Spinal Control. The Rehabilitation of Back Pain

309 Pages·2012·19.517 MB·English

by  Paul W. Hodges, Jacek Cholewicki and Jaap H van Dieen (Eds.)

| 1 |
Chapter

Introduction: convergence and divergence of
opinions on spinal control
Paul W. Hodges*, Jaap H. van Dieën† and Jacek Cholewicki‡
*NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, University of
Queensland, Brisbane, Queensland, Australia, †MOVE Research Institute Amsterdam, Faculty of Human Movement Sciences, VU University
Amsterdam, Amsterdam, The Netherlands and ‡Department of Osteopathic Surgical Specialties, College of Osteopathic Medicine, Michigan State
University, East Lansing, Michigan, USA
Low back and pelvic pain is a major issue facing the
CHAPTER CONTENTS
modern world. The economic burden of musculoskeletal
Models of spine control and experimental pain is second only to cardiovascular disease (Australian
approaches 2 Bureau of Statistics 2001) and of that burden, spinal com-
Motor control 2 plaints contribute the greatest percentage due to long-term
disability. Low back pain (LBP) is the most common
Proprioceptive systems 3
chronic pain in Australia (Blyth et al. 2001), and the most
Spinal control as a basis for design of
common work-related condition in Western society. Recur-
clinical treatments for low back and
rence and persistence of symptoms are major issues in LBP
pelvic pain 3
and are associated with the majority of its health care and
Convergence and divergence of opinions social costs. Persistent LBP is increasing and its prevalence
in ‘spine control’ 3 has doubled in the last 14 years (Freburger et al. 2009).
References 4 Although clinical guidelines promote the view that acute
LBP has a favourable prognosis with most people recov-
It is well defended and accepted that control of the spine ered in 6 weeks (Koes et al. 2001), systematic reviews of
and pelvis depends on the contribution of active, passive prospective trials suggest that 73% of people experience at
and control systems (Panjabi 1992). In this interpretation least one recurrence in 12 months of an acute episode, and
of spine physiology, ideal control relies on the appropri- pain and disability have only recovered by 58% at one
ate passive support, supplemented with muscle forces month (Pengel et al. 2003). Further recovery is slow
that are coordinated by the nervous system. Conversely, (Pengel et al. 2003; Henschke et al. 2009). Identification
changes in any of these systems can lead to less than of modifiable factors associated with LBP is a key objective
optimal control and this has formed the basis of a range in the international research agenda. However, reviews of
of rehabilitation strategies that aim to restore control and risk factors provide less than encouraging results (Linton
reduce pain and disability or the potential for further 2000; Pincus et al. 2002). Even factors that have been
pain or injury. Although the theoretical underpinning is purported to have the strongest relationship to outcome,
relatively straightforward, there is variable evidence for such as psychosocial aspects of distress (Pincus et al.
the many assumptions that underlie the understanding of 2002) and job satisfaction (Linton 2000), can only account
‘spine control’ and the way in which it may be modified for a small proportion of the variability (Linton 2000;
with pain and/or injury or the manner in which aspects Young Casey et al. 2008). There is no evidence for an
of spine control may be a precursor to development of association between biological factors such as trunk
pain and/or injury. An area of considerable variation in muscle strength or endurance, or range of motion and LBP
opinion is how this model can be applied to clinical outcome (Hamberg-van Reenen et al. 2007). However, in
practice for the treatment of people with low back and clinical practice and many fields of research, it has been
pelvic pain. proposed that ‘spine control’ is related to low back and
1
Spinal Control: The rehabilitation of back pain
pelvic pain and investigation of this promising notion is models and consideration of systems engineering aimed
worthy of a concerted research effort. at understanding the mechanisms by which the spine is
There are considerable promising data of changes in controlled to meet the demands of everyday activities. A
spine control as a potential candidate factor underpinning key issue is that different models rely on different assump-
the development and persistence of low back and pelvic tions and lead to different conclusions about the optimal
pain from cross-sectional studies (Hodges and Richardson mechanisms for spine control and about the consequences
1996; MacDonald et al. 2009) and some longitudinal of changes in control for the health of the system and,
studies (Cholewicki et al. 2005). Positive outcomes from therefore, lead to different extrapolations from science to
clinical trials, that have been summarized and subjected clinical practice. The first part of this book (Chapters 2–4)
to meta-analyses in a number of systematic reviews (Fer- takes a look at the state-of-the-art research in terms of
reira et al. 2006; Macedo et al. 2009), provide additional modelling and novel experimental approaches that aim to
strength to the argument that consideration of ‘spine provide insight into the mechanisms for control of this
control’ in the management of low back and pelvic pain complex system.
is worthwhile and promising.
