title: The effect of muscle fatigue and low back pain on lumbar movement variability and complexity.
abstract: BACKGROUND
  Changes in movement variability and complexity may reflect an adaptation strategy to fatigue. One unresolved question is whether this adaptation is hampered by the presence of low back pain (LBP). This study investigated if changes in movement variability and complexity after fatigue are influenced by the presence of LBP. It is hypothesised that pain free people and people suffering from LBP differ in their response to fatigue.
  
  
  METHODS
  The effect of an isometric endurance test on lumbar movement was tested in 27 pain free participants and 59 participants suffering from LBP. Movement variability and complexity were quantified with %determinism and sample entropy of lumbar angular displacement and velocity. Generalized linear models were fitted for each outcome. Bayesian estimation of the group-fatigue effect with 95% highest posterior density intervals (95%HPDI) was performed.
  
  
  RESULTS
  After fatiguing %determinism decreased and sample entropy increased in the pain free group, compared to the LBP group. The corresponding group-fatigue effects were 3.7 (95%HPDI: 2.3-7.1) and -1.4 (95%HPDI: -2.7 to -0.1). These effects manifested in angular velocity, but not in angular displacement.
  
  
  CONCLUSIONS
  The effects indicate that pain free participants showed more complex and less predictable lumbar movement with a lower degree of structure in its variability following fatigue while participants suffering from LBP did not. This may be physiological responses to avoid overload of fatigued tissue, increase endurance, or a consequence of reduced movement control caused by fatigue.
authors.affiliation: Zurich University of Applied Sciences, Department of Health, Institute of Physiotherapy, Technikumstrasse 71, 8400 Winterthur, Switzerland; University of Tampere, School of Medicine, Kalevantie 4, 33014 University of Tampere, Finland. Electronic address: christoph.bauer@zhaw.ch.
authors.forename: C M
authors.initials: CM
authors.lastname: Bauer
authors.affiliation: Zurich University of Applied Sciences, Department of Health, Institute of Physiotherapy, Technikumstrasse 71, 8400 Winterthur, Switzerland.
authors.forename: F M
authors.initials: FM
authors.lastname: Rast
authors.affiliation: Zurich University of Applied Sciences, Department of Health, Institute of Physiotherapy, Technikumstrasse 71, 8400 Winterthur, Switzerland.
authors.forename: M J
authors.initials: MJ
authors.lastname: Ernst
authors.affiliation: Zurich University of Applied Sciences, Department of Health, Institute of Physiotherapy, Technikumstrasse 71, 8400 Winterthur, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Meichtry
authors.affiliation: Zurich University of Applied Sciences, Department of Health, Institute of Physiotherapy, Technikumstrasse 71, 8400 Winterthur, Switzerland.
authors.forename: J
authors.initials: J
authors.lastname: Kool
authors.affiliation: University of Eastern Finland, Department of Applied Physics, P.O. Box 1627, 70211 Kuopio, Finland. Electronic address: saara.rissanen@uef.fi.
authors.forename: S M
authors.initials: SM
authors.lastname: Rissanen
authors.affiliation: UKK Institute for Health Promotion Research, Kaupinpuistonkatu 1, 33500 Tampere, Finland. Electronic address: jaana.h.suni@uta.fi.
authors.forename: J H
authors.initials: JH
authors.lastname: Suni
authors.affiliation: University of Tampere, School of Medicine, Kalevantie 4, 33014 University of Tampere, Finland; Pirkanmaa Hospital District, Physical and Rehabilitation Medicine Outpatient Clinic, Box 2000, 33521 Tampere, Finland. Electronic address: markku.kankaanpaa@pshp.fi.
authors.forename: M
authors.initials: M
authors.lastname: Kankaanpää
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D017116:Low Back Pain / Q000503:physiopathology*; D008161:Lumbosacral Region / Q000502:physiology* / Q000503:physiopathology; D008297:Male; D008875:Middle Aged; D009068:Movement; D018763:Muscle Fatigue; D018482:Muscle, Skeletal / Q000502:physiology* / Q000503:physiopathology
keywords: Biomechanical phenomena; Fatigue; Low back pain; Movement disorders