through Patrick JohnsonPosted on respectable 5, 2019September 10, 2021Posted in Biomechanics, Sit-to-StandTagged biomechanics, procedures, sit to stand1 discuss The Physics the Sit-to-Stand

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

You are watching: Why must the person either lean forward or slide their feet under the chair in order to stand up?

Sit-to-stand is just one of the necessary movements that Alexander an approach teachers use to teach. We usually leave the mechanically details of the movement aside and also focus on teaching inhibition and direction. Yet understanding the simple physics the sit-to-stand uses insights into an essential aspects of the motion that can assist in expertise the student’s postural habits. It may also inspire brand-new ways of interacting with an ext technically minded students as well as doctors and physical therapists. Furthermore many teachers currently use words and concepts from physics and also engineering, such together gravity, momentum, stability, efficiency, mechanical advantage, stiffness, and also mobility, in their description of Alexander Technique.

This post introduces the physics of sit-to-stand v respect to teaching and learning Alexander Technique. It also demonstrates how various executions that sit-to-stand can an obstacle students in physically various ways. 6 “games” the you deserve to play while analysis are had to offer you a feeling for what is gift discussed. We’ve tried to save the level of technical detail manageable for most readers. If the message gets overwhelming at times, feel free to skip front to the following game and also return later to the text.

Many of the values of sit-to-stand have the right to be applied directly come the reverse movement (stand-to-sit) though there room some crucial differences. To save the write-up concise, we will focus on sit-to-stand in our explanations and also note a couple of of the an essential differences v stand-to-sit.

## Some general body physics

At the heart of every actions, according to Newtonian mechanics, are forces. A pressure is a press or a pull. As you relocate through the world, every readjust in her state of motion is associated with a arsenal of forces that cause the change. These pressures can be separated into outside forces which plot on the body from outside and also internal forces which act within the body.

Two type of external forces impact you, gravity and contact forces. Gravity pulls every part of friend down in the direction of the ground. Many of us know this. Less renowned is that the ground additionally pushes ~ above you through a contact force. If you space sitting in a chair right now both the chair and floor are pushing increase on your bum and also your feet. An additional contact force is friction which prevents your feet from sliding together you stand up. As the surname implies, call forces only work-related when the objects are in call with every other. By comparison gravity is constantly pulling girlfriend down, even if you space jumping in the air.

You can visualize the effect of the external forces by imagining what would happen if they were not there. If heaviness were no there, the slightest motion would send your body floating up turn off the ground. If the call forces were not there friend would fall down through the floor together if it had actually no substance, or in the case of friction you would certainly not be able to avoid sliding any kind of which means along the floor.

Internal forces are generated inside the body by bones, muscles, tendons, ligaments, and other tissue. Few of these pressures consume energy, such together the pressure from muscles. Rather consume no energy, such together the forces from bones, ligaments, and tendons. Without the internal forces from the body, it would certainly collapse as result of the outside forces gravity and contact forces.

The balance of all of these forces, both internal and also external, determines whether and also how motion will happen. Well balanced forces are connected with a state that either remainder or continuous motion. Imbalanced forces are linked with readjust in the state that motion. To comment on how this works we must very first define momentum.

The momentum of things describes both the motion and its resistance to adjust of motion. The much more mass and/or speed things has, the an ext momentum. A car passing a cyclist has an ext momentum both because it has more mass and because that is travelling faster. The car will thus be lot harder to slow down than the bike. Momentum also has direction. A auto heading south and an identical automobile heading north at the exact same speed have opposite momenta. The human body is a bit more facility than the automobile because it deserve to fold, bend, and also twist in so countless ways. In motion, the different parts that the body have the right to be relocating in different directions with different speeds and also masses. Hence every component of the body can have its very own momentum. For example at the beginning of the tipping phase of sit to was standing the head has actually a forward momentum if the feet have hardly any momentum

Forces and momentum room intimately related. A change in momentum needs a net force. If a moving auto tries to unexpectedly skid to a stop, an external force—the frictional force from the soil rubbing on the tires—is necessary to slow-moving it down. The tendency for objects to resist alters in momentum is dubbed inertia. In the instance of the automobile trying come stop, if the roadway is really icy and also slippery the automobile continues forward due to the fact that the frictional force is gone and inertia maintains that motion. In sit come stand, a combination of forces, both internal and also external, are required to rate up, slow-moving down, or change the direction of the various parts in motion, when inertia deserve to be supplied to lug a motion forward.

