This essay by Tim Cacciatore discusses his 2014 study, “Neuromechanical Interference of Posture on Movement: Evidence from Alexander Technique Teachers Rising from a Chair,” published in the Journal of Neurophysiology. The study documents the striking difference between healthy, untrained subjects and Alexander teachers in standing up unusually slowly and smoothly from a chair. Alexander teachers had little difficult standing smoothly, but healthy, untrained subjects couldn’t help but lurch out of the chair. Why is this? And what does the difference between the two groups say about how postural coordination facilitates or interferes with movement? The essay was originally published in AmSat Journal in 2015.
Our experimental study “Neuromechanical Interference of Posture on Movement: Evidence from Alexander Technique Teachers Rising from a Chair,” was recently published in the Journal of Neurophysiology (Cacciatore, Milan, Peters, & Day, 2014). We studied whether healthy adults without any Alexander Technique training could mimic the smooth chair rise previously observed in Alexander Technique teachers, and if not, why not? The results have implications for movement neuroscience as well as the Alexander Technique.
Our previous work showed that untrained adults shift weight abruptly in comparison to Alexander Technique teachers (Cacciatore, Gurfinkel, Horak, & Day, 2011). However, it wasn’t known whether this was due to habit or choice, or whether untrained adults are in fact unable to shift weight as smoothly as Alexander Technique teachers can. To address this question, we created a clear and constrained task where subjects were instructed to rise from a chair very slowly at a uniform speed (the whole movement taking up to 8 seconds). Slow movements reduce momentum, so this task would highlight any tendency for untrained subjects to move abruptly.
Subjects started from a set of standard seated positions with their feet in each of three locations (back, mid, and forward) and performed a range of speeds (1, 2, 4, and 8 seconds). Feedback was given after the trial to ensure consistent movement times. Movements were recorded with motion analysis and force plates under the seat and feet. An inverse dynamic model was constructed of each subject; this enabled a relatively complete mechanical analysis of their movement, including the movement of all body segments and the timecourse of joint forces, torques, and whole body center-of-mass.
Results
While both groups could perform chair-rises at speeds that closely matched the instructed movement times, there were clear and profound differences between groups, especially for slow (4, 8 seconds) movements with the feet forward. For these movements, the teachers moved the body forward slowly and evenly while gradually increasing weight on their feet. In contrast, untrained adults began by slowly inclining the trunk, but then abruptly sped up and weighted their feet just before liftoff. (See Figure 1) This lurching motion is of course familiar to Alexander Technique teachers as “jumping out of the chair.” Even for faster chair-rises, untrained adults used roughly twice the forward body velocity and foot-loading rate as the teachers. Untrained adults were well aware of their abrupt coordination prior to liftoff and were told repeatedly by the experimenters to prevent it, but they were still unable to rise smoothly from the chair.
Further analysis revealed that untrained adults’ lurch at liftoff (increased forward velocity and rate of foot loading) was closely related to distance between the body’s center of mass and under-feet pressure at liftoff. The further back the subject was at liftoff, the greater the velocity required to bring the mass forward and balance over the feet, and thus the greater the jump out of the chair. This simple relationship stems from the physics of balancing, and a single linear fit explained all the data extremely well—including data from both the untrained subjects and the Alexander Technique teachers (accounting for 91.5 % of the variance). Within this relationship however, untrained subjects were shifted towards more rearward liftoff positions, faster feet-loading rates, and faster forward velocities in comparison to the teachers. This suggests that the difficulty experienced by untrained subjects in rising smoothly was due to an inability to move their bodies forward at liftoff without using momentum.
So the question then becomes: Why didn’t untrained subjects merely lean further forward and rise smoothly? Their difficulty was not due to range of motion, as the range required was well within normal limits. It was not due to strength, as both groups used similar hip and knee joint extensor torques, and, moreover, the more difficult, slower movements required less hip and knee extensor torque than the faster ones. While a standard Alexander Technique answer might be that untrained subjects were end-gaining, untrained subjects produced the same movement times as teachers, which suggests they were not rushing. (This is not an argument against end-gaining, just that other factors are involved). More importantly, the “lurch” parameters (forward velocity and feet-loading) fluctuated closely and systematically with foot position and movement speed, which suggests that mechanical factors contributed to the difficulty in rising smoothly. To investigate this more closely, we developed a computer simulation.
Model Simulation
Previous work has shown that postural stiffness differs between Alexander Technique teachers and untrained subjects (Cacciatore, Gurfinkel, Horak, Cordo, & Ames, 2011). We therefore explored how stiffness affects chair-rise coordination. To do this, we constructed a forward neuromechanical model of the body (different from the inverse models described above) so that we could systematically vary stiffness and examine the effect of the change on sit-to-stand coordination. The model could be made to stand up successfully by activating simulated muscles with realistic timings (Figure 2A). However, even with the minimum stiffness, there was substantial resistance to forward movement from activating hip and knee extensors to take the weight of the body by liftoff. Thus, there was an inherent conflict between achieving forward balance over the feet and supporting the body weight against gravity that peaked at liftoff. We found that stiffness had a marked effect on the model’s coordination by exacerbating this conflict: Increasing hip or knee stiffness by as little as 5% further hindered hip or knee flexion and prevented the model from achieving forward balance over the feet, causing it to fall backwards into the chair at liftoff (Figure 2B).
