Motor rehabilitation after hemiparetic stroke is essential to soften physical disability
(Furlan, 2014). Nevertheless, current interventions are mostly designed for well recovered
individuals and often exclude stroke survivors with rather limited motor ability (Sterr
and Conforto, 2012). Given that, and further advancing our research agenda in this
arena (Sterr et al., 2002; Sterr and Freivogel, 2003, 2004; Sterr, 2004; Sterr et
al., 2006; Sterr and Saunders, 2006), we recently tested the efficacy of a 2-week
modified constraint-induced (CI) therapy program in chronic stroke individuals with
very low-functioning upper limb hemiparesis (Sterr et al., 2014a). We tested the influence
of both the intensity of daily motor training (90 vs. 180 minutes) and the restraint
of the less affected upper limb (restraint vs. no restraint) on treatment outcomes.
Sixty-five individuals were randomly assigned to four experimental conditions (90
minutes of training with or without restraint, and 180 minutes of training with or
without restraint). They were assessed at baseline and after the intervention (2 weeks
before, immediately before and after, 6, and 12 months after). Across the cohort,
motor function improved significantly, and treatment benefits were largely sustained
over the 12 months of follow-up. Analysis of the different treatment variants, however,
revealed interesting yet unexpected findings, particularly with regards to the relationship
between intensity (amount) of daily training and motor outcomes. As suggested by previous
work (Sterr et al., 2002), longer sessions of daily training were expected to yield
better outcomes than short sessions, a finding in line with the theory that massed
practice is essential for neuroplasticity processes driving the functional improvements
induced by CI therapy. However, this was not entirely the case. While we found some
differences suggesting greater benefit of longer training sessions, the picture was
not as clear as one might expect. This pointed to an interaction between training
intensity and motor outcomes in low-functioning chronic stroke that appears to be
different from that seen in less severe chronic hemiparesis, where the concept of
‘the more the better’ often holds true (
Figure 1
). We argued that this intensity-outcome relationship is moderated by variables that
highly depend on the level of residual recovery. A key candidate for this moderation
is fatigue. Fatigue is identified as rather common, yet obscure problem in stroke
survivors (Wu et al., 2015). Post-stroke fatigue is multifactorial and seems to result
from a complex interaction among biological, psychosocial, and behavioral factors
(Wu et al., 2015). Here, we discuss the role of fatigue in motor rehabilitation of
low-functioning chronic stroke using the framework recently suggested by Kluger et
al. (2013). Although relatively different from, yet not antithetic to other fatigue
models (e.g., Wu et al., 2015), we believe their framework provides conceptual and
mechanistic support to our hypothesis. According to that framework, neurological,
including post-stroke fatigue encompasses two domains: Perception of fatigue and fatigability.
Perception of fatigue refers to a subjective sense of effort or exhaustion, whereas
fatigability is related to an objective decline in performance. Although these two
types of fatigue might be largely interrelated (e.g., an increased sense of effort
would usually contribute to impair performance), they might also act independently
and still significantly affect the individual's engagement with activities posing
high motor and/or cognitive demands. This is because those two types of fatigue are
likely to be caused by different, yet potentially interacting factors. For instance,
perception of fatigue could be induced by homeostatic (e.g., metabolic stimuli, such
as depletion of energy reserves in skeletal muscle and/or brain tissue) and/or psychological
(e.g., decreased motivation) mechanisms, while fatigability could occur due to declines
in skeletal muscle force production and/or deficits in task-related neural processing
(Kluger et al., 2013). Based on that, we propose that low-functioning chronic stroke
survivors are highly susceptible to get into a complex fatigued state, which renders
motor training ineffective. This state is more likely to be reached by individuals
undergoing longer training sessions. Essentially, we elaborate here on the possibility
that a combination of general deconditioning and compromised neural processing might
greatly increase both perception of fatigue and fatigability in those individuals,
which substantially reduces their engagement with motor training and thereby decreases
the likelihood for neuroplasticity processes driving behavioral improvements.