The critical terms to know about are torques and rotational motion. Much of the motion of the body requires rotation of objects roughly joints. In these cases, it can be simpler to talk around torques rather than forces and also rotational momentum/inertia quite than straight momentum/inertia. Precisely defining these terms goes past the border of this article. But for our objectives it is enough to remember the torque and also rotational momentum/inertia room to rotating objects what forces and also momentum are to objects moving in a directly line.

## Slow, smooth sit to stand

We start our discussion of sit-to-stand by considering an extremely slow continuous movement. Picture a Tai Chi understand moving very slowly. Not only is the activity slow however there are likewise no jerks or sudden shifts during the movement. In this case the momentum and also changes in inert are practically zero. With small momentum, all pressures must stay about balanced transparent the movement. This way that together the movement gradually flows forward, the mover deserve to pause at any moment through no adjustments and stay in balance. This absence of momentum and subsequent balance of forces makes the physics easier. That also way that the direction of the motion does not influence the physics. I.e. Really slow sit-to-stand and stand-to-sit are, from a physics perspective, identical. Physicists call this type of incredibly slow smooth movement “quasistatic”, which method practically no moving.

Game 1: shot moving quasistatically from sitting to standing and also vise versa now. Be sure that you room really moving progressively with no jerks and that you are never ever “throwing” yourself or falling from one balance point to another. Inspect that you have the right to stop at any kind of moment v no adjustment. Shot in details to eliminate any kind of jerks at the minute that her bum leaves the chair (in this post we will call this moment liftoff).

Quasistatic motion requires that your center of massive stay over your basic of assistance at every times—i.e. That you continue to be continually in balance. Approximately speaking, the facility of mass is the point around which massive is evenly distributed. In the sitting position it is located roughly around the belly button yet it will transition its place as the body alters shape during the movement. Only once the center of mass is directly above its base of support have the right to the thing balance in ~ rest. The basic of support in sitting is the area characterized by the sit bones and the two feet, if in standing it is the single rectangle defined by the outer edges that the 2 feet. To remain in balance transparent the slow movement, you must first move your facility of massive forward, across the sitting base of support,until that is above your feet. Only then have the right to you lift her bum off from the chair there is no falling. If you try to liftoff prior to the facility of fixed reaches the feet you will certainly either should suddenly lurch forward in preparation (to create forward momentum) or rather you will loss backward come the chair.

Fig. 1. Cartoon of sit to stand v the center of mass (green dot) for 2 positions, prior to the activity starts (left) and also just after liftoff (right). In quasistatic motion the facility of massive must move to above the feet before you deserve to stand.

Getting the center of mass over the feet is achieved by tipping the trunk (Fig. 1). The closer the feet are to the chair the less the trunk demands to pointer forward to get the facility of mass above the feet. The tribe tilt at lift off is additionally reduced when the load is in her heels rather than the center or balls the the feet. One benefit of sit-to-stand contrasted to stand-to-sit is that the distance between the bum and also the feet can be chosen before the motion thus specifying what kind challenges that the mover will certainly face.

Game 2: try quasistatic sit-to-stand now with various feet positions. Remember to move an extremely slowly and smoothly transparent the movement. Notice that the further the feet room positioned from the chair the additional you must tip forward and also the more challenging it is to maintain quasistatic movement. Notice what changes when girlfriend initiate liftoff ~ above the heels that the feet as opposed come the balls that the feet. Now shot quasistatic was standing to sit. An alert that the depth the the trunk tilt determines how far back the bum have the right to land ~ above the chair.

