In 2000, Morris described the theoretical framework supporting the use of PT in PD. The model was based on the pathophysiology of movement disorders in basal ganglia disease, on scientific evidence, and on personal observations of PT interventions in PD. Morris described task-specific strategies for improving the performance of activities (e.g., gait, turning in bed, manual activities, and transfers such as sitting down), the prevention of falls, and the maintenance of physical capacity (e.g., muscle force and aerobic capacity). These strategies incorporate the use of external cues and cognitive movement strategies.

External cues include visual, auditory, or proprioceptive stimuli that are either presented rhythmically or serially (to improve the continuation of gait) or as isolated “one-off” cues (to initiate movements such as gait or transfers). By applying external stimuli instead of the usual internal cues (which normally happens in the healthy brain), alternative circuitries in the brain can be engaged to accomplish the task, while avoiding the defective basal ganglia circuitry [44].

With cognitive movement strategies, complex movements are broken down into separate components. Subjects are trained to perform each of the components separately, and to pay conscious attention to their execution. Mental rehearsing is part of this training. Cognitive movement strategies are mainly used to improve the performance of complex movements such as transfers or manual activities. Morris also stressed the importance of involving the caregiver to optimize therapy.

Goodwin et al. [45], in a systematic review reported the results of PT interventions in patients with PD. The majority of the studies showed that the PT was beneficial for the improvement of functionality, QoL, muscular strength, gait velocity, and balance. Keus et al. [41] reinforce that PT treatment should be based on the following recommendations: (1) external cue strategies; (2) cognitive strategies for movement, in which complex movements are divided into simple subcomponents to be memorized by the patients and executed in a specific order, under conscious control—before the execution of a task, the patient should be guided to practice it mentally (with the objective of overcoming the deficits of sequential movement automatization caused by disorders of the basal ganglia); and (3) balance and gait training with dual tasks and memorized cues. The PT approach to gait and balance disorders in PD has attracted the attention of the scientific community and health professionals over the last few years, because it is known that these disorders are determinants of the decrease in QoL and increase in mortality [46].

The beneficial effects of external cues in the improvement of gait and the decrease in freezing of gait (FOG) episodes are well established in the literature [47]. People with PD could benefit from external cues that are largely used to compensate for the deficiency of the basal ganglia in selected cases and trigger internally learned motor programs [48]. External cues act as potential facilitators of cognitive gait control by selective attention and increase the gait prioritization, mainly during the performance of complex tasks [49, 50]. For example, using a metronome or “clapping hands” to the rhythm of the steps or, taping horizontal lines on the floor that the patient must cross at each step passage.

People with PD demonstrate impaired postural responses. Impairments include hypometric force production, difficulty modifying and scaling postural responses based on the initial context and poor use of the hip strategy [75]. Abnormal proprioceptive–motor integration also contributes to impaired feet-in-place (feet are stationary, no step is performed) postural responses. People with PD tend to over-estimate the size and strength of their movements; even though they may perceive that they are moving with appropriate size and strength, they actually demonstrate hypometric movements and do not reach far enough, walk far enough, or step far enough toward a target when performing without visual feedback [76–78]. Postural instability and falls increase the morbidity and mortality in patients with PD [79]. Early recognition of balance impairments and falls risk contribute to a tailored rehabilitation program, which can minimize complications [80].

Postural adjustments are adaptive responses that maintain postural control. They can be evaluated using static and dynamic posturography, which consists of measuring forces exerted against the ground. Posturography has not only experimental but also clinical relevance, because it can quantify balance disorders, especially in individuals with PD [81–84]. Static posturography assesses body sway under quiet stance tasks. It usually quantifies the center of foot pressure displacements performed on eyes open, eyes closed and dual-task conditions [83, 84]. Dynamic posturography provides data on voluntary motor control, while patients perform tasks or stand still on a platform with external perturbations. It involves latencies of postural responses and body sway mechanisms [81, 85, 86]. Studies approaching PD balance have shown conflicting results; some found increased body sway [87, 88] or no differences in body sway between healthy controls and patients [87]. Others found a reduced or normal sway with a poor correlation with clinical balance tests [89]. These conflicting results stemmed from differences in disease severity, levodopa treatment conditions, and methods of postural instability quantification. In dynamic assessments, most studies evaluated postural control with moving or unstable platforms to observe the responses to external perturbations [86].

