler. It likewise means that the direction of the movement does not impact the physics. I.e. exceptionally slow-moving sit-to-stand also and also stand-to-sit are, from a physics perspective, the same. Physicists contact this sort of very slow smooth motion “quasistatic”, which means practically not moving.

Game 1: Try moving quasistatically from sitting to standing and also vise versa now. Be certain that you are really moving progressively with no jerks and also that you are never before “throwing” yourself or falling from one balance suggest to an additional. Check that you deserve to speak at any kind of moment via no adjustment. Try in particular to get rid of any kind of jerks at the moment that your bum leaves the chair (in this short article we will contact this moment liftoff).

Quasistatic motion requires that your facility of mass stay above your base of assistance at all times—i.e. that you stay continually in balance. Roughly speaking, the facility of mass is the point around which mass is evenly dispersed. In the sitting position it is located roughly approximately the belly button yet it will certainly change its place as the body transforms shape during the motion. Only when the facility of mass is straight over its base of assistance can the object balance at rest. The base of support in sitting is the location identified by the sits bones and the two feet, while in standing it is the single rectangle characterized by the external edges of the two feet. To continue to be in balance throughout the sluggish activity, you should first move your facility of mass forward, throughout the sitting base of support,till it is above your feet. Only then have the right to you lift your bum off from the chair without falling. If you try to liftoff prior to the facility of mass reaches the feet you will certainly either should suddenly lurch forward in preparation (to geneprice forward momentum) or else you will certainly fall backward to the chair.

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Fig. 1. Cartoon of sit to stand via the facility of mass (green dot) for 2 positions, before the activity starts (left) and also just after liftoff (right). In quasistatic activity the facility of mass need to move to above the feet before you have the right to stand.

Getting the facility of mass over the feet is completed by tipping the trunk (Fig. 1). The closer the feet are to the chair the less the trunk requirements to guideline forward to gain the facility of mass above the feet. The trunk tilt at lift off is likewise reduced once the weight is in your heels quite than the center or balls of the feet. One advantage of sit-to-stand compared to stand-to-sit is that the distance in between the bum and the feet have the right to be chosen prior to the activity hence defining what type challenges that the mover will certainly face.

Video Game 2: Try quasistatic sit-to-stand now through different feet positions. Remember to move exceptionally gradually and also smoothly throughout the motion. Notice that the further the feet are positioned from the chair the even more you should pointer forward and also the more complex it is to preserve quasistatic movement. Notice what transforms once you initiate liftoff on the heels of the feet as opposed to the balls of the feet. Now attempt quasistatic stand also to sit. Notice that the depth of the trunk tilt determines just how far back the bum deserve to land on the chair.

We deserve to think of the slow-moving movement as a force equivalent task. Gravity would collapse the spine forward and fold the body at the hips and also knees (flexion) were tright here no inner muscle action. To proccasion this collapse and manage the movement, activation of extensor muscles is essential in the trunk, hip, and leg to enhance gravity. Note that in principle noflexoractivation of the trunk, hip, or knee flexors is required for the movement to occur. Up through liftoff gravity drives the activity and the extensors control it. In technological terms, the hip and knee extensor muscle contractions are eccentric. After the pelvis leaves the chair, these extensors continue to work-related to unfold the body to the standing place. Hence both the tipping forward phase and also the unfolding to standing phase call for consistent extensor activity.

Tipping the trunk to shift the center of mass over the feet shifts even more weight to the feet, pushing the feet onto to the floor. This is well balanced by the external call force from the floor upwards. This call force have the right to be measured through a bathroom scale.

Video Game 3: You have the right to feel and/or meacertain the contact pressure from the ground. Place a bathroom scale under your feet and watch the call pressure, measured by the scale, increase as the feet are weighted (you need an analogue scale or a digital scale that continually updates). When the sit-to-stand is perdeveloped progressively and smoothly you (or a friend) can track the continuous boost until you reach around your body weight at liftoff. If you don’t have actually a scale, simply revolve your attention to the changing pressure on the bottom of the feet.

Interestingly, you have the right to additionally weight and also unweight the feet without tipping the trunk. Generating a horizontal force through the feet versus the floor, either forward or backwards, geneprices horizontal frictional forces from both the floor and the chair. These generate torques that either push down or lift up the feet. It is beyond the scope of this short article to describe the physics of this in even more information, however you deserve to quickly feel this happening and/or meacertain it through a range. Perhaps you can number out exactly what is going on yourself.

Video Game 4: Sit in the chair with your feet on the ground or on a scale and also try to slide your feet forward (knee extension) while maintaining your feet stuck to the ground (don’t actually relocate the feet). You will certainly feel and/or measure more weight come onto the feet. Likewise if you pull your feet backwards you will certainly lesson the weight of the feet on the ground.

This type of static weighting and also unweighting of the feet without change of place is unessential in sit to stand also. As you competent in game 3, it is sufficient to lug the facility of mass forward for the feet to be weighted. No extra pushing is required. Additionally, if the bum leaves the chair while this added horizontal force is still active, the body will all of a sudden be either pushed backwards or forwards by the horizontal force from the floor.