The counter argument is that biological aspects are less
important than psychosocial aspects of pain, and that
MOTOR CONTROL
compromised spine control may be present but neither
sufficient nor necessary for the perpetuation of pain. Criti-
cism of the biological model of pain has come from a Motor control is a term that can be used to refer to all
number of sources. For instance, the lack of a one-to-one aspects of control of movement. This can extend from the
relationship between indications of structural damage on motivation within the frontal and other regions of the
diagnostic imaging and pain is commonly used as an argu- brain related to the decision to move, the sensory inputs
ment against the importance of mechanical injury in its to the system that provide information of the body seg-
origin. However, such argumentation could be used simi- ments’ current location and movement, the various levels
larly to deny the relation between smoking and lung of the nervous system that integrate inputs and plan
cancer; not every person with lung cancer is or was a outputs (from simple spinal cord mechanisms to complex
smoker, nor does every smoker develop lung cancer. A supraspinal integration and decision making), the motor
probabilistic model is more appropriate here and struc- output to the muscles (the effector organs of the system),
tural abnormalities are strong risk factors for LBP. and down to the mechanical properties of the tissues
Current evidence suggests we cannot reject the contribu- (including muscle mechanics and passive tissues that
tion of biological issues to development and persistence influence joint mechanics) that influence the manner in
of pain. The quality of ‘spine control’ which determines which motor commands to muscles relate to movement.
the nature and magnitude of loading on spinal structures There are many views of how consideration of motor
is likely to be a key factor in this equation. However, control can be applied to the issue of spine control, and
within the consideration of spine control there are differ- how the nervous system meets the challenge to control the
ent interpretations and opinions. There are differing opin- spine and pelvis when considered in the context of the
ions regarding the most appropriate theoretical models to entire human body function. Drawing on the develop-
understand the systems; this extends to biomechanical/ ments in modelling of spine biomechanics highlighted in
engineering models, neurophysiological models of control Chapters 2–4, this view of spine control involves not only
of motor output and sensory input, and clinical models control of the spine movement and position that is specific
extrapolating from research and clinical practice to formu- to the demands of the task, but also the contribution of
late effective treatments for back pain. This book aims to the spine to other physiological functions such as breath-
provide a state-of-the-art review of the current understand- ing and maintaining whole body equilibrium, to name
ing of these issues, the areas where opinions converge and but a few functions that the nervous system must consider
diverge, and a road map for consideration of how to concurrently.
resolve the critical questions in the field. Perhaps the most debated aspect of motor control, as it
relates to spine control, is how and why motor control is
altered in people with pain and injury. Fundamental ques-
tions remain unresolved. Are there issues in motor control
MODELS OF SPINE CONTROL AND
that can predispose an individual to development of pain
EXPERIMENTAL APPROACHES
and/or injury? Does motor control adapt in response to
pain and injury or is this a factor in the persistence and
There are fundamental differences in how people define recurrence of pain? Which aspects of motor control are the
and model spine control leading to different interpreta- most critical for low back and pelvic pain, if at all? Which
tions of what is optimal. Although early models relied on aspects of motor control, if any, should be addressed in
static methods, more recent approaches propose dynamic patients with low back and pelvic pain? Part 2 of this book
2
Introduction: convergence and divergence of opinions on spinal control Chapter | 1 |
(Chapters 5–11) tackles these fundamental issues to perpetuates misunderstanding of the scope of some phe-
provide a comprehensive view of the current state of nomena (e.g. Levin 2002; Lederman 2010). In some ways
knowledge. it may be considered that the indirect debate and criticism
within the field is its own worst enemy.
We have reached a critical point in time at which it is
necessary to consider where the divergence and conver-
PROPRIOCEPTIVE SYSTEMS
gence of opinions lie in order to move the field forward.
