Effect of Cognitive, Impairment-Oriented and Task-Specific Interventions onBalance and Locomotion Control

Falls, defined as an unexpected event in which individuals come or drop down to the ground, floor, or lower level, occur at least once annually in 29% of community-dwelling adults 65 years or older, representing a global public health concern for our aging societies (Bergen, Stevens, & Burns, 2016). On the other hand, in persons with stroke (PwS), falls correspond to a significant secondary complication with 40% of individuals experiencing a serious fall within the first year after being discharged (Persson, Hansson, & Sunnerhagen, 2011). Fall-related injuries can cause devastating outcomes such as hip fracture and traumatic head injury, requiring hospitalization and an extended stay in a long-term care facility (Scheffer, Schuurmans, Van Dijk, Van Der Hooft, & De Rooij, 2008). Additionally, after a fall, it has been reported that a fear of falling develops in 21 to 39% of those who previously had no such fear, which may restrict their activity and affect their quality of life and participation (Scheffer et al., 2008). Compensatory postural response has been described as one of the most important components of balance restoration after experience a loss of balance or a fall. In this context, it has been well described that changes in muscle strength, muscle coordination, and joint mobility along with impaired sensorimotor integration contribute to poor postural control (Mansfield, Wong, Bryce, Knorr, & Patterson, 2015), which ultimately impact the ability to rapidly and appropriately generate corrective muscle forces to recover from balance disturbances (Jacobs & Horak, 2007) (Shumway-Cook & Woollacott, 2007). Such reactions are referred to as compensatory or reactive postural responses and are defined as the ability to recover from instability through a rapid postural muscle corrective response, step, or grasp (Maki & McIlroy, 1997). Thus, compensatory postural responses play a major role in the recovery of balance from small perturbations (Maki & McIlroy, 1997) (Jensen, Brown, & Woollacott, 2001) and are considered the most important defense against large magnitude balance perturbations (Shumway-Cook & Woollacott, 2007). Trip and slip like perturbations have been described as the major contributors to falls (Kelsey, Procter-Gray, Hannan, & Li, 2012). Among to this line, it has been shown that muscle weakness, gait and balance problems, poor vision, psychoactive medications, and home hazards corresponds to modifiable risk factors that are susceptible to influence in fall risk (Tinetti, Speechley, & Ginter, 1988). In the last years, it has been reported that multimodal exercise programs are effective for fall prevention, reporting that evidence-based recommendations call for tailored progressive exercise providing a high level of challenge to balance, mobility, and lower extremity strength training (American Geriatric Society, 2001). However, recently, the recognized importance of task-specific training targeting balance recovery mechanisms and postural responses has led to interest in perturbation-based training for falls reduction among older adults (Bhatt, Espy, Yang, & Pai, 2011; Madehkhaksar et al., 2018; Patel & Bhatt, 2015). Perturbation-based training is an emerging paradigm based on the principle of task specificity, which consists of unexpected, repeated perturbation to simulate the accidental nature of falls (Gerards, McCrum, Mansfield, & Meijer, 2017). Specifically, after a trip, the body rotates forward while translating in the same direction. In contrast, after a slip, the body rotates backward while translation of the body continues in the forward direction. Biomechanical studies have suggested that perturbation-based training improves reactions to postural perturbations in the laboratory, reduces the risk of falling following simulated trips and slips, and can be retained over an extended period (Pai, Bhatt, Wang, Espy, & Pavol, 2010; Pai, Bhatt, Yang, Wang, & Kritchevsky, 2014). On the other hand, in persons with stroke (PwS) hemiparetic gait is a persistent problem that limits mobility and imposes higher energy demands for performing basic daily activities (Macko et al., 2001; Silver, Macko, Forrester, Goldberg, & Smith, 2000). Gait and balance deficits contribute to more than 70% of PwS sustaining a fall within 6 months (Forster & Young, 1995), leading to higher risks for hip and wrist fractures (Dennis, Lo, McDowall, & West, 2002; Kanis, Oden, & Johnell, 2001). Limited ankle range of motion (ROM) for the affected side is a common sequela after stroke. It is caused by weakness of dorsiflexors (e.g., tibialis anterior, extensor halluces longus, and extensor digitorum longus) and stiffness of plantarflexors (e.g., gastrocnemius, soleus, tibialis posterior, flexor halluces longus, and flexor digitorum longus) (An & Won, 2016). Ankles are located close to the body’s base of support and assist in controlling balance (Karakaya, Rutbil, Akpinar, Yildirim, & Karakaya, 2015). Limited ankle ROM in most of PwS impairs balance control, becoming one of the major risk factors for falls (de Haart, Geurts, Huidekoper, Fasotti, & van Limbeek, 2004). Functional gait and symmetric gait rely on ankle ROM and well controlled contraction of dorsiflexors and plantarflexors (An & Won, 2016). Additionally, normal gait requires a minimum 10° of dorsiflexion (An & Won, 2016), and plantarflexors (e.g., gastrocnemius and soleus) commonly generate forward propulsion during the push-off phase in locomotion (Liu, Anderson, Schwartz, & Delp, 2008; Neptune, Kautz, & Zajac, 2001). Due to limited ankle ROM and abnormal contraction of dorsiflexors and plantarflexors, most PwS manifest slow walking speed, reduced cadence, and shortened step length, which are common indicators of abnormal gait patterns, which in turn increase the risk of falling (An & Won, 2016; Olney & Richards, 1996). In this context, the recovery of impaired ankle motion and paretic muscle activity (e.g., dorsiflexors and plantarflexors) to improve balance control and gait performance has received attention in the last years in stroke rehabilitation (Winstein et al., 2016; Yana, Saracoglu, Emuk, & Yenilmez, 2017; Zeng, Zhu, Zhang, & Xie, 2018), and more studies in this area are needed in order to improve the efficacy of the current therapeutic strategies in stroke rehabilitation. In addition to kinematic, biomechanics and sensorimotor functions, it has been well established that cognition and motor control interact extensively (Peterka, 2002). Along to this line, most of the situations in everyday life often involve cognitive-motor interaction, e.g. walking while talking, texting on a cell phone, or thinking about one’s shopping list. Consequently, the assessment of cognitive functions such as cognitive-motor dual-tasks, attention, and mental fatigue is of great interest for gaining a better understanding of cognition/motor control interplay and for improving the diagnosis, prevention and management of cognitive impairment and falls (Kelsey et al., 2012). 1.2 Statement of the Problem In order to perform an efficient compensatory motor response after an external perturbation or to develop anticipatory mechanism to maintain the balance control while performing a motor task, a complex integration of sensory information from the visual, vestibular, and proprioceptive systems is required (Peterka, 2002). Proper sensory integration relies on the central nervous system to (1) depend on the optimal combination of sensory sources for balance and (2) reweight the sensory contributions as sensory conditions change. However, even when peripheral vestibular, visual, and somatosensory systems are intact, difficulty performing static and dynamic balance tasks under challenging surface and/or visual conditions could occur due to deficits in weighting/reweighting use of vestibular, visual, and somatosensory information for balance when the sensory conditions change (Peterka, 2002). For example, standing or walking on compliant foam or irregular surfaces requires the nervous system to rely more on vestibular and visual inputs while down weighting the normal dependence upon somatosensory inputs (Peterka, 2002). When healthy subjects stand on foam with eyes closed, they away 50% more because they must rely on vestibular inputs, which have higher sensory noise than somatosensory inputs (Van Der Kooij & Peterka, 2011). Thus, a person who cannot reweight reliance on sensory inputs properly when the sensory conditions change will be less stable than someone who can reweight sensory information for balance control. Although both perturbation-based training and therapeutic interventions, in which the motor behavior is trained under different sensory conditions, have shown promising results in improving balance control and reduce the risk of falling in population at higher risk of experience a fall, the effect of the combination of these two training strategies on postural control has not been largely reported and remain unexplored. Another sensorimotor problem that affects general mobility and the efficacy of compensatory motor responses after experience a loss of balance is related to the neuromuscular weakness that could be seen in elderly population, but, in particular, in persons with neurological diseases. Along to this line, it has been well described that persons with stroke shows delayed paretic limb muscle onset latencies following perturbations while standing on a moveable platform compared to their non-paretic limb and to healthy older adults (Dietz & Berger, 1984; Marigold, Eng, & Inglis, 2004). In addition, ankle, knee and hip torque responses are reduced on the paretic side following platform translations (Ikai, Kamikubo, Takehara, Nishi, & Miyano, 2003), which affect significantly the efficacy of the compensatory motor responses after an external-induced postural perturbation and increase the risk of falling in this population.