We have the right to think the the slow movement as a force corresponding task. Gravity would certainly collapse the spine forward and fold the body at the hips and knees (flexion) to be there no interior muscle action. To protect against this collapse and control the movement, activation the extensor muscles is essential in the trunk, hip, and also leg to complement gravity. Keep in mind that in principle noflexoractivation of the trunk, hip, or knee flexors is required for the movement to happen. Up v liftoff gravity drives the movement and also the extensors regulate it. In technical terms, the hip and also knee extensor muscle contractions room eccentric. After ~ the pelvis pipeline the chair, these extensors continue to occupational to open the body to the was standing position. Therefore both the tipping forward phase and the unfolding to standing step require consistent extensor activity.

Tipping the trunk to transition the facility of mass above the feet shifts more weight come the feet, advertise the feet onto come the floor. This is balanced by the external call force native the floor upwards. This call force have the right to be measured through a toilet scale.

Game 3: You have the right to feel and/or measure the call force from the ground. Location a bathroom range under your feet and also watch the contact force, measure by the scale, increase as the feet room weighted (you require an analogue range or a digital scale that continuous updates). As soon as the sit-to-stand is performed slowly and also smoothly friend (or a friend) can track the continuous increase till you reach about your body weight at liftoff. If you don’t have a scale, simply turn your fist to the changing pressure on the bottom of the feet.

Interestingly, friend can likewise weight and unweight the feet there is no tipping the trunk. Generating a horizontal force with the feet versus the floor, either forward or backwards, generates horizontal frictional forces from both the floor and the chair. These create torques that either press down or lift up the feet. That is past the border of this article to describe the physics that this in an ext detail, yet you can conveniently feel this happening and/or measure it with a scale. Perhaps you can figure out specifically what is walking on yourself.

Game 4: Sit in the chair with your feet ~ above the floor or top top a range and try to slide your feet forward (knee extension) while keeping your feet grounding to the floor (don’t actually relocate the feet). You will certainly feel and/or measure more weight come ~ above the feet. Likewise if you pull her feet backwards you will certainly lesson the weight of the feet ~ above the ground.

This type of static weighting and also unweighting the the feet without adjust of position is unnecessary in sit come stand. As you competent in video game 3, the is sufficient to carry the facility of massive forward because that the feet to it is in weighted. No extra advertise is needed. Furthermore, if the bum leaves the chair when this extra horizontal pressure is quiet active, the human body will all of sudden be either thrust backwards or forwards by the horizontal force from the floor.

While a student have the right to weight the feet without tipping forward she will certainly never be able to stand quasistatically without obtaining the facility of mass above the feet. If she tries, she will simply fall earlier into the chair. Yet if a teacher’s hand is carrying a pressure to keep the student from falling backwards, she can indeed gradually rise increase vertically. In this case the call force from the ground can be much more than the body load at elevator off relying on the feet position. In ~ liftoff, the student’s feet have to generate both a horizontal backward force to accomplish the pressure of the teacher’s hand and a vertical force to fulfill the full weight that the body. The vector sum of this two forces adds up to an ext than the student’s weight.

This hand assisted movement poses part interesting obstacles for both the student and also the teacher, beyond just the new of the trajectory. The college student is tested to continuously complement not only gravity and also the contact forces native the soil but likewise the force of the teacher’s hand. The teacher is also challenged; giving a secure force enhance to the college student is quite a subtle task. Both student and teacher need to smoothly ramp up the equivalent forces together, v no jerks or inert generating strategies. In ~ liftoff itself, the student demands to create a pressure greater 보다 their human body weight. Finally, every one of these challenges need come be completed while minimizing cocontraction (stiffening that the joints) which would just include unnecessary resistance to the motion.

In general, once the teacher pushes the college student forward with a hand that can readjust the physics fairly dramatically, enabling the teacher an ext control that the use of momentum and the pressures at liftoff.

## Using momentum smoothly

Thus much we’ve been pointing out the quasistatic limit. But we commonly use a fair little bit of inert to increase up out of a chair.

From a balance perspective, the role of momentum have the right to be compared to throw a round to a friend. The round is an initial in static balance in your hand. The movement of your hand generates momentum in the ball. This momentum allows it come fly off its support and also then land and decelerate ago into balance in the hand of your friend.