Points of convergence require clarification and where
Although sensation is a critical element of motor control, divergence remains, studies must be planned to test the
there are issues related to sensory function that require relative merits of the different ideas. It is possible that one
specific consideration. Deficits in proprioception have hypothesis is correct, that several are correct (but it
been described for many conditions related to pain and depends on the individual patient as to which alternative
injury in the musculoskeletal system. From deficits in the approach applies to them) or the research may lead to
acuity to detect input (Lee et al. 2010), to changes in the generation of new hypotheses.
organization of cortical areas associated with sensory func- A major aim of this book is to present the arguments
tion (Flor et al. 1997). A glaring issue in the low back and and consider the areas for divergence and convergence in
pelvic pain literature is why do some studies report differ- opinions. The state-of-the-art evidence on efficacy of exer-
ences in sensory function between patients with low back cise interventions for low back and pelvic pain is outlined
pain and healthy control subjects, whereas others do not? in Part 4 (Chapter 15), whereas the foundations for dif-
This could be explained by many reasons: differences ferent clinical ideas is presented throughout Chapters
between patient subgroups, differences between specific 2–14 with references to research and the justification for
parameters of sensory function that have been studied, the extrapolations that have been made from basic science
or other methodological issues (e.g. sample size and to clinical practice.
reliability/validity of measures). Resolution of this issue
and other issues (such as the question of which sources of
sensory information are used in the control of the spine,
CONVERGENCE AND DIVERGENCE
and how this is used) requires deeper understanding of
OF OPINIONS IN ‘SPINE CONTROL’
sensory function as it relates to the spine and pelvis. Any
extrapolation from research to clinical practice necessitates
an understanding of the state-of-the-art of this field. This The chapters that make up Part 5 (Chapters 16–20) forge
discussion forms the basis of Part 3 (Chapters 12–14). new territory in the debate regarding spine control and its
relevance for low back and pelvic pain. These chapters are
prepared by collaboration between key players in each
SPINAL CONTROL AS A BASIS FOR area of consideration in the book, and draw the line
between the convergence and divergence of viewpoints.
DESIGN OF CLINICAL TREATMENTS
These five chapters help resolve some of the misunder-
FOR LOW BACK AND PELVIC PAIN
standing within the field and provide a unique insight into
what is known, what is unknown and what are the priori-
Perhaps the biggest point of apparent divergence of ties for the future. Key issues that are addressed include:
opinion arises when the findings of research and the 1. Biomechanical modelling and engineering
observations from clinical practice are translated into approaches provide considerable promise to
clinical interventions for the management of low back and understand the relevance of spine control to low
pelvic pain. Many clinical programs have been proposed. back and pelvic pain. But can this information be
On the surface, these approaches have often been viewed used to understand the individual patient and design
as divergent and the unique aspects of each are often treatment? Chapter 16 considers this and other areas
emphasized to amplify points of difference. But how dif- of convergence and divergence of opinion in
ferent are they really? Do they share a common founda- modelling of spinal control.
tion with some specific distinctions based on different 2. Multiple groups are working on the challenge to
interpretations of the literature and clinical observations? subgroup individuals with low back and pelvic pain
Or are they diametrically opposed, mutually exclusive and for the targeting of interventions (Chapter 17).
incapable of being amalgamated into a single broader Although this approach is logical, different
approach? The debate has often been fuelled by presenta- approaches exist, it cannot a priori be assumed that
tion of simplified/reductionist views of an approach to a subgrouping improves outcomes, and there are many
single element (e.g. activation of deep abdominal muscles pitfalls and challenges for the development of
in a lying position) rather than presentation of an entire clinical methods and the subsequent validation of
concept, and work that misinterprets the literature and these approaches.
3
Spinal Control: The rehabilitation of back pain
3. Whether differences in motor control between clinical observations can be extrapolated to effective
patients and healthy controls are a cause or clinical interventions. Chapter 20 makes a major
consequence of low back and pelvic pain requires contribution by highlighting where the views
consideration (Chapter 18). This is not a trivial converge and diverge and presents a road map for
question to resolve as it requires complex how to progress knowledge in this sometimes-
experimental methods, and the fact that changes in contentious issue.
motor control could be either, neither or both cause Finally, Part 6 of the book (Chapter 21) highlights all that
and consequence. has been gained from identification of the state-of-the-art
4. If and how sensory function is affected in low back of understanding across the field of spine control and
pain and injury, if this is relevant for development or discusses ways that this has been applied to the design and
persistence of symptoms, and how this could be implementation of treatment for people with low back
addressed in low back and pelvic pain is far from and pelvic pain. The result is a multifaceted approach to
resolved. This issue is debated in Chapter 19. optimization of motor control that considers individual
5. An issue of considerable discussion in the literature differences within a multi-dimensional framework that
and a topic of surprising convergence of includes consideration of the bio-psycho-social model
fundamental concepts – but also considerable of pain.
divergence of opinions – is how experimental and
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Australian Bureau of Statistics, 2001. Henschke, N., Maher, C., Refshauge, K., keep hurting their back? Evidence of
Musculoskeletal Conditions in Herbert, R.D., Cumming, R., Bleasel, ongoing back muscle dysfunction
Australia: a snapshot. J. et al., 2009. Prognosis in patients during remission from recurrent
Blyth, F.M., March, L.M., Brnabic, A.J., with recent onset low back pain in back pain. Pain 142, 183–188.