Figure 1
Hypothetical relationship between training intensity and outcomes in chronic hemiparetic
stroke.
This figure illustrates the modulation of the optimum session length/training intensity
by residual recovery levels. Two assumptions are made. Firstly, as session length
increases, performance also increases, until it reaches its peak; increasing session
length further, however, results in performance deterioration, which presumably reflects
the impact of fatigue. Secondly, in stroke survivors with low-functioning hemiparesis
(red line), performance is not only lower in general, but critically, the optimum
training intensity is reached earlier than in those with high-functioning hemiparesis
(blue line). Optimum session length/training intensity: Mostly determined by both
the level of residual recovery and the fatigued status an individual achieves. Of
note, the latter is critically influenced by the first. Investigations of dosage effects
in motor rehabilitation should, therefore, not only carefully consider the level of
residual function, but also take measures of fatigue into consideration.
General deconditioning in low-functioning chronic stroke: The musculoskeletal and
cardiorespiratory systems are interdependent. Effective skeletal muscle work requires
that muscle fibers have not only relatively good levels of strength, but also adequate
supply of oxygen and nutrients. The first is achieved through regular doses of mechanical
load normally imposed to the musculoskeletal system during routine tasks, while the
latter is implemented by the cardiorespiratory system via the blood stream. On the
other hand, to maintain an effective cardiorespiratory system that ensures adequate
supply of oxygen and nutrients to working muscle fibers, regular physical activity
is mandatory. This is only possible through the contraction of skeletal muscles. After
hemiparetic stroke, individuals experience significant cardiorespiratory and musculoskeletal
deterioration. Pre-stroke age-related changes and/or comorbid cardiovascular diseases
often contribute to a deteriorated cardiorespiratory function already at the sub-acute
phase (Kilbreath and Davis, 2005). In parallel, the stroke-induced loss of voluntary
motor control results in an important deterioration of musculoskeletal function at
the same time. More specifically, the lack of selective control over spinal motor
units imposes major limitation on the individual's ability to generate muscle force
and coordinate contraction across muscle groups, which critically reduces their capacity
to use the paretic body side (Carr and Shepherd, 2011a). Combined to customarily low
inpatient and post-discharge physical activity levels, this initial impaired cardiorespiratory
and musculoskeletal status greatly increases the likelihood for an inactive lifestyle
after hemiparetic stroke (Carr and Shepherd, 2011b). Over time, the sustained immobility
of paretic limbs starts maladaptive plastic changes in skeletal muscles that contribute
to aggravate even more an already compromised musculoskeletal condition. Some of these
changes include: atrophy and shortening of muscle fibers, proliferation of connective
(non-contractile) tissue, increased stiffness and fat content, and reduced capillary
density and oxidative capacity (Carr and Shepherd, 2011a). Collectively, these changes
further subsidise the ongoing physical inactivity process and thereby the aggravation
of cardiorespiratory function (Kilbreath and Davis, 2005; Carr and Shepherd, 2011a,
b). Thus, chronic stroke individuals often become trapped in a self-perpetuating cycle
of general deconditioning, where an early deteriorated cardiorespiratory and musculoskeletal
condition fosters physical inactivity and paretic limbs disuse, which in turn contributes
to deteriorate cardiorespiratory and musculoskeletal function even further. This is
likely to elevate both perception of fatigue and fatigability during motor activities
by accelerating depletion of skeletal muscle energy reserves and causing rapid declines
in force production, respectively. While the first is likely a compound of an impaired
cardiorespiratory system – which may fail to effectively supply contracting muscles
with oxygen and nutrients – and a deteriorated musculoskeletal function – in terms
of reduced capillary density and oxidative capacity limiting muscle energy production
–, the latter mostly results from an impaired neuro-musculoskeletal system, owing
primarily to both a reduced ability to activate spinal motor units and skeletal muscle
plastic changes. Of note, it is plausible to assume that general deconditioning tends
to be more pronounced in low-functioning chronic hemiparesis, as here the likelihood
for paretic limbs immobility and an inactive lifestyle is even higher due to greater
physical (and often cognitive) limitations.
