title: An acoustic startle alters knee joint stiffness and neuromuscular control.
abstract: Growing evidence suggests that the nervous system contributes to non-contact knee ligament injury, but limited evidence has measured the effect of extrinsic events on joint stability. Following unanticipated events, the startle reflex leads to universal stiffening of the limbs, but no studies have investigated how an acoustic startle influences knee stiffness and muscle activation during a dynamic knee perturbation. Thirty-six individuals were tested for knee stiffness and muscle activation of the quadriceps and hamstrings. Subjects were seated and instructed to resist a 40-degree knee flexion perturbation from a relaxed state. During some trials, an acoustic startle (50 ms, 1000 Hz, 100 dB) was applied 100 ms prior to the perturbation. Knee stiffness, muscle amplitude, and timing were quantified across time, muscle, and startle conditions. The acoustic startle increased short-range (no startle: 0.044 ± 0.011 N·m/deg/kg; average startle: 0.047 ± 0.01 N·m/deg/kg) and total knee stiffness (no startle: 0.036 ± 0.01 N·m/deg/kg; first startle 0.027 ± 0.02 N·m/deg/kg). Additionally, the startle contributed to decreased [vastus medialis (VM): 13.76 ± 33.6%; vastus lateralis (VL): 6.72 ± 37.4%] but earlier (VM: 0.133 ± 0.17 s; VL: 0.124 ± 0.17 s) activation of the quadriceps muscles. The results of this study indicate that the startle response can significantly disrupt knee stiffness regulation required to maintain joint stability. Further studies should explore the role of unanticipated events on unintentional injury.
authors.affiliation: Department of Athletics, University of the Sciences, Philadelphia, Pennsylvania, USA.
authors.forename: A I
authors.initials: AI
authors.lastname: DeAngelis
authors.affiliation: Department of Health and Exercise Science, Appalachian State University, Boone, North Carolina, USA.
authors.forename: A R
authors.initials: AR
authors.lastname: Needle
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, Delaware, USA.
authors.forename: T W
authors.initials: TW
authors.lastname: Kaminski
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, Delaware, USA.
authors.forename: T R
authors.initials: TR
authors.lastname: Royer
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, Delaware, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Knight
authors.affiliation: Department of Kinesiology and Applied Physiology, University of Delaware, Newark, Delaware, USA.
authors.forename: C B
authors.initials: CB
authors.lastname: Swanik
mesh_terms: D000161:Acoustic Stimulation; D000293:Adolescent; D000328:Adult; D004576:Electromyography; D005080:Exercise Test; D005260:Female; D064368:Healthy Volunteers; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D007719:Knee Joint / Q000502:physiology*; D008297:Male; D052097:Quadriceps Muscle / Q000502:physiology*; D013216:Reflex, Startle / Q000502:physiology*; D013997:Time Factors; D055815:Young Adult
keywords: Anterior cruciate ligament; columnar buckling; injury prevention; joint stability