In the United States, the number of adults ≥65 years of age is estimated to increase from 53 million in 2018 to 88 million in 2050. As this population increases in size, the financial burden on the healthcare system will also increase, and effective preventive and/or therapeutic approaches will be desperately needed to help diminish the increased burden associated with the increase in the number of older adults. Among the various age-related diseases, neurological disorders with or without concomitant cognitive decline are particularly relevant given their adverse impact on people’s quality of life [1]. Neurological disorders can have detrimental effects on gait and mobility, as they may increase the risk of falls and disability [2], leading individuals to maintain a sedentary lifestyle and have an increased fear of falling. However, improvements in walking ability provide a positive impact on the quality of life and health of older individuals, particularly as daily walking significantly reduces the risk of cardiovascular disease, osteoporosis, diabetes, and other chronic diseases in this aging population [3].

Stroke, one of the most common neurological disorders, is a leading cause of long-term disability in older adults. Brain damage due to stroke can lead to symptoms such as cognitive and motor impairments including pain, paralysis, poor balance, spasticity, muscle weakness, and ineffective gait patterns [4]. More than 80% of post-stroke survivors suffer from chronic walking dysfunction [5]. Adults with stroke are prone to injuries leading to falls and often require rigorous rehabilitation during the subacute and chronic phases. The goal of early stroke rehabilitation is to restore the ability to perform activities of daily living (ADL) such as walking, feeding, or toileting. In particular, regaining normal walking function is one of the most important concerns for people who have suffered a stroke and results in a significant amount of time spent focusing on re-learning how to walk [6]. Studies have shown that patients who partake in early intensive rehabilitation have better outcomes, especially in regard to regaining independent ambulation [7]. However, there is an extensive gait rehabilitation literature which makes it difficult for clinicians to choose optimally effective treatment plans.

Another common neurological disorder is Parkinson’s disease (PD). PD is a common neurodegenerative disorder that occurs in about 1% of adults over the age of 60 years old [8]. Adults with PD suffer from impaired basal ganglia function, leading to disturbances in gait and balance, to name a few. Major motor impairments include bradykinesia (slowness in movement), postural instability, rigidity, and resting tremor. Individuals with PD often have trouble picking their feet up while walking which leads to taking small, shuffling steps known as “Parkinsonian gait”. This type of gait impairment is associated with increased falls and can negatively impact the quality of life [9]. Like adults with stroke, adults with PD also undergo comprehensive rehabilitation programs to improve walking ability but often face difficulty re-gaining normal gait patterns.

Adults living with these conditions must undergo sustainable forms of short-term and long-term rehabilitation to regain function and perform the activities of daily living. Many studies have demonstrated that treadmill training is a common approach for improving mobility and gait. Treadmill training (TT) has been shown to promote ‘normal’ walking patterns, as it facilitates the proper movement and timing of the lower limbs, thus eliminating the need for compensatory gait mechanisms [10]. It can also improve spatiotemporal gait parameters such as stride length, swing time, and cadence. HThis systere matic review focused on studies including treadmill intervention effects on spatiotemporal gait parameters of people with neurological disorders. Types of treadmill interventions were body weight support (BWS) TT, and TT with sensory cues or biofeedback. Although TT is a popular rehabilitation activity, there is an insufficient literature regarding its effectiveness in improving gait parameters in patients with neurological disorders [5]. Previous reviewers have not undertaken a quantitative synthesis of intervention effects on spatiotemporal gait parameters in common neurological populations [11]. If they carried out a quantitative synthesis, then it was not carried out on all gait parameters [5], and they also did not compare interventions in one neurological population to another to find out the best suitable intervention for the specific gait impairment [12].

2.1. Pure Treadmill Training

Neurorehabilitation TT programs are goal-based, repetitive, and include intensive motor learning components for adults with or without PD and stroke. IFrom our systematic review and meta-analysis, it can be seen that pure TT increases stride length in adults with PD ^[13][14][15][25,33,41], and step length in adults with stroke ^[16][22], which can be beneficial in helping them to develop a compensatory strategy for everyday activities.

2.2. Treadmill Training with an Incline or Speed-Dependent Treadmill Training

Speed-dependent treadmill training is a repetitive, intensive TT in which the belt speed is increased or decreased incrementally by 10% based on the individual’s performance. Incline-based TT is a repetitive, intensive training in which incline level is increased incrementally with the belt speed based on the performance of individuals. The results  from our systematic review indicate that speed-dependent TT was more effective to increase stride length and gait speed in individuals with stroke in comparison with pure TT ^[17][23]. Speed- and incline-dependent (mixed) TT were also effective in improving gait speed, cadence, and stride length for patients with PD ^[18][40]. A likely rationale behind the effectiveness of incline/speed-dependent TT is that walking on inclined surfaces increases lower extremity muscle activity and may be an excellent means to improve endurance and strength. Furthermore, using inclined surfaces also decreases the monotony associated with repeated TT programs by varying the type of stimulus received in sessions ^[18][40]. After these studies, adults were successful in improving their quality of life and decreasing gait-freezing episodes, which further highlights the significance of implementing mixed TT programs

2.3. Treadmill Training with Sensory Feedback

Treadmill training with sensory feedback is a task involving training on a treadmill while providing sensory feedback such as auditory cues, visual cues, rhythmic cues, or a combination of these. HeFre, researchersom our systematic review, we found that auditory cues had the strongest effect on step width and step length in adults with PD ^[19][35]. rWesearchers also found improvements in cadence after TT with rhythmic auditory stimulation in adults with stroke ^[20][26]. TT with auditory stimulation from functional music leads to greater improvement in functional gait. This could be due to an increased symmetry in movement with music tempo feedback ^[20][26].

