# Lower- Limb Exosuits for Rehabilitation or Assistance of Human Movement: A Systematic Review
## 3 Review
### Exosuit Technology
The functionality of an exosuit depends highly on the mechanical components selected to actuate and transmit forces to the body. To transfer movement to the lower extremities during the performance of tasks, particularly Bowden cables and pneumatic actuators are used. Bowden cables stand out as a readily available technology that has been shown to be effective in exosuits [43]. They allow for application with little space and can adapt to complex surfaces. Another material for force transmission in small spaces might be shape memory alloys (SMA). Thin cables of these alloys can be used to act as contracting muscles when applying heat or electricity. In exosuits for lower limbs, no publication was found using this material, while in exosuits for upper limbs, SMA-based actuators have been used already [44; 45]. Another material for this application might be an active mechanical metamaterial that can provide soft, bendable \"smart matter\" [46] that might adapt and actuate in an exosuit. Main sensor technologies to provide input in active exosuit controllers were (i) force, pressure or strain (11 studies) and (ii) Inertial Measurement Units (IMU) or gyroscopes (8 studies) (see Figure 2e). Studies with passive exosuits did not employ sensors. Sensors for the monitoring of load or position of the addressed articulations are easily available as commercial products and are easy to implement in the active exosuits. The use of sensors with a continuous variable output is preferable because the measured parameters of load, movement or position themselves are continuous during movement. While electroencephalogram (EEG) and electromyography (EMG) measurements are better suited to detect the user's movement intention, in the reviewed publications EMG was only used for clinical assessment. No exosuit used the EMG data as an input for control, most likely because these physiological measurements need an advanced computational analysis. Control schemes ranged from having an (i) automatic closed-loop control that detected the movement intended by the user and adjusted the actuators accordingly, and (ii) open-loop control, where forces and displacements were delivered by stretching or compressing elastic material during movement. All active exosuits used a closed-loop automatic control scheme, while all passive exosuits used no external control. It is preferable to use an automatic control that reacts to the intended movement of the user and adjusts the actuator's force or position accordingly by analyzing sensor input. For the automatic control, the necessary real-time analysis still faces challenges, for example, the use of adequate sensor sampling frequencies, the availability of processing power on low-power wearable devices or reducing the delay due to sensor data packetizing during data transmission [47]. Most studies stated that exosuits must be lightweight, but of the 19 studies included, none using passive exosuits stated their weight (52% of all studies). Reporting the weight of the studied exosuit should be highly encouraged for future publications because the wearability of exosuits is derived mostly from their reduced weight. The weight of active exosuits, categorized by force transmission type, was the lightest in pneumatic systems (0.26 kg in tethered use) compared to Bowden cable systems (up to 5 kg untethered). Compared to active rigid exoskeletons with up to 25 kg [48], active exosuits have a reduced weight, thanks to the lack of a rigid frame. Nevertheless, they rely on electric motors and pneumatic actuators for moving the users' body, which add weight to the exosuit, and their power output and consumption determine the assisting power of actuation and the autonomy of the suit. Actuation systems and power supply units must be dimensioned carefully to reduce weight while maintaining function. Even though newer types of actuators and power sources might help to minimize weight even further in the future, the use of passive exosuits or combination of active and passive systems might be considered.