title: Novel Methods for Sensing Acoustical Emissions From the Knee for Wearable Joint Health Assessment.
abstract: We present the framework for wearable joint rehabilitation assessment following musculoskeletal injury. We propose a multimodal sensing (i.e., contact based and airborne measurement of joint acoustic emission) system for at-home monitoring.
  
  
  
  We used three types of microphones-electret, MEMS, and piezoelectric film microphones-to obtain joint sounds in healthy collegiate athletes during unloaded flexion/extension, and we evaluated the robustness of each microphone's measurements via: 1) signal quality and 2) within-day consistency.
  
  
  
  First, air microphones acquired higher quality signals than contact microphones (signal-to-noise-and-interference ratio of 11.7 and 12.4 dB for electret and MEMS, respectively, versus 8.4 dB for piezoelectric). Furthermore, air microphones measured similar acoustic signatures on the skin and 5 cm off the skin (∼4.5× smaller amplitude). Second, the main acoustic event during repetitive motions occurred at consistent joint angles (intra-class correlation coefficient ICC(1, 1) = 0.94 and ICC(1, k) = 0.99). Additionally, we found that this angular location was similar between right and left legs, with asymmetry observed in only a few individuals.
  
  
  
  We recommend using air microphones for wearable joint sound sensing; for practical implementation of contact microphones in a wearable device, interface noise must be reduced. Importantly, we show that airborne signals can be measured consistently and that healthy left and right knees often produce a similar pattern in acoustic emissions.
  
  
  
  These proposed methods have the potential for enabling knee joint acoustics measurement outside the clinic/lab and permitting long-term monitoring of knee health for patients rehabilitating an acute knee joint injury.
authors.forename: Caitlin N
authors.initials: CN
authors.lastname: Teague
authors.forename: Sinan
authors.initials: S
authors.lastname: Hersek
authors.forename: Hakan
authors.initials: H
authors.lastname: Toreyin
authors.forename: Mindy L
authors.initials: ML
authors.lastname: Millard-Stafford
authors.forename: Michael L
authors.initials: ML
authors.lastname: Jones
authors.forename: Geza F
authors.initials: GF
authors.lastname: Kogler
authors.forename: Michael N
authors.initials: MN
authors.lastname: Sawka
authors.forename: Omer T
authors.initials: OT
authors.lastname: Inan
mesh_terms: D000328:Adult; D001314:Auscultation / Q000295:instrumentation*; D001696:Biomechanical Phenomena / Q000502:physiology; D006801:Humans; D007719:Knee Joint / Q000503:physiopathology*; D008297:Male; D018670:Monitoring, Ambulatory / Q000295:instrumentation*; D011474:Prosthesis Design; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*; D055815:Young Adult