Human skin, principal strain, lumbar skin, vertebrae, collagen fiber orientation, skin adhered wearables, low back pain


Increasing efforts to leverage skin mounted wearables has exposed a gap in our understanding of skin strain in the lumbar region. Lumbar skin is known to be both inhomogenous and anisotropic, and experiences dramatic material strain during activities of daily living (ADLs). Previous efforts to quantify strain fields during ADLs has been limited to a few activities (primarily flexion) and have neglected inhomogeneity or anisotropy. The purpose of the present work was to address this gap by measuring large-deformation skin strain tensors in healthy volunteers during a cadre of functional movements.

36 tightly packed retro-reflective markers were mounted to the surface of the lumbar region of 30 volunteers in order to measure the inhomogeneous, anisotropic skin deformation during 11 functional movements. Large deformation strain tensors were computed based on the marker motion, from which principal strains and principal strain directions were derived. Principal strain rates were also calculated for the regions of highest strain.

The largest principal strains (72% - 103%) occurred during sagittal movements (Flexion, Flexion right/left, and Sit To Stand) in the lower lumbar region, with a vertically oriented principal strain orientation located between 35 and 60 mm laterally from the central line of spinous processes. Skin strain data between the T11 and T12 vertebrae showed that transverse movements (Rotation right/left and Extension right/left) caused principal strain orientations directed towards the left/right ribcage respectively with up to 27% principal strain magnitude. Frontal plane movements caused vertically oriented principal strains that increased the more lateral of location from the spinal axis, with 15% at a location 30 mm lateral and 30% at a location 85 mm lateral. Principal strain rates were also measured which were between 23% and 151% strain per second depending on the movement. The variation of principal skin strains in the thoracolumbar region between participants was only significant for movements involving flexibility: Flexion (12.2%) and Flexion right/left (15.1%/8.07%), or technique: Sit To Stand (13.9%). Motions that did not involve participant flexibility had minimal inter-subject variability (0.077% - 2.29%). All motions had minimal intra-subject variability (0.031% - 1.67%). Additionally, principal strain orientations for functional movements were consistent between repetitions.

Of note, the magnitudes and orientations of the largest principal strains during the measured ADLs agrees with the facet geometry of lumbar vertebrae, which allow greater spinal flexibility in the sagittal plane and the facet geometry of the thoracic vertebrae, which allow for greater spinal flexibility in the axial plane. Additionally, the lateral location of maximum strain agrees with the location of the erector spinae muscles which are positioned bilateral to the spinal vertebrae. Additionally, upper lumbar locations had principal strain orientations directed supero-laterally during sagittal movements, which agrees with the collagen fiber orientation which is supero-medial (e.g., perpendicular to the collagen fiber orientation).

These data may be helpful in the design of skin-adhered wearables, as well as in pre-operative incision planning in the context of expected post-operative motion (e.g., after the patient leaves the hospital), and post-operative wound healing.

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Ira A. Fulton College of Engineering


Mechanical Engineering

University Standing at Time of Publication

Graduate Student