Compromised neural processing in low-functioning chronic stroke: The adult brain can
be thought of as having both primary, specialized, and secondary, less specialized
circuits/networks. Primary circuits are critical for the generation of behavior and
therefore are normally recruited. Secondary circuits, however, are not essential for
behavior expression, but because they have some capacity to contribute to it, they
may be recruited under special circumstances, such as when primary circuits can no
longer afford behavioral/task demands, which can happen after stroke (Kleim and Schwerin,
2010; Ward, 2011). In that case, enlisting secondary networks often allows for the
brain to compensate for damage and preserve behavior integrity to varying degrees
(Kleim and Schwerin, 2010; Ward, 2011). The extension of the recruitment of secondary
brain networks largely depends on the remaining availability of primary circuits –
i.e., the less the sparing of the latter due to more severe damage, the more extensive
the recruitment of the first (Ward, 2011). Besides, the functional relevance of shifting
activity to secondary circuits in that context relies essentially on how well these
circuits can characterize the relevant behavior, which is primarily dictated by their
pattern of neuronal connections (Ward, 2011). Because secondary brain networks usually
share only part of the highly specific connections displayed by their primary counterparts,
as reliance on these networks increases, the ability to maintain behavior integrity
is progressively lowered (Kleim and Schwerin, 2010; Ward, 2011). After hemiparetic
stroke, primary brain circuits normally controlling skillful motor behaviors are disrupted
to different degrees (Frey et al., 2011). The resulting compromised neural processing
state manifests itself not only at the motor execution level (see previous paragraph),
but also at the more cognitive level. For example, studies by our group have shown
deficits in up-stream motor processes, such as motor preparation (Dean et al., 2012).
Therefore, it is not only the movement execution-related mental effort that is increased
during motor tasks in order to preserve behavior integrity, but critically, also the
mental effort associated with the processing of other movement-related information.
In chronic stroke, this translates into a pattern of widespread brain activation,
which is characterized by enhanced activity in potentially spared primary networks
and recruitment of many secondary circuits (Ward, 2011). This has come mostly from
studies utilizing functional magnetic resonance imaging to investigate changes in
brain activity after stroke. Because of the theoretical/methodological assumptions
underlying interpretations of this type of data (Ward, 2009), it seems reasonable
to conclude that the pattern of increased brain activation described above reflects
a condition of elevated neuronal metabolism in chronic hemiparetic stroke, which favors
rapid depletion of brain energy reserves. This, in turn, might elevate perception
of fatigue during motor tasks. Besides, an imposed reliance on secondary, less specialised
motor control networks is very likely to also increase fatigability in chronic stroke
individuals owing to deficits in motor task-related neural processing. Importantly,
compromised neural processing is likely to be more aggravated in low-functioning chronic
hemiparesis, as here (1) individuals often recruit more secondary circuits and therefore
have stronger brain activation (Ward, 2011), causing higher neurometabolic demands
and hence more elevated perception of fatigue, and (2) the reliance on a less specialized
brain system is stronger (Ward, 2011) – which reflects itself in a low-functioning
status –, making task performance even more challenging and thus causing greater fatigability.
In our CI therapy study, because we did not measure fatigue, the interpretation outlined
in this article can only be speculative. However, given the prevailing characteristics
of our sample, the aspects of the intervention that was delivered, and the mechanisms
underpinning the processes of fatigue described before, we nonetheless feel that this
is a relevant interpretation. The very low-functioning motor status and long chronicities
prevailing in our cohort are rather suggestive of pronounced cardiorespiratory and
musculoskeletal deterioration, and hence important general deconditioning. When combined
to a physically demanding motor training regimen such as CI therapy, this may well
have exacerbated perception of fatigue and fatigability in our sample. Moreover, although
we did not obtain specific information about stroke lesion size and location from
our cohort, investigations on the association between motor status and the integrity
of movement-related primary brain circuits (Sterr et al., 2010, 2014b) show that poor
motor function at the chronic phase highly depends on the overlap of the stroke with
those circuits, which in turn is very likely to cause overreliance on secondary neural
substrates for movement control, and thereby rather compromised neural processing.