Let’s see how this image applies to smoothly making use of momentum to quickly rise from a chair (Fig. 2). You begin by permitting gravity to generate momentum in the torso by maintaining the hip extensor activity low during the start of the tipping phase. The forward momentum the the trunk enables you to launch the trunk from the chairbeforethe facility of mass is over the feet. After ~ liftoff, momentum carries the human body forward while the rearward, turn off balance, position of the center of massive slows this motion so the the facility of mass involves a soft landing over the feet. This usage of momentum accelerates the as whole movement and also limits the amount the the trunk needs to reminder forward. You have the right to generate plenty of inert this way, using just gravity v no net activation the the flexors, to wake up in about 1.5 seconds (Cacciatore 2014).

Fig. 2. Inert generated during the tilt phase, between the left and also middle position, enables liftoff to happen before the center of fixed is above the feet (middle figure). The center of mass “lands” over the feet together the motion slows down (right). A call force from the teacher’s hand ~ above the ago can have actually a similar effect.

Adding inert to the movement likewise requires extr forces because that overcoming inertia. Together mentioned, transforming the inert of an object, it is in it at rest or moving, calls for a force. In the round throwing analogy both slowing down and speeding up the round both require contact forces indigenous the hands. Likewise, a rotating thing will require torques to rate or slow down rotation. For example a lengthy springy rod being waved ago and soon bends and also springs back. Inertia produced by momentum bends the rod while the springiness the the pole resists bending.

Just favor the ball and the rod, increasing and decelerating the stems in sit to stand calls for forces. Acceleration throughout the tipping front phase have the right to be completed using heaviness alone. In this situation the trunk falls forward as result of release the hip joint resistance. Deceleration is then forced to extend into liftoff. This deceleration calls for extensor task in the back and hips to conquer both gravity and slow the flexion motion. Favor the springy rod, tipping activity the trunk creates forwards inertia which often tends to flex the spine as shortly as the i know good extensors slow-moving the tipping motion. The earlier extensors prevent this flexion hence maintaining spinal size throughout liftoff.

Thus extensors need to smoothly both to readjust to gravitational forces and to get over the inertia of the trunk. If the extensors under or over compensate the mover will either collapse or arch the earlier respectively. Utilizing momentum is a tradeoff: the much more vertical tribe reduces the gravitational torques but extr extensor torques are necessary to slow the body’s forward momentum.

Game 5: move from sit-to-stand very first quasistatically, and then making use of momentum smoothly. Notification that because that the same foot position, the trunk deserve to tip less forward when using momentum. Notification however that substantial direction of the spine is required to protect against the trunk from collapsing in both cases—in the quasistatic case to withstand gravity and also in the momentum situation to furthermore resist front inertia.

Momentum offers second range of difficulties for mobilizing and also stabilizing the body versus external forces in various positions. Relying on what the teacher desires to test, the student deserve to be coached or guided v the motion along a range of trajectories at different speeds.

While momentum have the right to be offered to traject the facility of mass with positions of revolution instability, similar strategies space not therefore smooth when sitting down. If the student throws himself off balance together he heading come the chair he has actually no selection but to land v a thump ~ above the chair. We will leave the physics that this because that the reader to think around further.

## Using extra momentum

Often a student will certainly generate a burst of momentum even when they are clearly instructed to move smoothly. Having actually a student do sit-to-stand very slowly can make this evident. Despite the instruction come stand slowly the college student lurches at the moment just prior to liftoff. One plausible explanation for this is that the student depends on the burst of inert to get over stiffening in the i know good joints, efficiently throwing himself v the allude of stiffness <1>. In stand-to-sit a similar pattern causes the college student to fall back into the chair rather than landing smoothly. The alternative, in this model, would certainly be to complimentary the i know well joints to enable the movement to happen without the extra burst. This is quite a challenge for many students, especially in the excessive feet front position.

Another excess use of momentum in sit-to-stand is the activation that hip and also knee flexors throughout the tilt phase in stimulate to advice the tribe forward. This strategy is not crucial even for a brisk paced movement. Gravity increases the trunk v plenty of rate to was standing up conveniently without activating flexors <1>.

