during quiet stance with muscle onset defined as activity greater than 3 standard deviations. EMG amplitude was normalized and expressed as a percentage of ...
JUMP PROTOCOL DIRECTION DOES NOT AFFFECT DYNAMIC POSTURAL STABILITY OR EMG IN HEALTHY SUBJECTS
Measurements
•Subjects reported to a research laboratory for a single test session. •Maximum vertical jump height was assessed with a Vertec vertical jump tester (Sports Imports, Columbus, OH) and defined as the difference between the maximum height touched and the standing reach height (vanes at 1.27cm ). •A Myopac EMG system (Run Technologies, Laguna Hills, CA) was used to collect EMG signals of the vastus medialis (VM), semimembranosus (SM), lateral gastroc (LG), and tibialis anterior (TA) from 200ms prior to and after touch down. A tri-axial force plate collecting at 800Hz was used to synch EMG data with touch down. •The jump protocol started with subjects in a standing position 70 cm from the center of a force plate for each direction tested (forward, diagonal, lateral) and required them to jump off both legs and touch an overhead marker placed at a position equivalent to 50% of the subject’s maximum vertical leap. Each subject was to land on the test leg, stabilize as quickly as possible and balance for 3 seconds (Figure 1).
•Muscle activation, and amplitudes were determined by digitally processing the raw analog signals using a symmetric root mean square algorithm with a 10ms time constant. Baseline EMG measures were obtained during quiet stance with muscle onset defined as activity greater than 3 standard deviations. EMG amplitude was normalized and expressed as a percentage of the maximal voluntary contractions obtained during the maximal vertical jump testing. Average preparatory EMG activity was measured during the 200ms prior to touchdown and reactive EMG activity was measured during the 200ms time period following touch down. Means were calculated from three trials taken in each direction.
Statistical Analyses •All dependent variables were analyzed using a oneway repeated measures analysis of variance (ANOVA) on direction: forward, diagonal, lateral. •An alpha level of .05 was initially set for all statistical tests (n=13). A Bonferonni adjustment was used to control experiment wise error and set the adjusted alpha level equal to .004.
Twenty-six subjects [10 males (223.9 yrs, 70.97.6 kg, 176.8 cm) and 16 females (20.6.5 yrs, 65.69.1 kg, 166.45.9 cm)] volunteered to participate in this investigation. No subject had a history of acute lower extremity or head injuries for the previous three months nor did they suffer from any equilibrium disorders or chronic lower extremity pathologies.
Conclusions
Figure 1: Starting and finishing position for the sagittal jump landing protocol.
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•The results indicate that there are no neuromuscular control differences as measured by EMG and the DPSI when the direction of a jump landing is manipulated in healthy subjects.
25 20 15 10 5 0 180
Results •Activation times showed no significant differences. •Preparatory EMG amplitude showed no significant differences. •Reactive EMG amplitude showed no significant differences. •Dynamic Postural Stability showed no significant differences (Forward = .73..27; Lateral =.72.28; Diagonal =.77.45 ). •All results can be seen in Figure 2
Subjects
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40
% of MVC
ACL and lateral ankle injuries cause a financial impact of >$4 billion dollars a year. As a result, a recent impetus to conduct more functional and dynamic testing has emerged to better understand the role of the dynamic restraints and the muscular stiffness created to protect the joints of the lower extremity. The previously established techniques of elctromyography (EMG) and dynamic postural stability allow the indirect measure of muscular stiffness during jump landing tasks. As it has been indicated that subjects with greater and earlier preparatory muscle activity in the lower extremity has lower dynamic postural stability scores. However, few investigations have examined how neuromuscular control is affected when performing jump-landing tasks in different directions (e.g. forward (F), lateral (L), diagonal (D)). Examining the differences in dynamic postural stability among different jump landing directions could reveal previously overlooked information regarding neuromuscular control. Therefore the purpose of this investigation is to examine EMG and dynamic postural stability when completing a jump landing protocol in multiple directions.
Procedures
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Time (ms)
Purpose
% of MVC
Naugle KE, Wikstrom EA, Tillman MD, Schenker SM, Borsa PA Athletic Training/Sports Medicine Research Laboratory, University of Florida, Gainesville, FL
100 80 60 40 20 0 Forward
Diagonal VM
SM
LG
Lateral TA
Figure 2: Top: Preparatory EMG amplitudes. Middle: Reactive EMG amplitudes. Bottom: EMG activation times (time from touch down in ms).