Jorm, L.R., Williamson, M., Cousins, Australian primary care: inception Macedo, L.G., Maher, C.G., Latimer, J.,
M.J., 2001. Chronic pain in Australia: cohort study. BMJ 337, a171. McAuley, J.H., 2009. Motor control
a prevalence study. Pain 89, Hodges, P.W., Richardson, C.A., 1996. exercise for persistent, nonspecific
127–134. Inefficient muscular stabilization of low back pain: a systematic review.
Cholewicki, J., Silfies, S., Shah, R., the lumbar spine associated with Phys Ther 89, 9–25.
Greene, H., Reeves, P., Alvi, K., et al., low back pain: a motor control Panjabi, M.M., 1992. The stabilizing
2005. Delayed trunk muscle reflex evaluation of transversus abdominis. system of the spine. Part I. Function,
responses increase the risk of low Spine 21, 2640–2650. dysfunction, adaptation, and
back pain injuries. Spine 30, Koes, B.W., van Tulder, M.W., Ostelo, R., enhancement. J Spinal Disorders 5,
2614–2620. Kim Burton, A., Waddell, G., 2001. 383–389.
Ferreira, P.H., Ferreira, M.L., Maher, Clinical guidelines for the manage- Pengel, L.H., Herbert, R.D., Maher, C.G.,
C.G., Herbert, R.D., Refshauge, K., ment of low back pain in primary Refshauge, K.M., 2003. Acute low
2006. Specific stabilization exercise care: an international comparison. back pain: systematic review of its
for spinal and pelvic pain: a Spine 26, 2504–2513; discussion prognosis. BMJ 327 (7410), 323.
systematic review. Aust J Physiother 13–14. Pincus, T., Burton, A.K., Vogel, S., Field,
52, 79–88. Lederman, E., 2010. The myth of core A.P., 2002. A systematic review of
Flor, H., Braun, C., Elbert, T., Birbaumer, stability. J Bodywork Movement Ther psychological factors as predictors of
N., 1997. Extensive reorganization of [review]. 14, 84–98. chronicity/disability in prospective
primary somatosensory cortex in Lee, A.S., Cholewicki, J., Reeves, N.P., cohorts of low back pain. Spine. 27,
chronic back pain patients. Neurosci Zazulak, B.T., Mysliwiec, L.W., 2010. E109–E120.
Lett 224, 5–8. Comparison of trunk proprioception Young Casey, C., Greenberg, M.A.,
Freburger, J.K., Holmes, G.M., Agans, between patients with low back pain Nicassio, P.M., Harpin, R.E.,
R.P., Jackman, A.M., Darter, J.D., and healthy controls. Arch Phys Med Hubbard, D., 2008. Transition from
Wallace, A.S., et al., 2009. The rising Rehabil 91, 1327–1331. acute to chronic pain and disability:
prevalence of chronic low back pain. Levin, S.M., 2002. The tensegrity-truss as a model including cognitive,
Arch Intern Med 169, 251–258. a model for spine mechanics: affective, and trauma factors. Pain
Hamberg-van Reenen, H.H., Ariëns, Biotensegrity. J Mech Med Biol 2, 134, 69–79.
G.A., Blatter, B.M., van Mechelen, W., 375–388.
Bongers, P.M., 2007. A systematic Linton, S.J., 2000. A review of psycho-
review of the relation between logical risk factors in back and neck
physical capacity and future low pain. Spine 25, 1148–1156.
back and neck/shoulder pain. Pain MacDonald, D., Moseley, G.L., Hodges,
130, 93–107. P.W., 2009. Why do some patients
4
| 2 |
Chapter

Spine systems science: a primer on the
systems approach
N. Peter Reeves and Jacek Cholewicki
Department of Osteopathic Surgical Specialties, College of Osteopathic Medicine, Michigan State University, East Lansing, Michigan, USA
parts and how these interactions affect the behaviour of
CHAPTER CONTENTS
the system as a whole. For instance, predicting the behav-
The rationale for the systems approach 7 iour of a new aeroplane design would be impossible by
Spine (in)stability: an unstable term 8 designing the parts of the system in isolation and not
considering how these individuals parts would interact
What is stability? 8
with one another.