While a student deserve to weight the feet without tipping forward she will never have the ability to stand also quasistatically without obtaining the center of mass above the feet. If she tries, she will simply fall back into the chair. However before if a teacher’s hand is moving a pressure to save the student from falling backwards, she can indeed slowly increase up vertically. In this instance the contact pressure from the ground deserve to be even more than the body weight at lift off depending on the feet position. At liftoff, the student’s feet need to generate both a horizontal backward pressure to meet the pressure of the teacher’s hand and also a vertical force to accomplish the full weight of the body. The vector sum of these two pressures adds as much as more than the student’s weight.

This hand aided activity poses some amazing difficulties for both the student and the teacher, beyond just the novelty of the trajectory. The student is challenged to repetitively match not only gravity and also the contact forces from the ground however additionally the force of the teacher’s hand. The teacher is additionally challenged; giving a secure force complement to the student is fairly a subtle task. Both student and teacher must smoothly ramp up the equivalent forces together, with no jerks or momentum generating methods. At liftoff itself, the student needs to generate a force greater than their body weight. Finally, all of these difficulties have to be accomplished while minimizing cocontraction (stiffening of the joints) which would certainly simply include unnecessary resistance to the activity.

In general, once the teacher pushes the student forward with a hand also it can readjust the physics fairly considerably, permitting the teacher more regulate of the usage of momentum and the pressures at liftoff.

Using momentum smoothly

Therefore far we’ve been mentioning the quasistatic limit. But we generally usage a fair little of momentum to rise up out of a chair.

From a balance perspective, the function of momentum can be compared to throwing a sphere to a friend. The sphere is first in static balance in your hand also. The movement of your hand geneprices momentum in the round. This momentum enables it to fly off its assistance and also then land and also deceleprice earlier into balance in the hand also of your friend.

Let’s check out exactly how this picture applies to smoothly utilizing momentum to conveniently rise from a chair (Fig. 2). You begin by permitting gravity to generate momentum in the torso by maintaining the hip extensor action low in the time of the start of the tipping phase. The forward momentum of the trunk permits you to launch the trunk from the chairbeforethe center of mass is over the feet. After liftoff, momentum carries the body forward while the rearward, off balance, place of the center of mass slows this activity so that the facility of mass pertains to a soft landing over the feet. This usage of momentum increases the in its entirety motion and also boundaries the amount that the trunk needs to tip forward. You can generate plenty of momentum this method, making use of only gravity via no net activation of the flexors, to stand also up in about 1.5 secs (Cacciatore 2014).

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Fig. 2. Momentum produced throughout the tilt phase, in between the left and also middle position, permits liftoff to occur prior to the center of mass is over the feet (middle figure). The center of mass “lands” over the feet as the motion slows dvery own (right). A contact pressure from the teacher’s hand on the earlier have the right to have actually a comparable impact.

Adding momentum to the motion also needs additional pressures for overcoming inertia. As discussed, altering the momentum of an item, be it at rest or moving, requires a pressure. In the round throwing analogy both slowing down and increasing the round both require call pressures from the hands. Likewise, a rotating object will call for torques to speed or slow-moving down rotation. For example a lengthy springy rod being waved ago and also forth bends and springs ago. Inertia produced by momentum bends the rod while the springiness of the rod resists bending.

Similar to the round and also the rod, speeding up and also decelerating the trunk in sit to stand also requires pressures. Acceleration in the time of the tipping forward phase deserve to be achieved making use of gravity alone. In this instance the trunk falls forward because of release of hip joint resistance. Deceleration is then required to extend right into liftoff. This deceleration calls for extensor task in the earlier and also hips to get over both gravity and slow-moving the flexion movement. Like the springy rod, tipping movement the trunk creates forwards inertia which has a tendency to flex the spine as quickly as the hip extensors slow-moving the tipping movement. The back extensors prevent this flexion for this reason maintaining spinal size throughout liftoff.

Hence extensors have to smoothly both to adjust to gravitational pressures and also to conquer the inertia of the trunk. If the extensors under or over compensate the mover will certainly either collapse or arch the back respectively. Using momentum is a tradeoff: the more vertical trunk reduces the gravitational torques yet added extensor torques are essential to sluggish the body’s forward momentum.

Game 5: Move from sit-to-stand first quasistatically, and also then using momentum smoothly. Notice that for the exact same foot position, the trunk can pointer less forward when making use of momentum. Notice yet that significant direction of the spine is needed to prevent the trunk from collapsing in both cases—in the quasistatic situation to stand up to gravity and also in the momentum instance to even more resist forward inertia.

Momentum offers a second range of obstacles for mobilizing and also stabilizing the body versus external forces in various positions. Depfinishing on what the teacher desires to test, the student have the right to be coached or guided through the motion along a range of trajectories at various speeds.

While momentum deserve to be used to traject the center of mass via positions of static instability, comparable strategies are not so smooth when sitting dvery own. If the student throws himself off balance as he heading to the chair he has no option however to land with a thump on the chair. We will leave the physics of this for the reader to think around better.

