Experimentally measured lumbar skin strains during functional movements

There has been an influx of skin-adhered wearables that have begun to show promise for their ability to continuously monitor spinal kinematics based on skin deformation for assessing spine-related problems including low-back pain. However, a lack of information regarding the amount of stretch (or strain) that lumbar skin experiences when wearers perform uniplanar or multiplanar movements makes the designing of these wearables difficult. In this study, skin motion was measured using a relatively dense grid of small reflective markers during 6 uniplanar, 4 multiplanar, and 1 activity of daily living (ADL) movements of increasing functionality. These data were used to compute dynamic, inhomogeneous, anisotropic strain fields of lumbar skin based on large deformation strain theory. Of particular note, macroscopic principal strains were highest in Flexion, reaching averages as high as 103 %, with strain rates up to 151 % per second. Principal strain orientations were movement dependent. Males exhibited higher principal strains than females during Flexion (p = 0.0027) and Sit To Stand (p = 0.0453) motions. Repeatability was high between repetitions, ranging from 71.1 % (extension) to 97.2 % (Sit To Stand motion). Skin strain fields were sensitive to both underlying spinal geometry and dermal collagen fiber orientations. The results of this study are relevant to the precision of spinal-specific wearables when placed on different regions of the lumbar skin and may also have clinical relevance to choice of surgical incision orientation and wound care in the lumbar region.