How is stability achieved? 9 Starting in the twentieth century, it became apparent
Performance 9 that society and modern technology was becoming
Robustness 10 increasingly more complex, and as a result, a new approach
was required to analyze and resolve problems. Out of
Lessons from balancing a stick 10
necessity, the branch of science known as systems science
Impairment in feedback control of the
emerged. With a systems approach, it was now possible to
spine 12
study complex systems in a way that not only included
Controller-related impairment 12 their parts, but also how these parts interacted to affect the
Plant-related impairment 13 behaviour of the entire system. Over time, the application
The systems approach applied to LBP 14 of systems science spawned the field of systems medicine,
and with this, expanded medicine beyond the realm of
Final remarks 14
reductionism. Systems medicine is defined as the applica-
References 14
tion of the systems approach to the prevention of, under-
standing and modulation of, and recovery from
developmental disorders and pathological processes in
THE RATIONALE FOR human health (Clermont et al. 2009). It adopts principles
of systems science to focus on uniquely human attributes,
THE SYSTEMS APPROACH
such as genetics, environment and behaviour (Federoff
and Gostin 2009). Benefits of the systems approach
The prevalent method for studying clinical conditions is include individualized, multi-dimensional treatment that
based on a reductionist approach in which the problem is is both time- and space-sensitive, and which can explore
broken down into smaller and smaller parts to isolate ele- synergistic effects (Ahn et al. 2006).
ments of the condition. This type of approach is well Depending on the nature of the clinical condition, both
suited for containable diseases such as local infection, but reductionist and systems approaches can be advantageous.
less helpful when the problem is multi-factorial and more Which approach is better suited for studying low back
dispersed. Reductionism becomes less helpful when the pain (LBP)? One of the landmark hypotheses in spine
process of dividing a problem into its parts leads to a loss research is Panjabi’s proposed stability-based model of
of important information necessary to solve the problem spine dysfunction leading to chronic pain (Panjabi, 1992a,
(Ahn et al. 2006). The loss of information stems from the 1992b). As he stated, the behaviour of the spine system is
omission of the interaction effect between these smaller affected by the interaction between various subsystems:
7
Part | 1 | Models of the spine
the passive subsystem representing the spinal column, the
active subsystem representing the spinal muscles, and the
control subsystem representing the neural elements. He
suggested that impairment in one or more subsystems can
be accommodated by the other systems, but only up to a
certain level. Inability to adequately compensate leads to
chronic dysfunction and pain. This hypothesis has gained
popularity and has led to a paradigm shift in LBP treat- A
ment. Unfortunately, this hypothesis has not been rigor-
ously tested. This is partly because the reductionist
approach we currently employ cannot be used to test
such a hypothesis. To do so requires a systems approach
since the focus of the problem is not on isolated subsys-
tems, but instead on the interactions between these
subsystems.
Although systems medicine uses the general principles
of the systems approach to study clinical conditions, it
B
typically does not adopt mathematical systems theory,
which represents the formal end of the approach. This
Figure 2.1 (A) Unstable and (B) stable ball position.
theory is useful in framing the problem and developing
models to study general properties of systems such as
stability, performance, robustness and goal-directedness. the concept of stability, and in the process have developed
Model validity is always a concern with this type of more rigorous definitions (Reeves and Cholewicki 2003).
approach. But if models can be developed that accurately Using concepts from systems science, we will describe
reflect the spine system, significant insight can be gained. what stability is and how it is achieved.
Moreover, given the interdisciplinary nature of systems
science, this type of framework could help integrate data
What is stability?
from the spine research community thus leveraging our
expertise and resources. Stability of a system, whether it is stationary or moving, is
The goal of this chapter is to serve as a primer to tested by applying a small perturbation and observing the
develop a common understanding regarding the systems new behaviour. If the new behaviour is approximately the
approach. Given that all systems must be stable to fulfil same as the old, the system is stable. If the new behaviour
their intended goal, the first part of the chapter will becomes indistinguishable from the old behaviour, the
use systems theory to address the questions: what is sta- system is asymptotically stable. Finally, if the new behav-
bility, and how is it achieved? Other characteristics of iour differs significantly from the old behaviour, the
systems such as performance and robustness will also system is unstable.
be described. Later in the chapter, we will use a stick- Using a ball on a hill as an example (Fig. 2.1a), there is
balancing task to describe concepts of control, which no size of perturbation that can be applied that will keep
will then be used to elucidate possible spine system the ball in the original undisturbed position; hence this
impairments. Finally, we will present a possible roadmap, system is unstable. Whereas a ball in a valley (Fig. 2.1b),
which can be used to integrate future spine research following a perturbation, will oscillate and settle in the
efforts in the community. undisturbed position – assuming that there is some fric-
tion in the system.