In clinical practice, balance tests are used for assessing functional mobility and postural instability in patients with PD. The Movement Disorder Society (MDS) -commissioned task force [90] assessed the clinimetric properties of existing rating scales, questionnaires, and timed tests that assess posture, gait, and balance in PD and made recommendations on their utilization and the need for modifications or replacement. A literature review was conducted. Identified instruments were evaluated systematically and classified as “recommended,” “suggested,” or “listed.” Inclusion of rating scales was restricted to those that could be readily used in clinical research and practice. The results of their findings were: one rating scale was classified as “recommended” (the UPDRS-derived Postural Instability and Gait Difficulty Score) and two as “suggested” (Tinetti Balance Scale, Rating Scale for Gait Evaluation). Three scales requiring equipment (Berg Balance Scale, Mini-BESTest, Dynamic Gait Index) also fulfilled the criteria for “recommended” and two for “suggested” (FOG score, Gait and Balance Scale). Four questionnaires were “recommended” (Freezing of Gait Questionnaire, Activities-specific Balance Confidence Scale, Falls Efficacy Scale, Survey of Activities, and Fear of Falling in the Elderly–Modified) and four tests were classified as “recommended” (6-min and 10-m walk tests, Timed Up-and-Go, Functional Reach).

Bloem et al. [90], identified several questionnaires that adequately assess FOG and balance confidence in PD and a number of useful clinical tests. However, they found no scale suitable for all clinical purposes to assess gait, balance, and posture, as none of the instruments adequately and separately assesses all constructs, that is, gait (including FOG), balance, and posture. It is also unlikely that a single unidimensional scale can be developed for all three constructs (gait, balance, and posture) because of the potential heterogeneity in the underlying pathophysiology. Given that these concepts are interrelated but independent constructs, it is recommended to assess them simultaneously, but to ensure that separate scores are obtained for the constructs. An ideal future scale should therefore include separate sections for gait, balance, and posture and should specifically address FOG and fear of falling.

Educational status should be taken into consideration when assessing balance, because both executive function and educational level influence balance tests in older adults [91]. People with fewer years of formal education have more difficulty choosing adequate motor strategies for performing tasks. Meanwhile, individuals with higher education, because of their more efficient synaptic and neural networks, may find better motor solutions to imposed motor difficulties. Souza et al. [92] evaluated executive function and functional balance (assessed by the Berg Balance Scale [BBS]) in PD patients (HY score between 2 and 3) and healthy elderly people. Each group of participants was divided by educational status: low (4–10 years of education) or high (11 years or more). The participants with PD and a high educational status performed better on the BBS than did those with PD and low educational status.

The PT program should involve muscle resistance training for quadriceps, hamstrings, and foot extensors to improve posture, muscle stiffness in addition to gait and balance parameters [93, 94]. In PD, there is an imbalance between agonist muscles that facilitate opening movements (e.g., extensors, supinator muscles, external rotators, scapula, and pelvic abductors) and antagonist muscles that facilitate closing movements (flexors, pronators, internal rotators , and adductors), as shown by the difficulties encountered in performing quick, alternating movements in pronation–supination or flexion–extension [95]. PT programs must also include passive stretching of antagonist muscles and strengthening of agonist muscles [95]. Some exercise s for active mobilization in axial rotation should be implemented to fight stiffness, which affects the trunk. Tai Chi exercises can also help to reduce balance disorders [78].

As the disease progresses, complex motor sequences, such as transfers, may no longer be performed automatically [42]. Transfers that are particularly problematic include rising from, and sitting down onto a chair, getting in or out of bed, and turning over in bed [96]. A common problem during sit-to-stand transfers is that PD patients fail to lean forward far enough when standing up, thus falling back into the chair [41]. Cognitive cues would act as a potential facilitator of transfers. Motor strategies are designed to keep the gestures simple and easy, such as performing varied and repeated exercises dedicated to a precise daily activity to optimize training and facilitate transfers in the patient’s daily life. Cued training could improve motor control during daily activities, such as the sit-to-stand task and rising from the bed in people with PD (Figs. 2.3 and 2.4). For example, sit-to-stand can be divided into four phases: “open legs,” “move forward,” “move upward,” and “stand up.”

