Comfort Thresholds for Repetitive Forces
Condition: Healthy
Study type: Interventional

Summary: 
      Exoskeletons can be used for rehabilitation, as assistive device for patients, or to support
      workers during strenuous tasks. To fulfil their purpose, they need to apply forces to the
      user's musculoskeletal system. The forces are transmitted at skin level through an interface,
      often in the form of cuffs. Adverse events causing discomfort and injuries to the skin and
      underlying tissue can be attributed to those interaction forces. While there is some
      information about safe limit values for impact forces or pressure and shear applied for short
      durations, little is known regarding comfort and safety thresholds for repetitive forces
      applied over long durations as is the case in exoskeleton use. This study therefore aims at
      gaining new knowledge on safe and limit values, based on discomfort (staying below pain
      threshold), for continuous repetitive shear and normal forces applied through a cuff.

      Therefore, the primary objective is to determine comfort thresholds for prolonged exposure to
      repetitive normal and shear stress exerted to the human thigh via a cuff with straps, using
      different force patterns comparable to those exerted during exoskeleton use. The secondary
      objectives are to determine the feasibility of the experiment, the influence of subject
      characteristics on comfort thresholds, the occurrence of skin injuries or other negative
      signs, and whether characteristics of muscle activity can be related to discomfort.
    

Detailed description: 
      Rationale: Exoskeletons can be used for rehabilitation, as assistive device for patients, or
      to support workers during strenuous tasks. To fulfil their purpose, they need to apply forces
      to the user's musculoskeletal system. The forces are transmitted at skin level through an
      interface, often in the form of cuffs. Adverse events causing discomfort and injuries to the
      skin and underlying tissue can be attributed to those interaction forces. While there is some
      information about safe limit values for impact forces or pressure and shear applied for short
      durations, little is known regarding comfort and safety thresholds for repetitive forces
      applied over long durations as is the case in exoskeleton use. This study therefore aims at
      gaining new knowledge on safe and limit values, based on discomfort (staying below pain
      threshold), for continuous repetitive shear and normal forces applied through a cuff.

      Objective: The primary objective is to determine comfort thresholds for prolonged exposure to
      repetitive normal and shear stress exerted to the human thigh via a cuff with straps, using
      different force patterns comparable to those exerted during exoskeleton use. The secondary
      objectives are to determine the feasibility of the experiment, the influence of subject
      characteristics on comfort thresholds, the occurrence of skin injuries or other negative
      signs, and whether characteristics of muscle activity can be related to discomfort.

      Study design: The study is a cross-sectional intervention study with one measurement session,
      where participants will provide a continuous comfort rating during exposure to repetitive
      normal and shear forces.

      Study population: The aim is to include 20 healthy participants in total, matching the age
      groups 18-35 and 55-85.

      Intervention: A pre-defined set of prolonged repetitive force patterns will be applied to
      each participant's thigh though a contact force which is similar to cuffs commonly used in
      gait exoskeletons. The forces will be applied by a custom made actuated test device and are
      based on normal use of exoskeletons.

      Main study parameters/endpoints: The main study parameter is the level of discomfort as
      indicated continuously by the participants through a slider with a visual analogue scale. The
      scale reaches from no discomfort at all to maximum imaginable discomfort. A separate switch
      which releases all forces if activated by the participant is used to detect onset of pain.

      Nature and extent of the burden and risks associated with participation, benefit and group
      relatedness: The study is non-therapeutic, so no direct benefits for the participants are
      involved. The procedures are non-invasive and safety measures are taken to avoid excessive
      force exertion to the participants or other unsafe situations. Therefore, subjects suffer no
      negative effects or disadvantages, except for the invested time and the discomfort
      experienced during the force execution. Pain will be avoided as participants can trigger a
      release of the forces at any time. Any changes to the participants' skin or other negative
      signs will be monitored and followed up on until they have resolved.
    

Intervention name(s): Application of repetitive forces mimicking exoskeleton use
Intervention type(s): Other
Intervention description(s): The investigational treatment consists of applying a predefined set of continuous and repetitive forces to the participant's thigh. It is applied using a custom device that has been designed specifically for this study. The forces to be applied are based on forces measured during exoskeleton use and will not exceed pain pressure thresholds reported in literature. The participants will hold an enabling switch at all times which they can activate to release the force exerted to their thigh. Alternatively, the experimenter can activate an emergency switch if the situation seems unsafe.
Intervention model: Single Group Assignment
Intervention model description: The study is a cross-sectional intervention study with one measurement session, where participants will provide a continuous comfort rating during exposure to repetitive normal and shear forces.
Masking: None (Open Label)
Primary purpose: Other
Keywords: Exoskeleton; Safety; Comfort; physical Human-Robot Interaction