This definition of stability is generic and applicable to
SPINE (IN)STABILITY: any system. It is important to point out that stability is
context dependent, which may explain some of the confu-
AN UNSTABLE TERM
sion when applying it in the context of the spine system
(Reeves et al. 2007b). For instance, are we interested in
As a community, we have the right to develop our own set mechanical stability of the spine, such as controlling the
of definitions that we deem ‘useful’. However, if these defi- displacement of individual vertebrae following a perturba-
nitions lead to confusion and unnecessary debate, or are tion, or are we interested in performing tasks without
not stable with time, then the community may deem them injuring spinal tissue or experiencing pain? At this point,
‘not useful’. A case can be made that the term spine (in) it is much easier to address mechanical stability of the
stability has not been enlightening, as illustrated by the spine, but the same framework could be applied to address
lack of consensus about its definition (Nachemson 1985, injury and pain. For this chapter, we will primarily focus
Reeves et al. 2007a). Other disciplines have struggled with on mechanical stability.
8
Spine systems science: a primer on the systems approach Chapter | 2 |
State of
Input Output Input the system
Σ Σ
Plant Plant
Feedback
control for
each vertebra
Feedback
k
controller
Feedback
A Spine system B Spine system
control
Figure 2.2 (A) Simple and (B) complex examples of feedback control of the spine. Reproduced from, Reeves, P.N., Narenda, K.S.,
Cholewicki, J., 2007b. Spine stability: the six blind men and the elephant. Clinical Biomechanics 22, 266–274, with permission from Elsevier.
How is stability achieved? significantly more complex. Multiple feedback signals
come from sensory receptors that convey information
Given that the spine has similar characteristics to an
about the state of the entire system (Fig. 2.2b). These
inverted pendulum, it can be shown to be unstable; there-
signals are processed by the feedback controller (CNS),
fore some form of control must be applied to ensure that
which in turn, generates many control signals to be
it is behaving in a stable manner. The principal approach
applied to the different segmental levels. Unlike the feed-
for stabilizing any system is feedback control. With feed-
back gain represented by k in the simple system, the con-
back control, information concerning the output of the
troller consists of a sophisticated network of neural
system is fed back and used to modify the input (Fig.
connections, which applies the logic for transferring
2.2a,b). Using systems terminology, the isolated system is
sensory information into control input.
called the plant, which represents the osteoligamentous
There are a number of feedback pathways for the spine
spine. The logic by which the control input is generated
system: intrinsic properties of the joint and muscles, which
from the output is the controller, which represents a feed-
apply resistive forces to the spine instantaneously; reflexes,
back controller. The plant (osteoligamentous spine)
which apply their control input after short, medium and
together with the controller (feedback control) is the
long delays following a disturbance; and voluntary correc-
overall system (spine system). A simple example of a feed-
tions, which also take time to respond. It is important to
back control spine system is shown in Figure 2.2a.
note that delays in feedback control can destabilize the
In Figure 2.2a, the control input to the plant is propor-
system. The longer the delay the more problematical it is.
tional to its output. This is indicated by the feedback gain
This will be discussed in more detail later.
denoted by k. Feedback can be positive or negative. If posi-
tive, the system is unstable since the force applied to the
Performance
system is in the same direction as the displacement (i.e.
ball on top of the hill). For stability, negative feedback is Once the stability of a system is established, the interest
used so that the force is applied in the opposite direction shifts to its performance. Performance reflects how closely
to the displacement (i.e. ball in the valley). Therefore, the and rapidly the disturbed position of the system tends to
goal of feedback control for the inherently unstable spine the undisturbed position. Accuracy and speed are impor-
is to give it stable behaviour. To do this, the controller’s tant attributes of any control system. Following a perturba-
negative feedback must be larger than the positive feed- tion, a system performing well will have behaviour that
back of the unstable spine. Stated differently, the overall resembles the undisturbed behaviour, indicating that the
system (spine and feedback controller) must have negative error between a disturbed and undisturbed system is
feedback. minimal. For asymptotically stable systems, a system per-
Figure 2.2a shows the simplest form of feedback control forming well will also converge to the undisturbed posi-
for the spine system. In reality, the spine system is tion in a short time interval.