Given the high cognitive/motor processing demands of our intervention, this may well
have contributed to further aggravate perception of fatigue and fatigability in our
cohort. Accordingly, a recent cross-sectional study with chronic stroke survivors
revealed an inverse correlation between levels of fatigue and the excitability of
movement-related primary brain networks (Kuppuswamy et al., 2015). Nevertheless, it
is difficult to determine whether the poor motor status of our sample indeed reflected
a severe neurological damage or, instead, the manifestation of maladaptive musculoskeletal
changes that may have evolved over time as a result of inactivity/immobility. A more
sensible appreciation of this point assumes that these two phenomena interact and
jointly contribute to the fatigued status, as well as the residual recovery an individual
achieves. Clearly, further research is needed to properly investigate the proposed
mechanisms and their interaction.
Fatigue, motor training, and neuroplasticity in low-functioning chronic stroke: Motor
training contributes to restore motor behaviors lost due to hemiparetic stroke by
providing the brain with neural signals that drive functionally relevant neuroplastic
changes within spared primary/secondary motor control networks (Nudo et al., 1996).
However, to do that, trained motor tasks need to be not only actively and repetitively
practiced, but also challenging enough to stimulate individuals to go beyond the current
state of their motor capacity and thereby achieve the adaptive brain reorganization
driving behavioral improvements (Nudo, 2003). The CI therapy intervention is grounded
on this principle (Morris and Taub, 2006). In our study, the individuals receiving
longer daily sessions indeed spent more time in active, repetitive motor practice
than those receiving shorter sessions, but treatment outcomes did not coherently reflect
this. One could henceforth conclude that 90 minutes of daily training is enough. But
we do not take this position. Rather, we argue that those in the group receiving longer
daily sessions are more likely to reach an exacerbated fatigued status – characterized
by both elevated perception of fatigue and fatigability –, which adversely directs
motor training towards the repetition of movements that are more accommodated to their
motor impairments, and hence reduces the neural activation required for the relevant
neuroplastic changes mediating motor improvements (Nudo, 2003). Furthermore, an increased
fatigued status is also likely to negatively impact on motivation and compliance,
which in our study could have not only contributed to aggravate an already existing
condition of elevated perception of fatigue in the individuals exposed to the more
intensive CI therapy protocol (see mechanisms of fatigue), but also directly affected
their ability to engage with the training as well as their commitment to it. Thus,
the fact that those individuals may have spent their extra training time practicing
tasks while they were physically and/or mentally too fatigued to do so effectively,
might explain the limited added value of the more intensive 30 hour training regimen.
Conclusion: We believe the results from our study, when interpreted under the perspective
presented in this article, harbor important implications for post-stroke motor rehabilitation
research. Two of the many challenges in this field have been to define the optimal
intensity of motor training-based interventions (Cooke et al., 2010) and to account
for potential individual differences in motor outcomes after such interventions. Taking
critical modulators such as fatigue into consideration is very important here. This
is because not only it might explain individual differences to some extent, but also
it will contribute to prevent misconceptions around the intensity-outcome relationship
of those interventions. Because fatigue is very likely to be more pronounced in low-functioning
chronic stroke, studies with this group have an even stronger mandate to take it into
consideration when both, seeking for optimal training intensity-related parameters
as well as interpreting motor outcome measures.
This work was supported by the MRC, UK (G0200128, awarded to AS) and the CAPES Foundation,
Ministry of Education, Brazil (BEX 0996/14-9, awarded to LF).