title: Enhancing muscle force and femur compressive loads via feedback-controlled stimulation of paralyzed quadriceps in humans.
abstract: OBJECTIVE
  To compare paralyzed quadriceps force properties and femur compressive loads in an upright functional task during conventional constant-frequency stimulation and force feedback-modulated stimulation.
  
  
  METHODS
  Crossover trial.
  
  
  METHODS
  Research laboratory.
  
  
  METHODS
  Subjects (N=13; 12 men, 1 woman) with motor-complete spinal cord injury.
  
  
  METHODS
  Subjects performed 2 bouts of 60 isometric quadriceps contractions while supported in a standing frame. On separate days, subjects received constant-frequency stimulation at 20Hz (CONST) or frequency-modulated stimulation triggered by a change in force (FDBCK). During FDBCK, a computer algorithm responded to each 10% reduction in force with a 20% increase in stimulation frequency.
  
  
  METHODS
  A biomechanical model was used to derive compressive loads on the femur, with a target starting dose of load equal to 1.5 times body weight.
  
  
  RESULTS
  Peak quadriceps force and fatigue index were higher for FDBCK than CONST (P<.05). Within-train force decline was greater during FDBCK bouts, but mean force remained above CONST values (P<.05). As fatigue developed during repetitive stimulation, FDBCK was superior to CONST for maintenance of femur compressive loads (P<.05).
  
  
  CONCLUSIONS
  Feedback-modulated stimulation in electrically activated stance is a viable method to maximize the physiologic performance of paralyzed quadriceps muscle. Compared with CONST, FDBCK yielded compressive loads that were closer to a targeted dose of stress with known osteogenic potential. Optimization of muscle force with FDBCK may be a useful tactic for future training-based antiosteoporosis protocols.
authors.affiliation: Physical Therapy and Rehabilitation Science, Carver College of Medicine, The University of Iowa, Iowa City, IA, USA.
authors.forename: Shauna
authors.initials: S
authors.lastname: Dudley-Javoroski
authors.forename: Andrew E
authors.initials: AE
authors.lastname: Littmann
authors.forename: Shuo-Hsiu
authors.initials: SH
authors.lastname: Chang
authors.forename: Colleen L
authors.initials: CL
authors.lastname: McHenry
authors.forename: Richard K
authors.initials: RK
authors.lastname: Shields
mesh_terms: D000328:Adult; D000465:Algorithms; D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D015519:Bone Density; D018592:Cross-Over Studies; D004599:Electric Stimulation Therapy; D005246:Feedback; D005260:Female; D005269:Femur / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology; D010243:Paralysis / Q000503:physiopathology* / Q000534:rehabilitation*; D052097:Quadriceps Muscle / Q000503:physiopathology*; D013119:Spinal Cord Injuries / Q000503:physiopathology* / Q000534:rehabilitation*; D016474:Weight-Bearing / Q000502:physiology*