Moreover, in adults with stroke, reswearchers found that visual cues with treadmill training have the strongest effect on cadence, paretic step length, and non-paretic step length ^[21][22][16,17]. There was an improvement in motivation due to an increase in the control of movement among these adults, because they can visualize the accuracy of the task on the screen during the training ^[22][23][17,50]. Visual biofeedback is useful in training patients post-stroke because it provides information about the accuracy and performance of tasks. Visual feedback provides an effective way to alter gait patterns and improve the frequency of steps, symmetry, and the coordination of gait in people with stroke ^[24][21][14,16].

Given that gait is a complex sensorimotor behavior that involves the coordination of neural networks, bones, muscles, and joints, it is not surprising that sensory information can aid and even influence gait performance ^[25][51]. Work from Mahoney and colleagues examining healthy older adults demonstrated that the successful ability to integrate concurrent visual and somatosensory information is associated with faster gait velocity, longer strides, a smaller percentage of the gait cycle spent in double support, and less stride length variability compared with those with unsuccessful multisensory integration abilities ^[26][27][52,53].

2.4. Treadmill Training with Bodyweight Support

Bodyweight support TT is locomotor training on a treadmill with partial body weight supported (PBWS) with an overhead harness, a pelvic belt, and thigh straps. PBWS is effective in improving mobility outcomes in adults with stroke and spinal cord injuries ^[26][52]. The load percentage in PBWS also has additional gains in improving gait functionality ^[27][53].

HeFre, researchersom our systematic review, we found that TT with PBWS can create better gait kinematics, symmetry, velocity, and endurance ^[28][29][27,28]. PBWS is beneficial for overground gait as well as patients with PD ^[13][25]. Ribiero et al. also found that TT with PBWS can also increase gait speed and step length in adults with stroke ^[30][29]. It was shown that TT with PBWS was able to activate central pattern generators in the spinal cord, which produces constant rhythm in walking in post-stroke survivors, thus helping to increase measures such as step length and cadence [31]. TT with PBWS is not only effective in people with stroke, but it is also effective in improving mobility in people with PD, although the effects appeared to be only short-term ^[32][39]. Fortunately, people with PD were able to tolerate a higher treadmill speed. In contradiction, Trigueiro et al. did not find any differences based on the PBWS weight load in people with PD ^[33][44], which may be attributable to the small sample size. Surprisingly, there were fewer studies using TT with PBWS as an intervention in people with PD.

2.5. Other Treadmill training: Curved TT and Perturbation TT

Curved walking involves the coordination of whole-body movement and the complex integration of multiple sensory systems and motor output to respond to balance demands [34]. Curved walking improves balance and speed compared with straight walking and has been effective in improving gait speed (by increasing step length) for people with PD. Perturbation treadmill training involves slips and trips while walking on a treadmill. Perturbation TT showed a significant increase in overground walking speed and gait stability in people with PD. It also reduces variability in step length, stride time, double limb support, and cadence ^[35][38], and stride length, stance, and swing time ^[36][43].

2.6. Biomechanical and Physiological Mechanisms behind Impact of TT in Adults with PD and Stroke

In adults with PD, TT improves functional capacity, balance, and quality of life after even short-term treadmill training ^[37][54]. TT has an impact on the overground walking economy ^[38][55]. TT also improves turning performance, the strength of lower limbs, and the sensory organization balance of individuals with PD ^[39][56]. Some of the biochemical effects of TT on PD are a decrease in inflammatory status, memory improvements through an increase in brain-derived neurotrophic factor levels, a reduction in stress hormone levels, and a decrease in neurodegeneration through the stimulation of neural plasticity ^[40][57].

In adults with stroke, TT has been shown to improve lower limb function and gait performance, which in turn promotes stroke recovery by inducing further brain plasticity through activation of the sensorimotor cortex, supplementary motor cortex, and cingulate motor area ^[41][58]. Furthermore, TT improves the fitness reserve by increasing peak VO₂ levels while lowering the energy cost needed for hemiplegic gait and further enhancing motor functions ^[42][59]. Recent studies have shown that bi-hemispheric activation happens in adults with stroke through exercise training, which further increases motor recovery ^[43][60]. Lastly, aerobic training has been shown to decrease weight, body mass index, blood pressure, total cholesterol, and triglycerides in adults with stroke ^[43][60].

Although rwesearchers do see these changes at physiological and biomechanical levels in adults with stroke and PD, these changes depend on the intensity and duration of exercise in each population. Here, the researchersThrough this review, we tried to show the importance of treadmill training in different neurological populations by its ability to improve specific spatial–temporal gait parameters. researchersWe were successful in showing the impact of treadmill training on spatial–temporal gait parameters. researchersWe found that although treadmill training is effective on its own, the use of sensory stimulation in unison with TT provides a more effective therapy for neurological populations, such as PD and stroke.

2.7. Adverse Effects of Treadmill Training

While the majority of TT studies did not report dropouts, 7/32 studies reported drop-outs due to personal reasons (e.g., problems with transport, or acute symptom or injury not directly attributable to intervention), while only a single study reported subjects dropping out due to leg pain, fear of falling, or subjective intolerance to training.