9
Part | 1 | Models of the spine
Robustness
From a practical standpoint, the issue of spine stability is Velocity
probably not a major concern. The overall spine system
appears to be stable. What is more pertinent is the
question, is the spine robust? For instance, is the spine
sufficiently robust to recover from both small and large
disturbances? Or is a person’s control of the spine robust
enough to accommodate various types of impairment?
Robustness reflects the level of tolerance a system has to
disturbances or changes in the system properties. For
example, some individuals may be less tolerant to degen-
erative disc disease or whole body vibration than others.
Lessons from balancing a stick
There are many subtle nuances of control that can be
explained with the example of balancing a stick in your
hand (Reeves et al. 2011). Because the stick has inverted
pendulum characteristics (like the spine), it is unstable
and requires some form of feedback control to keep it
upright. A B
We can test stability of the upright stick by basically
Figure 2.3 (A) Stick is positioned to the right of the hand
doing nothing. If you keep your hand in the same posi-
with zero velocity. (B) Stick is positioned to the right of the
tion, the stick will eventually fall over. Therefore, to stabi-
hand, but is moving to the left. Reproduced from Reeves, N.P.,
lize the stick, you need to move your hand in a controlled
Cholewicki, J., 2010. Expanding our view of the spine system.
fashion in order to keep the centre-of-pressure (COP)
European Spine Journal 1, with kind permission of Springer Science
acting on the hand under the centre-of-mass (COM) of the
and Business Media.
stick. Although it may not be obvious, how you move your
hand is based on feedback control.
As discussed earlier, feedback control uses information
about the state of the system to apply control. For stick
balancing, this means you must track the stick and use this feedback control. We use position-related feedback,
information to determine where to move your hand. referred to as stiffness, and velocity-related feedback,
Therefore, if you cannot track the stick, say for instance referred to as damping, to control and stabilize a system
you close your eyes, than it becomes impossible to keep with mass. This is an important observation that has sig-
the stick upright. Or stated in a more general sense, if we nificant ramifications for how we study the spine system.
cannot track the state of the system, we cannot stabilize it. To demonstrate our limited view of the spine system,
When we apply feedback control, there is a minimum we recently performed two PubMed searches using the
amount of information that must be obtained to stabilize following terms: (i) ‘stiffness’ AND ‘spine’ AND ‘stability’
the system. For instance, it is obvious that we use our and (ii) ‘damping’ AND ‘spine’ AND ‘stability’. Searches
visual system to track the position of the stick, but what (i) and (ii) yielded 234 and 5 hits respectively. If you
is less obvious is the fact that we must also track the veloc- exchange ‘stability’ with ‘instability’ you will obtain similar
ity of the stick. To demonstrate, we will present two stick- results. This exercise shows that we have an incomplete
balancing scenarios (Fig. 2.3a,b). In the first scenario, the picture of the spine system. We need to expand our defini-
stick is positioned to the right of the hand and is stationary tion of stability from the current static representation to
(velocity = 0). In this situation, you would want to move include dynamics. This transition will be essential if we
your hand to the right to bring the COM under the COP. plan to investigate control aspects of LBP (Reeves and
In the second scenario, the stick is in the same position, Cholewicki 2010).
but is moving to the left. It is unclear in this case which Another necessary condition for stability is controllabil-
direction to move the hand and it depends on the velocity ity. Now let us consider the task of balancing two sticks,
of the stick. If the stick is moving slowly, you would want one set in series (Fig. 2.4a) and another in parallel (Fig.
to move your hand to the right. But if the stick is moving 2.4b). In these examples, all sticks are identical, having the
really fast, to catch the stick would require moving your same mass and length. Which of these two conditions can
hand to the left. This simple experiment shows that two be controlled? It is not obvious, but only the sticks in
independent sets of data, referred to as states, are used for series can be controlled. Even though we cannot apply
10
Spine systems science: a primer on the systems approach Chapter | 2 |
θ
2
θ θ1 θ2
1
A B
Figure 2.4 (A) Inverted pendulums in series are controllable and hence can be stabilized. (B) Identical inverted pendulums in
parallel are not controllable and cannot be stabilized. Reproduced from Reeves, N.P., Narenda, K.S., Cholewicki, J., 2011, Spine stability:
lessons from balancing a stick. Clinical Biomechanics 26, 325–330, with permission from Elsevier.
control directly to the top stick, by moving the hand, we independently changed. Consequently, if one stick is
can change the states of the bottom stick, which in turn bumped or has a different starting position, there is no
can change the states of the top stick. What is important possible way to bring both sticks to the upright position.