Ever have a student the pulls the chair with them in ~ liftoff so that the chair advice forward? The college student is likely pulling the pelvis forward (and help to flex the hip) utilizing knee flexion. This can be thought of together either a momentum trick—to litter the trunk quickly above the feet after ~ liftoff—or a sliding trick—the college student chooses to slide the height of the chair with them to gain the center of mass over the feet quite than simply bending at the hips. Or it might be a remnant of the fabricated pushing or pulling the the legs the we questioned earlier. Either way it is a strangely counterproductive activity since knee flexion additionally unweights the feet.

Sometimes a teacher will take a student’s stems backwards native vertical prior to taking castle forwards into the motion. This deserve to be pedagogically very useful for emphasizing a number of aspects of great use (for instance testing the integrity of the trunk and freedom that hips in a behind tilt position). It likewise challenges the stems flexors, rather than extensors, to counteract the collapsing pressure of heaviness on the spine. But the teacher need to realize that they may be generating extra momentum at the vertical position that would certainly otherwise no be there. It permits the college student to increase up v a more vertical stems than would be possible without the extra inert generating preparation.

Most teachers intuitively usage a variety of pathways to take the college student from sit-to-stand—slow, fast, deep, shallow, with and also without behind tilts, with and without support from the teacher’s hands. This selection allows the teacher to location a wide range of needs on the student’s system and thus check out the student’s use and weaknesses therein from a variety of perspectives.

## Discussion

Sit-to-stand exam students’ an answer to gravity, momentum, and also contact forces. Carry out they collapse? execute they stiffen? perform they usage momentum efficiently? What mechanical strategies room they employing? The answers to this questions give both the student and also teacher understanding into fundamental psychophysical patterns. In this short article we have actually aimed come clarify the mechanical side of this questions.

Other components in sit-to-stand beyond the border of this short article include the usage of the arms or tribe flexion/extension to generate momentum and also transfer forces and also the duty of the fishing eye flexors and extensors to surname a few. Nor have actually we discussed any type of underlying neuromuscular instrument or pedagogical methods which are at the heart of Alexander method and certainly an ext mysterious indigenous a clinical perspective.

The straightforward physics can be very useful once working with students. For instance understanding the quasistatic an obstacle can do postural problems and also moments the extra lurching an ext evident because that both the student and also the teacher specifically when merged with playing through the position of the feet. Expertise the range of difficulties that can be introduced by an altering the speed of movement, place of feet, liftoff moment, and also varying the push from the teacher’s hand can additionally bring diversity to a lesson. Finally the language that physics deserve to be valuable for interacting with certain kinds the students and also teachers.

The physics additionally gives insight into the complexity of the matching task. Perfectly equivalent the various pressures at the feet, chair, and also teacher’s hands requires requires consistent smooth mediate of the totality body’s response. Offered this complexity, that is unlikely the the student can finish the task by simply choreographing the movement—i.e. By consciously deciding what and when to press or pull. Any type of imperfectly matched pressures will an outcome in small jerks in movement or the level the resistance i beg your pardon the teacher or student will have the ability to either watch or feel. That is as such likely that the response being trained wake up at all levels of the student’s system, from reflexive to higher level fine processes. Speculating further about the nature that this learning procedure is past the scope of this article.

See more: 1) Given The Arithmetic Sequence An = 4 − 3(N − 1), What Is The Domain For N?

Finally, knowledge the physics clarifies how various positions and executions challenge the body in various ways. In the past there have been debates amongst teachers from different streams around the “best” pathways and also feet location in sit-to-stand. Grasping the physics can overview discussion an ext towardshowone meets the various challenges, rather than the particular parameters the the difficulty itself.

Game 6: take a moment to think around the different sit to stand pathways you have actually your student execute throughout your lessons. Carry out you highlight details challenges an ext than others? are there any pathways that you might explore to an obstacle your student in brand-new ways? How could you incorporate few of the language the physics come appeal to your more technically minded students?

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