Manual activities may become difficult to perform because of the complex motor sequences required. The coordination, efficiency, and speed of reach and dexterity of movements are often diminished; therefore, therapies may be directed at addressing loss of mobility, difficulties in reaching, grasping, and manipulating objects. and self-care activities such as eating and dressing. Nonstandardized techniques, commonly observational analysis or nonstandardized timed tasks, were most often used to assess impairments and activity limitations. There are limited formal clinical guidelines for treating and evaluating impairment of manual activities in PD patients.

Studies on physical exercises with PD animal models have shown many plastic processes involved with neuroprotection mechanisms, such as angiogenesis improvement [97], increased anti-inflammatory responses [98], decreased inflammatory responses [99], and improvement of mitochondrial functions [100]. The neuroprotective effect of exercises in PD was also reported in rats trained on a treadmill [99–101].

It is possible that the main factors related to the neuroprotective effect of the physical exercise are neurotrophic factors such as the brain-derived neurotrophic factor (BDNF) . These factors are essential to cellular differentiation, neuronal survival, migration, dendritic arborization, synaptogenesis, and synaptic plasticity [102]. BDNF expression is decreased in PD animal models [99–101] and in PD postmortem brains [103]. On the other hand, physical exercise is able to recover BDNF levels both in animal models [99–101, 104] and in patients with PD [105].

According to Petzinger et al. [106], exercise is beneficial for PD rehabilitation because it incorporates many aspects of practice important for goal-directed motor skill learning. These elements include repetition, intensity, and challenge, which together with skill training lead to improvement in motor performance.

Although there is a wide spectrum of exercise modalities, most of the studies share common elements, including goal–based practice for the acquisition of a skill, and reinforcement (learning through feedback). The feedback in turn serves to challenge patients beyond self-selected levels of perceived capability, maintaining motivation, and facilitating the engagement of individuals to become cognitively aware of movements that were previously automatic and unconscious [106].

Aerobic training is encouraged and all patients should exercise in their best medicated state. Shu et al. [107] conducted a meta-analysis study to evaluate the effectiveness of aerobic exercise for PD and suggested that aerobic exercise (including treadmill training, dancing, walking, and Tai Chi) might significantly improve motor action, balance, and gait, including gait velocity, stride/step length, and walking ability.

In relation to resistance training, a recent systematic review suggested that resistance training might have a positive effect on both muscle strength outcomes and functional outcomes. Resistance training was shown to increase fat-free mass, muscle strength, and improve mobility and performance in PD patients [108].

Most of the research target the motor symptoms of PD, but Reynolds et al. [109] reviewed the literature searching for the efficacy of aerobic and strengthening exercise interventions relative to mood, cognition, and sleep. The author concluded that aerobic and strengthening exercises offer especially promising treatment strategies for alleviating nonmotor symptoms of PD.

Despite a large number of studies demonstrating that exercise improves motor performance in PD, there is no consensus on which frequency and duration of treatment, number of repetitions, and difficulty/complexity of exercises should be endorsed [45]. Most of the studies adopt a global treatment program of 6–8 h distributed over 6–12 weeks, with 1–1.5 h of training per week. It has been suggested that intensive, specific exercise might improve rehabilitation outcomes for PD patients. To be considered an intensive program the exercise should be practiced 2–3 h/week for 6–14 weeks, totaling 12–42 h of treatment [110].

Based on the guideline of the American College of Sports Medicine (ACSM) van der Kolk and King [43] suggest the following exercises for PD: the program should include aerobic, strengthening, flexibility, and balance training. Aerobic training should be done 5 days/week for 30 min of moderate intensity or 3 days/week for 20 min at vigorous intensity. Strengthening should be performed 2 days/week, 8–10 exercises involving major muscle groups and 2 sets of 10–15 repetitions. Flexibility exercises should be done for 10 min each time. Balance training should be introduced for those who are at risk for falls. However, many PD patients present postural instability precociously; thus, balance exercise should be applied in accordance with patients’ needs.