is that movement of the hand will change the position and If, however, one stick is slightly longer than the other, it
velocity of the bottom stick in a way that is different than now becomes possible to control both sticks. This is
the upper stick. This means we have independent control because the sticks will have different natural frequencies,
over the two sticks. Thus, we can use the system’s states to meaning they will tend to move at different speeds, which
generate feedback control to bring both sticks upright, then allows for independent control over the states of the
whereas the states of the two sticks in parallel cannot be system. One point to note: the closer in lengths the two
11
Part | 1 | Models of the spine
sticks are, the faster and more forceful control must be to are limited to controlling slower moving systems. And as
stabilize the system. Also in the case of the sticks in series, demonstrated with a fast moving stick, once the dynamics
we would need fast and forceful control. Can a human of the plant are outside the bandwidth of the controller,
balance two sticks? Most likely not. The dynamics of the the system will become unstable.
plant (two sticks) are such that it may require more power
than a human controller can apply. This is an issue of force
saturation. Also, timing of control becomes an issue. The
IMPAIRMENT IN FEEDBACK
human controller may not be able to respond fast enough.
CONTROL OF THE SPINE
Next, we would like to spend some time discussing
performance issues. Recall that how we move our hand to
keep the stick upright is determined by the position and Using lessons from balancing a stick, we can now discuss
velocity of the stick. This also implies that the force applied the issue of impairment in feedback control of the spine.
to the stick through the hand will be proportional to the As shown in Figure 2.5, there are a number of sources for
size and rate of stick movement. Consequently, any impairment. In terms of the controller, these include poor
impairment in control that causes larger and faster stick proprioception, faulty control logic, longer delays and
movement also requires more stabilizing force. This can decreased resolution in the regulation of muscle force. In
be demonstrated by balancing the stick and focusing addition, degenerative changes to the osteoligamentous
either at the top or near the bottom of the stick. You spine (plant) can also be considered as impairments in
should notice that focusing higher on the stick makes it feedback control (Reeves et al. 2007a, 2007b).
easier to balance. You should also notice that focusing
lower on the stick results in larger and faster stick move-
Controller-related impairment
ments, which in turn, requires more effort to stabilize the
stick. What is causing this impairment? It stems from dif- If we cannot track the spine precisely, the information
ferences in visual resolution. For a given angular displace- used by the CNS will contain noise, which in turn means
ment, more linear displacement occurs at higher focal control applied to the spine will not be precise. Do people
points than lower focal points. Therefore, you are more with LBP have impaired proprioception? It is unclear from
sensitive to displacement of the stick when you focus at a the literature. Some studies report impairment in trunk
higher point on the stick. When you can track the stick proprioception with LBP (Field et al. 1997; Gill and
more precisely, less noise enters the system, and the preci- Callaghan, 1998; Brumagne et al. 2000; Leinonen et al.
sion of hand positioning improves, which in turn reduces 2002, 2003; O’Sullivan et al. 2003), while others find no
effort to balance the stick. such impairment (Lam et al. 1999; Koumantakis et al.
We will next discuss the issue of delays in feedback 2002; Descarreaux et al. 2005; Åsell et al. 2006; Silfies
control. Using a weight attached with elastic bands to a et al. 2007). This could represent heterogeneity in study
stick, you can balance the stick with the weight in different
positions. Which is easier to balance, a stick with the
weight at the top or near the bottom? You should find it
harder to balance as the weight moves closer to the hand, Plant
and in fact, balancing will become impossible at some
critical height. As you move the weight closer to the hand,
you should notice that the stick will have larger and faster
Muscle force
oscillations. With the mass near the bottom of the stick,
the stick will tend to fall faster, reflecting a higher natural
frequency of the system. Conversely, with the mass at the
top of the stick, it will take longer to fall, reflecting a lower
n
natural frequency. ptio
So what causes the stick to become unstable? Force satu- ce
o
ration may be a culprit. It is possible that the large and pri
o
fast movements of the stick require stabilizing forces that Pr
are greater than your arm and shoulder muscles can gener-
ate. But in this case, instability most likely stems from
delays in feedback control. Because your controller has
inherent delays, as the stick (plant) moves faster, the CNS
Delay Control logic
(controller) does not have enough time to register the
states of the system, process this information and then
send commands to move the hand. Controllers with Figure 2.5 Sources for impairment in feedback control of
longer delays respond slower, which in turn means they the spine.
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