(24) ELECTROMYOGRAPHY NORMALIZATION ERROR IN DROP JUMP LANDINGS

Electromyography (EMG) is commonly used to measure muscle activity in explosive lower extremity movements such as jumping, landing, and cutting. EMG, measured in voltage, presents raw data on an arbitrary scale. While maximal voluntary isometric contraction (MVIC) is most often cited in explosive lower extremity EMG research for raw data normalization, there is no standardized approach to EMG normalization.

Purpose: The present study examined EMG normalization error with an MVIC during a drop jump movement.

Methods: Sixteen participants (10 males and 6 females, age: 22.8±1.4 yr, body mass: 74±14 kg, height: 173±9.7 cm) completed the single group measure study. Surface EMG electrodes (Delsys Trigno Wireless EMG Systems, Inc., Boston, MA, USA) placed on their right-side recording electromyographic signals from the rectus femoris (RF), vastus medialis (VM), semitendinosus (ST), biceps femoris (BF), medial gastrocnemius (MG), lateral gastrocnemius (LG), tibialis anterior (TA), gluteus medius (GMed), and gluteus maximus (GMax) muscles. MVICs were performed for each muscle in a randomized order. The edge of two force plates (Bertec Corporation, Columbus, OH) were set up at a distance of half the participant’s body height from the 30cm high step. Participants completed 10 drop jumps with 1-minute rest between each jump, landing one foot on each of the two force plates. EMG data were Fourier band pass filtered and smoothed using root mean square sliding window. EMG normalization was first calculated using the highest value from the MVICs to represent EMG data recorded during the drop jump as %MVIC. Second calculation used the highest value recorded from the 10 drop jumps to represent the EMG data as % activity maximum. One-sample t-test was used to compare maximum values during the drop jump normalized to %MVIC to a set value of 100% for each muscle, observations confirmed using the one sample Wilcoxon ranked sign test. 

Results: A significant difference from 100% activity maximum was observed for %MVIC for the RF (M = 241%, SD = 140, p = .002), VM (M = 174%, SD = 50, p < .001), MG (M = 232%, SD = 111, p = .002), LG (M = 171%, SD = 73, p = .002),TA (M = 210%, SD = 172, p = .021), GMax (M = 180%, SD = 125, p = .027), but not for the BF (M = 127%, SD = 105, p = .37), ST (M = 170%, SD = 145, p < .12), or GMed (M = 139%, SD = 100, p < .18). 

Conclusions:  Six of the nine muscles demonstrated EMG values greater than 100%, indicating a greater value than the recorded maximum MVIC. MVICs underestimated maximal activity by 71%-140% on average with 20% of the measures having an error 160% or greater relative to MVIC. PRACTICAL APPLICATION: These data indicate MVIC significantly underestimates muscle activation in drop jumps. The practitioner should be conscious of this error when conducting new and interpreting current EMG literature involving maximal muscle activity in explosive lower extremity movements.


Acknowledgements: "None"