We also studied the effect of trunk stiffness on the model, as Alexander Technique teachers exhibit less spinal bending than untrained adults during weight shift ((Cacciatore, Gurfinkel, Horak, & Day, 2011), and thus likely have a stiffer spine during this behavior. We found that trunk stiffness could cause sit-back failures when it was either too high or too low: Very high trunk stiffness reduced forward movement by hindering forward trunk flexion, whereas low trunk stiffness prevented the transmission of upper body forces across the spine, reducing the force that flexes the leg joints during weight shift.
Can stiffness account for untrained adults’ need to lurch when leaving the chair? We addressed this question by testing whether greater forward velocity and delayed, abrupt weight shift could compensate for higher stiffness and make the stiff model stand successfully. Indeed, delaying weight shift in the model increased its forward momentum and thus the driving force that flexed its hips and knees. This higher driving force was sufficient to stretch the stiffer hip and knee extensors, as they supported the body during weight shift, enough to carry the model’s body mass forward over the feet, resulting in a successful chair-rise (Fig 2C). Thus, untrained adults’ larger forward velocity and abrupt weight shift may be necessary compensations for their greater leg stiffness.
Conclusions
Up through liftoff, slow (>2 seconds) chair-rises are essentially passive. The energy to bring the body forward comes not from muscles but from gravity, which can also accumulate as forward momentum. Thus, the body’s resistance (stiffness) determines the first half of sit-to-stand. The neural control of resistive behavior is special in that it can result solely from reactive rather than predictive neural control. Our discovery that postural stiffness (i.e., related to anti-gravity support) can explain untrained adults’ movement coordination suggests that postural mechanisms contribute to this resistance (which we call the postural frame) and, importantly, that posture can act to interfere with movement. It is interesting that the optimal stiffness distribution for smoothly rising from a chair is for a relatively stiff trunk to transmit forces across the spine to leg joints with low stiffness to minimize the resistance to movement. This is consistent with Alexander’s description of the ideal passive seated behavior: moving “just as a door moves on its hinges” about the hip joints (Alexander, 1923), which precisely describes high trunk stiffness and low leg-joint stiffness. While the neural mechanisms that underlie postural stiffness are not well understood, they likely include low-level reflexive mechanisms and spatial processes such as the body scheme (Gurfinkel, 1994) and may include high-level cognitive processes like executive function (Weiss, C., Tsakiris, M., Haggard, P., & Schutz-Bosbach, S., 2014).
We expect that postural stiffness shapes movement in general and not just sit-to-stand. Most of the main Alexander Technique procedures (sitting down in a chair, “monkey,” “squat,” “lunge” and “going onto toes”) have a critical moment where anti-gravity and balance constraints conflict, as they do in rising from a chair. Stiffness would thus presumably have a similar effect on these other movements. It is possible that these procedures are particularly useful in the Alexander Technique due to the conflicts, as they may act to highlight the effect of postural stiffness on movement.
In summary, this study: 1) provides strong evidence that untrained adults cannot mimic Alexander Technique teachers’ smooth sit-to-stand coordination; 2) provides a plausible mechanism whereby leg and trunk stiffness explains the abrupt coordination of untrained adults just before liftoff; 3) suggests that the Alexander Technique affects movement coordination through postural stiffness; and 4) provides basic evidence that the postural system can interfere with movement coordination. Stronger evidence for the hypothesis that postural stiffness interferes with a smooth weight shift could be obtained by studying the relationship of an individual’s leg and trunk stiffness to his/her lurch out of the chair. Causal evidence could be obtained by altering postural stiffness and observing a change in movement coordination. This appears analogous to chair work where postural and tensional adjustments on a seated student tend to decrease lurch at liftoff.
References
Alexander, F.M. (1923) Part II, Chapter IV, Illustration in Constructive Conscious Control of the Individual. New York: EP Dutton & Company. (p. 112)
Cacciatore, T.W., Gurfinkel, V.S., Horak, F.B., Cordo, P.J., & Ames, K.E. (2011). Increased Dynamic Regulation of Postural Tone through Alexander Technique Training. Human Movement Science 30. (pp. 74–89).
Cacciatore, T.W., Gurfinkel, V.S., Horak, F.B., & Day, B.L. (2011) Prolonged Weight-shift and Altered Spinal Coordination During Sit-to-stand in Practitioners of the Alexander Technique. Gait & Posture 34. (pp. 496–501).
Cacciatore, T.W., Mian, O.S., Peters, A., & Day, B.L. (2014). Neuromechanical Interference of Posture on Movement: Evidence from Alexander Technique Teachers Rising from a Chair. Journal of Neurophysiology 112. (pp. 719–729).
Gurfinkel, V.S. (1994). The Mechanisms of Postural Regulation in Man. Soviet Science Reviews, Section F, in Physics & General Biology 7 (pp. 61–89).
Weiss, C., Tsakiris, M., Haggard, P., & Schutz-Bosbach, S. (2014). Agency in the Sensorimotor System and Its Relation to Explicit Action Awareness. Neuropsychologia 52 (pp. 82–92).