The motor control test (MCT) and adaptation (ADT) tests are assessments of the automatic postural control system at the sub-cortical level (basal ganglia and cerebellum) in response to horizontal (backward/forward) translation or rotational (toes up/toes down tilt). They measure:

response latency, the time between the perturbation and the body’s response to maintain balance.
symmetry between left and right sides.
central motor programming, the brain’s ability to organize movement into patterns.
the body’s ability to compensate for these perturbations.
postural sway, the coordinated small forward and backward movement of the body around the center of mass

The automatic single-loop spinal reflex pathway (M1) is the most rapid, while M2 and M3 are long latency reflex loops involving multiple connections (synapses) and pathways to the brain (primary motor cortex and cerebellum)[1,3,4,5]. Increasing the number of connections increases the latency time. These semi-automatic, long-latency movement patterns take place before voluntary action occurs. The semi-automatic triggered reaction is the ability to inhibit a movement once it has been started; for example, braking your car quickly once you made the decision to “go” in traffic. Because central motor programs modify the feedback reflex reactions, perturbation training can improve these semi-automatic responses[4]. Table 1 describes the sequence of events following perturbation in this synthesis of the work of two giants in the world of motor control: Nikolai Bernstein and R.A. Schmidt[1,3].

Bernstein’s Brain Skyscraper	Schmidt’s Category	Latency Range	Complexity
Sub-cortical	M1	30-50 ms	monosynaptic stretch reflex
Sub-cortical	M2	50-80 ms	Spinal reflex Supraspinal (1o Motor cortex, cerebellum) semi-automatic long-loop reflex
Sub-cortical/cortical	Triggered Reaction	80-120 ms	Motor habit, pre-programmed responses occurs too fast to be voluntary
Cortical	Voluntary Reaction Time M3	160-180 ms	Voluntary action

Table 1: Sequence of motor behavior during automatic, semi-automatic, and voluntary postural control[1,3,5]

Figure 1: Timeline: onset of sequence of events in postural motor control. (Adapted from Latash, 2012; Horak & Nashner, 1986)

How do we measure pre-programmed reactions to postural perturbations?

The motor control test (MCT) measures the long-latency responses (M2-M3, triggered reaction) that occur after the forceplate moves. The MCT latency scores are based on the rapid changes in the CoP that occur when postural muscles respond to control the COG after abrupt forceplate translations. The Bertec forceplate sampling frequency of 1000 times/second captures 120-140 CoP samples during a 120-140 ms latency interval, making it highly sensitive in detecting the high frequency components of CoP motion.

Can we train semi-automatic programs?

In a word, yes.

These central motor patterns can be modified by prior experience (e.g., when we take an extra step at the bottom of a staircase while expecting another stair)[4] or instructions given before training. There is evidence to support specificity of training: to improve these semi-automatic postural reactions, training must replicate the postural demands of the motor control task. Perturbation training shifts neuromuscular responses from hip-knee strategies to ankle strategies without changing M1 latency times, meaning that training has resulted in a more efficient postural control strategy to respond to sudden balance challenges[6]. Perturbation training using tilt boards, balls, and foam to change the surface or by pushing on the body often result in exaggerated protective responses[7] but fail to address environmental context or perceptual challenges to balance. Immersive virtual reality (IVR) force plate balance training programs overcome these limitations by allowing for precise selection of training variables and measuring changes otherwise invisible to the eye.

What postural motor control questions should I consider in planning treatment?

Motor control and postural stability depend on the interaction between each of these domains8:

Biomechanical: strength, limits of stability; what strategy is used to maintain balance: ankle or hip?
Movement strategies: are they reflexive, voluntary, or somewhere in between?
Sensory strategies: how do sensory integration and reweighting between somatosensory, visual, and vestibular systems occur?
Spatial orientation: accuracy of perception and verticality (see the recent blog post by Dr. Charlie Shearer: Subjective Visual Vertical (SVV) and risk of fall (ROF))
Dynamic control of functional, gait and sport movements
Cognitive processing in complex environments and divided attention; how does this impact dynamic postural stability in daily life and sport?

Our next blog will discuss how to design and implement immersive virtual reality (IVR) perturbation training programs to improve semi-automatic responses so crucial in fall prevention and balance training programs. For an in-depth look on how to conduct voluntary and centrally programmed motor control tests, visit Bertec’s Interactive and On-Demand Education offerings.

References:

1. Latash, M. L. (2012). Fundamentals of motor control. Academic Press.

2. Magill R.A., & Anderson D.I.(Eds.), (2018). Motor control theories in Motor Learning and Control: Concepts and Applications, 11e. McGraw Hill. Available from: https://accessphysiotherapy.mhmedical.com/content.aspx?bookid=2311&sectionid=179409127

3. Petrynski, W. (2016). A scientific evening with N.A. Bernstein and R.A. Schmidt. Kinesiology. 25. 11-24. 10.5604/17310652.1226481. Available from: https://www.researchgate.net/publication/312519120_A_scientific_evening_with_NA_Bernstein_and_RA_Schmidt [accessed 11 Jan, 2023]

4. Forgaard CJ, Franks IM, Maslovat D, Chua R (2016) Perturbation Predictability Can Influence the Long-Latency Stretch Response. PLoS ONE 11(10): e0163854. https://doi.org/10.1371/journal.pone.0163854

5. Horak, F. B., & Nashner, L. M. (1986). Central programming of postural movements: adaptation to altered support-surface configurations. Journal of neurophysiology, 55(6), 1369-1381.

6. Hill, C. M., Wilson, S., Mouser, J. G., Donahue, P. T., & Chander, H. (2018). Motor adaption during repeated motor control testing: Attenuated muscle activation without changes in response latencies. Journal of electromyography and kinesiology: official journal of the International Society of Electrophysiological Kinesiology, 41, 96–102. https://doi.org/10.1016/j.jelekin.2018.05.007

7. Horak, F. B. (1987). Clinical measurement of postural control in adults. Physical therapy, 67(12), 1881-1885.

8. Horak F. B. (2006). Postural orientation and equilibrium: what do we need to know about neural control of balance to prevent falls?. Age and ageing, 35 Suppl 2, ii7–ii11. https://doi.org/10.1093/ageing/afl077