Limitations due to physiological parameters
This review identified that lower limb maximum muscle torques and forces, as well as leg extensor power, decreased with increasing age. For daily movements, increased joint torque and power requirements were identified for walking inclines and climbing stairs compared to level walking (Fig.
7). Thus, it is expected that both movement tasks will most likely challenge older adults and mobility-impaired individuals. In [
66], reduced quadriceps strength was identified as a reason for reduced stair climbing cadence in older adults. Additionally, older adults reached 75% of their maximum possible extensor moment in stair climbing, while younger adults reached 53% [
67]. Thus, the effort of older adults is greater and muscle fatigue may occur earlier. We expect similar effects in user effort for level walking and walking inclines. Furthermore, limited muscle power is linked to incident disability, mortality, falls, hospitalization, and health care resource consumption [
46].
This review identified a loss of VO
2max with increasing age or due to diseases. As the oxygen consumption at self-selected walking speed is below the VO
2max of most older adults (Fig.
4), these individuals should be able to handle the effort for short periods of time. With increasing locomotion time, sub-maximal values of VO
2max must be considered. For intervals of three minutes, walking or running in the Bruce GXT test, values above 70% of VO
2max were categorized as hard [
154]. A study on carrying loads on different terrain in men and women showed that, for all different conditions, the self-selected pace of the subjects required 45% of the individual VO
2max [
155]. This value seems to be the acceptable working limit for a duration of one to two hours. For young soldiers carrying loads over six hours for multiple days, the self-selected pace was approximately at 30%-40% of the VO2max [
156]. If these percentages of the VO
2max are assumed as continuous limits for level walking, it might explain part of the reductions in maximum, maximum six-minute, and self-selected walking speed of older adults and those that are mobility-impaired. In addition to some percentile of older adults without observable limitations, in comparison to young adults there will be some percentile with great restrictions, similar to the distribution for the steps per day (Fig.
6). In comparison to level walking, oxygen requirements for stair climbing and walking inclines (with a speed of young adults) are above the maximum for most older adults (Fig.
4). To perform both tasks, older adults need to reduce their speed, similar to the strategy employed by mountain runners [
151]. Studies of individuals with respiratory, cardiovascular, and neurological diseases showed clear reductions for VO
2max to levels of less than the half of unimpaired subjects of the same age group (Fig.
4). In addition, maximum (six-minutes) and self-selected level walking speed of the impaired populations analyzed were below the mean self-selected level walking speed of the unimpaired controls (Fig.
5). Thus, these groups are likely to struggle to perform daily locomotion tasks at self-selected speeds compared to unimpaired individuals of the same age.
Older adults showed only small reductions in self-selected walking speed compared to the reductions in maximum muscle force, maximum power, and VO2max. Thus, maximum physiological parameters seem to impact maximum performance (e.g. maximum walking speed) to a greater degree than movements that only require medium level effort (e.g. preferred walking speed). Typically daily locomotion is done at speeds up to the self-selected walking speed, which should require a medium level effort. But the number of steps per day decreased much more with increasing age than the physiological values (e.g. force, VO2max). This suggests that not only physiological, but other factors, such as not having a need to work, might play an important role in the reduction in steps per day.
Improving the functional capacity
Based on the physical and functional parameters analyzed in this work, we identified several mobility-related losses, due to aging and diseases, that have the potential to be improved. Functional improvements can include upright standing and locomotion, increasing locomotion speed, steps per day, reaction time, improving balance (risk of falling), or improving gait patterns, which includes the reduction of asymmetries.
We found that most functional tasks are affected by the same physical deficits, including muscle strength, muscle power, and VO2max. Consequently, with reduced levels, other factors such as fatigue, effort, pain, or joint stress have the potential to increase.
As physical and functional parameters are highly related to each other, it is not surprising that losses due to aging or disease in one area also reduce capabilities in other areas. For example, individuals with cardiovascular diseases (PVD) suffer from increased reaction times and fall rates, or individuals with respiratory diseases (COPD) suffer from reductions in maximum muscle power. Thus we believe that improvements in the physical capabilities have the potential to improve a wide range of functional parameters.
The authors see two possible options to improve mobility-related functional parameters (e.g. steps per day), and consequently, secondary parameters as well (e.g. pressure sores, body mass index).
The first potential solution is physical training, as physical inactivity was identified as a major cause for physical losses. Training directly targets the improvement of a specific capacity and can partially prevent or help to recover from physical losses.
The second potential solution would bypass the human physical losses to directly improve the mobility by improving the functional capacity. Next to the training approach, this approach is required as this review identified that there will be an inevitable loss of capabilities, especially for older adults from the age of above 70 yrs and for mobility-impaired individuals.
Until now, changes in the environment or the use of assistive devices, such as crutches or walkers, have been used and investigated to compensate for inevitable losses in physical and functional capabilities. Alternatively, assistive devices can also be used during rehabilitation as training devices.