Using additional momentum

Often a student will certainly geneprice a burst of momentum even when they are clearly instructed to move smoothly. Having a student percreate sit-to-stand incredibly slowly deserve to make this evident. Regardless of the instruction to stand also gradually the student lurches at the minute simply before liftoff. One plausible explacountry for this is that the student depends on the burst of momentum to overcome stiffening in the hip joints, successfully throwing himself via the allude of stiffness <1>. In stand-to-sit a comparable pattern causes the student to loss earlier right into the chair rather than landing smoothly. The alternative, in this design, would be to cost-free the hip joints to permit the activity to take place without the extra burst. This is rather a difficulty for the majority of students, specifically in the excessive feet forward place.

Anvarious other excess usage of momentum in sit-to-stand is the activation of hip and knee flexors during the tilt phase in order to acceleprice the trunk forward. This strategy is not crucial even for a bthreat paced motion. Gravity increases the trunk with plenty of speed to stand up easily without activating flexors <1>.

Ever have a student that pulls the chair with them at liftoff so that the chair tips forward? The student is likely pulling the pelvis forward (and helping to flex the hip) utilizing knee flexion. This deserve to be believed of as either a momentum trick—to throw the trunk conveniently over the feet after liftoff—or a sliding trick—the student chooses to slide the height of the chair via them to acquire the center of mass over the feet quite than simply bfinishing at the hips. Or it might be a remnant of the man-made pushing or pulling of the legs that we disputed previously. Either means it is a strangely counterproductive activity since knee flexion also unweights the feet.

Sometimes a teacher will certainly take a student’s trunk backwards from vertical prior to taking them forwards right into the motion. This have the right to be pedagogically very valuable for emphasizing a number of elements of great usage (for example testing the integrity of the trunk and also liberty of hips in a backward tilt position). It likewise challenges the trunk flexors, quite than extensors, to counteract the collapsing force of gravity on the spine. But the teacher should realize that they may be generating additional momentum at the vertical position that would otherwise not be there. It permits the student to rise up through a more vertical trunk than would certainly be possible without the extra momentum generating preparation.

Many teachers intuitively usage a selection of pathmethods to take the student from sit-to-stand—sluggish, quick, deep, shallow, via and without backward tilts, through and without assistance from the teacher’s hands. This range enables the teacher to location a broad range of requirements on the student’s device and also hence discover the student’s use and also weaknesses therein from a variety of perspectives.

Discussion

Sit-to-stand tests students’ response to gravity, momentum, and call forces. Do they collapse? Do they stiffen? Do they use momentum efficiently? What mechanical tactics are they employing? The answers to these questions offer both the student and teacher understanding into basic psychophysical fads. In this short article we have aimed to clarify the mechanical side of these concerns.

Other components in sit-to-stand also past the scope of this short article encompass the usage of the arms or trunk flexion/expansion to generate momentum and move forces and also the role of the ankle flexors and extensors to name a few. Nor have we disputed any kind of underlying neuromuscular mechanisms or pedagogical methods which are at the heart of Alexander Technique and definitely even more mysterious from a scientific perspective.

The basic physics can be exceptionally helpful as soon as functioning via students. For example understanding the quasistatic challenge have the right to make postural problems and also moments of additional lurching even more noticeable for both the student and also the teacher especially once combined via playing with the place of the feet. Understanding the variety of challenges that have the right to be introduced by altering the rate of movement, position of feet, liftoff minute, and varying the push from the teacher’s hand can additionally carry diversity to a leschild. Finally the language of physics have the right to be beneficial for communicating with certain kinds of students and teachers.

The physics additionally offers understanding right into the complexity of the corresponding job. Perfectly matching the various pressures at the feet, chair, and also teacher’s hands needs needs constant smooth adjustment of the entirety body’s response. Given this intricacy, it is unlikely that the student deserve to complete the job by sindicate choreographing the movement—i.e. by consciously deciding what and when to push or pull. Any imperfectly matched forces will lead to tiny jerks in movement or the level of resistance which the teacher or student will have the ability to either watch or feel. It is therefore most likely that the response being trained occurs at all levels of the student’s device, from reflexive to better level fine procedures. Speculating additionally about the nature of this learning procedure is beyond the scope of this write-up.

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Finally, expertise the physics clarifies how different positions and also executions difficulty the body in different methods. In the previous there have actually been debates among teachers from different streams about the “best” pathways and also feet positions in sit-to-stand. Grasping the physics can guide conversation even more towardshowone meets the assorted obstacles, quite than the particular parameters of the challenge itself.

Video Game 6: Take a minute to think around the different sit to stand pathmethods you have your students execute throughout your lessons. Do you highlight particular obstacles even more than others? Are there any kind of pathmeans that you could discover to challenge your students in brand-new ways? How can you incorporate some of the language of physics to appeal to your more technically minded students?

T. W. Cacciatore, O. S. Mian, A. Peters, B. L. Day (2014) “Neuromechanical interference of posture on movement: proof from Alexander Technique teachers climbing from a chair”,Journal of Neurophysiology112 (3), 719–729