title: The relative timing of VMO and VL in the aetiology of anterior knee pain: a systematic review and meta-analysis.
abstract: BACKGROUND
  Anterior knee pain (AKP) is a common musculoskeletal complaint. It has been suggested that one factor that may contribute to the presence of AKP is a delay in the recruitment of the vastus medialis oblique muscle (VMO) relative to the vastus lateralis muscle (VL). There is however little consensus within the literature regarding the existence or nature of any such delay in the recruitment of the VMO within the AKP population. The purpose of this systematic review and meta-analysis was to examine the relative timing of onset of the VMO and VL in those with AKP in comparison to the asymptomatic population.
  
  
  METHODS
  The bibliographic databases AMED, British Nursing Index, CINAHL, EMBASE, Ovid Medline, PEDro, Pubmed and the Cochrane Library were searched for studies comparing the timing of EMG onset of the VMO and VL in those with AKP versus the asymptomatic population. Studies fulfilling the inclusion criteria were independently assessed. Heterogeneity across the studies was measured. A meta-analysis of results was completed for those studies where adequate data was supplied. Where comparable methodologies had been used, results were pooled and analysed.
  
  
  RESULTS
  Fourteen studies met the inclusion criteria; one prospective and thirteen observational case control. Eleven compared VMO and VL EMG onset times during voluntary active tasks while four investigated reflex response times. All used convenience sampling and did not state blinding of the assessor. Study methodologies/testing and assessment procedures varied and there was considerable heterogeneity within individual samples. Whilst a trend was identified towards a delay in onset of VMO relative to the VL in the AKP population during both voluntary active tasks and reflex activity, a substantial degree of heterogeneity across the pooled studies was identified (I2 = 69.9-93.4%, p < 0.01).
  
  
  CONCLUSIONS
  Findings are subject to substantial and unexplained heterogeneity. A trend was demonstrated towards a delayed onset of VMO relative to VL in those with AKP in comparison to those without. However not all AKP patients demonstrate a VMO-VL dysfunction, and this is compounded by normal physiological variability in the healthy population. The clinical and therapeutic significance is therefore difficult to assess.
authors.affiliation: School of Allied Health Professions, University of East Anglia, Norwich, Norfolk, NR4 7TJ, UK. r.chester@uea.ac.uk
authors.forename: Rachel
authors.initials: R
authors.lastname: Chester
authors.forename: Toby O
authors.initials: TO
authors.lastname: Smith
authors.forename: David
authors.initials: D
authors.lastname: Sweeting
authors.forename: John
authors.initials: J
authors.lastname: Dixon
authors.forename: Sarah
authors.initials: S
authors.lastname: Wood
authors.forename: Fujian
authors.initials: F
authors.lastname: Song
mesh_terms: D016022:Case-Control Studies; D006801:Humans; D007719:Knee Joint / Q000473:pathology / Q000502:physiology*; D018482:Muscle, Skeletal / Q000473:pathology / Q000502:physiology*; D010146:Pain / Q000209:etiology* / Q000503:physiopathology*; D013997:Time Factors