## 1 Introduction  
Parkinson's disease (PD) is a degenerative disorder of the central nervous system. It is the second most common neurodegenerative disease after Alzheimer-type dementia [1]. PD is characterized by the loss of dopaminergic neurons in the compact part of the _substantia nigra_ of the basal ganglia. This explains the symptomatology that characterizes PD patients in both motor (rigidity, tremors, bradykinesia, and postural reflex and gait disorders) and non-motor ways (disorders of the neuropsychiatric sphere and cognitive impairment or sleep impairment, among others) [2, 3]. The degeneration suffered by patients with PD leads to their personal, family, and social spheres being affected, which increases their social isolation. Although the basis of PD treatment is medical-surgical and pharmacological (particularly for levodopa), owing to these patients' functional impairment, which deteriorates over time, exercise therapy has been shown to prevent functional disability and also improve both some physical abilities and their quality of life [4, 5, 6, 7, 8, 9, 10, 11], thus limiting the negative influence of symptomatology and slowing the progression of the disease [10, 12]. Furthermore, it appears that high-speed, complex, and repetitive movements result in activity-dependent neuroplasticity [13, 14, 15, 16, 17, 18], or as [PERSON] et al. [19] pointed out in their recent study, intense exercise improves the functionality of dopaminergic neurons, which become neuromodulatory and neuroprotective against PD. Among the various methods of carrying out high-speed or intense physical activity, we came across some studies that analyzed the potential benefits of high-cadence cycling or forced exercise cycling [11; 13; 15; 18; 20; 21; 22; 23; 24]. [PERSON] et al. [11] pointed out that this method of cycling is more than just a passive increase in cadence; rather, it is a technique by which the patient is forced to be active and is assisted by the machine in acquiring the desired speed, which is higher than that which he or she would achieve on his or her own. Other benefits of this type of exercise in PD patients are aspects such as improved joint mobility ranges [21], improved gait [22; 24; 25], increased hand coordination [18; 22; 24], an increase in brain activity similar to that produced by medication [13; 15; 20; 26], a reduction in motor symptomatology such as rigidity, tremors, and bradykinesia [13; 18; 23; 24; 25; 27], improved muscular endurance and cardiovascular fitness [18; 28], improved sequential coordination of movements during gait transfers [29], and finally not presenting side effects from any medication. Moreover, these benefits have longer lasting effects over time (for at least 48 h, according to [PERSON] and [PERSON]) [24]. Despite all their positive aspects, most of these studies are short interventions of only a few sessions or even one standalone session delivered indoors [13; 20; 21; 23; 24; 27]. Also, as [PERSON] et al. [11] pointed out, cycling interventions have some drawbacks, such as the need for someone to assist the patient with his or her exercise, the difficulty in getting on and off the bike (in the case of tandem cycling), the monotony of the environment (indoors), the possible decrease in motivation and lack of adherence to the protocols when carried out alone (in the case of cyclooxygenase), and finally the cost (in both cases but especially for cyclooxygenase). Immersive virtual reality (IVR) is a possible solution to some of these drawbacks, as it could increase patient motivation and thus adherence to forced exercise programs using cyclooxygenase. IVR, via a head-mounted display (HMD), immerses patients into a virtual world where they can experience different environments and carry out tasks in first-person scenarios, which are comparable to real situations [30]. This technology has previously been used with PD patients and has achieved promising therapeutic results [5; 6], although not in combining high-intensity forced cycling and IVR interventions.