A novel assistive device concept that can address these two options for functional improvements are exoskeletons. Similar to crutches, exoskeletons can be used for daily assistance (compensation) and as a rehabilitation device (recovery). In addition to the improvement of the physical condition, improvements of secondary medical symptoms as well as other movement- and posture-related health outcomes are expected. These improvements will be beneficial for the users also when not wearing the exoskeleton. Compared to devices like crutches, they could also be used as a versatile training device to partially prevent losses similar to other physical exercise devices [
157]. In addition to prevention, the functional compensation, and rehabilitation from losses, exoskeletons provide the possibility to augment user capabilities to levels above that of normal human performance. For example, when using the Raytheon Sarcos’s XOS 2 robotic suit, the user should be able to lift 200 lb of weight for long periods of time without feeling the strain [
158]. So far it is unknown how different levels of assistance will influence the physical capabilities of the users. To prevent from further physical losses, the trade-off between exoskeleton assistance and physical user involvement has to be investigated. We can imagine that muscles might degenerate if the user completely relies on the external force assistance of an exoskeleton. On the other hand, too much effort may overload and fatigue the user. Variable assistance levels, controlled by parameters that indicate human effort (e.g. heart rate) might be a possible way to set an appropriate level of effort.
Thus far commercial exoskeletons have been primarily used in rehabilitation [
159]. A review on lower limb rehabilitation exoskeletons concluded that exoskeletons can be used to regain locomotion capability for impaired with neurological diseases. They can increase mobility, improve functioning, and reduce the risk of secondary injury by reinstating a more normal gait pattern [
159]. For the devices investigated in this review (most commonly ReWalk, HAL, Vanderbilt lower limb exoskeleton), user’s mobility benefited from the exoskeletons body weight support and the propulsion during walking.
Needs such as the compensation for lost locomotion speed or endurance and the reduction of fatigue and effort, may require exoskeletons, which are able to reduce the metabolic cost of walking by providing propulsion to the lower limbs. Examples for autonomous designs that are able to reduce metabolic cost of walking by assisting the hip are from Samsung [
33], Honda [
34], or Georgia Tech [
35]. An autonomous systems with ankle support was designed by MIT [
32]. Ankle and hip assistance was provided with the exosuit from Harvard [
16].
A reduction of gait asymmetries could potentially be addressed with unilateral systems like the ankle exosuit [
160,
161], or with bilateral systems similar to the Ekso-GT [
162], which has demonstrated improved gait metrics by providing propulsion at the deficient limb of people with stroke.
The risk of falling may be reduced by reducing fatigue and asymmetries, improving strength and power, or by using control algorithms within exoskeletons or assistive devices that improve balance or assist to recover from perturbations, as demonstrated in [
163]. As increased reaction times have been associated with falls [
164], artificial sensors in combination with assistive forces could also help to compensate for the human sensory losses.
To reduce joint stress and pain, exoskeletons have to reduce the forces on the cartilage and the bones. Increasing joint stability by antagonistic structures may further decrease pain while moving.
While there are many of gait rehabilitation exoskeletons for clinical environments, there are only a few exoskeletons available that are solutions for improving mobility in daily life for many of the mobility impairments discussed in this work. Necessary technological advances that will allow for greater widespread daily use include improvements to the actuators, sensors, batteries, and the human machine interface. Furthermore, it must be investigated how the control of such assistive devices can deal with different gait patterns, as found in individuals with diseases such as CP [
143]. Next to individual solutions, people with CP, PD, and other diseases require solutions to deal with symptoms like tremors, spasticity, and involuntary movements.
While we see a huge potential to improve the mobility of individuals with the help of lower limb exoskeletons, we believe there is still a lot of development required to create systems that fulfill the needs for the different populations with reduced mobility. Hardware and control complexity should be user-friendly and cover the needs of the desired target population.
Questioning the necessity of lower limb exoskeletons
It is hard to estimate, which level of fatigue, effort, pain, or fall risk would make individuals to choose to use an exoskeleton for daily life mobility assistance. Conventional training, medication, passive walkers or crutches, or even a reduction in movement speed may be preferred alternatives. For shorter distances in level environments in particular, a high amount of older adults without severe physical and functional deficits will not require a lower limb exoskeleton for assistance. The possible benefits of reduced effort or risk of falling might be rated lower compared to the effort of donning and doffing or charging of the exoskeleton. Further, financial expenses for the device could be disincentive for use.
To establish the usage, the advantages of exoskeletons must be perceived to be higher by the users compared to the disadvantages. We clearly see this for target populations with severe mobility impairments due to diseases. On the other hand, we could imagine that also young and healthy people might use such devices to augment their capabilities at the workplace or for activities such as hiking or running. User-friendly (e.g., robust, simple) exoskeleton solutions that work for these applications might also improve the accessibility for populations with moderate limitations in mobility.