Effects of Rate of Force Development on EMG Amplitude and Frequency Ricard MD, Ugrinowitsch C, Parcell AP, Hilton S, Rubley MD, Sawyer R, Poole CR

Size Principle (Recruitment Threshold) Small [Slow Twitch, Type I] are recruited first Small [Slow Twitch, Type I] are recruited first Intermediate [Type IIa] are then recruited Intermediate [Type IIa] are then recruited Large [Fast Twitch IIb] are recruited last Large [Fast Twitch IIb] are recruited last The vertical lines indicate when a motor unit has fired. When the lines are closer together, the motor unit has increased its firing rate. The vertical lines indicate when a motor unit has fired. When the lines are closer together, the motor unit has increased its firing rate. Fast twitch units are the first to be de-recruited, followed by intermediate and finally slow twitch units. Fast twitch units are the first to be de-recruited, followed by intermediate and finally slow twitch units. Notice that motor units are recruited and de-recruited at about the same force level. Notice that motor units are recruited and de-recruited at about the same force level. ST motor unit FT motor unit

In ballistic contractions all motor units are recruited at once, each fires at it’s maximal firing rate. In ramp contractions MUs are recruited: ST, INT, FT and the firing rates gradually increase throughout the contraction.

Motor units fire in a burst (high rate) at the outset of a ballistic contraction.

Effects of Motor Unit Synchronization Upon EMG Amplitude and Force Synchronization increases the EMG amplitude and the variability of the force. [From Yao (2000) #1996]

Motor unit synchronization lowers the median frequency of the surface EMG signal. [From Yao (2000) #1996] Effects of Synchronization Upon the Median Frequency of the EMG Signal

Van Cutsem: Ballistic training increases RFD & initial EMG activation.

Van Cutsem: Ballistic training increases RFD & lowers recruitment threshold for FT.

Van Cutsem: Ballistic training increases RFD & occurrence of doublets.

Purpose of the Study Compare EMG amplitude and frequency in a ramp and ballistic contraction Compare EMG amplitude and frequency in a ramp and ballistic contraction

Methods 16 Female sprinters 16 Female sprinters Each performed 3 ramps (2-3 s to peak) & 3 ballistic contractions Each performed 3 ramps (2-3 s to peak) & 3 ballistic contractions Plantar flexion torque measured with a Biodex Plantar flexion torque measured with a Biodex EMG of gastrocnemius and torque sampled at 1000 Hz (Keithley-Metrabyte 16 bit A/D) EMG of gastrocnemius and torque sampled at 1000 Hz (Keithley-Metrabyte 16 bit A/D) EMG amp & freq measured at 25, 50, 75 & 100% MVC EMG amp & freq measured at 25, 50, 75 & 100% MVC Short term FFT with resolution of 2 Hz. Short term FFT with resolution of 2 Hz.

EMG Amplitude and Frequency

Statistics Two factor ANOVA Two factor ANOVA Contraction (ballistic, ramp) Contraction (ballistic, ramp) Torque (25, 50, 75, 100% MVC) Torque (25, 50, 75, 100% MVC) SAS Proc Mixed with unstructured covariance SAS Proc Mixed with unstructured covariance Alpha set at 0.05 Alpha set at 0.05

Results No Difference in Mean Torque

RFD Significant Difference

Ballistic vs Ramp Contractions In ballistic contraction the amplitude is high and frequency is low at 25%, suggesting synchronization of MUs. In ballistic contraction the amplitude is high and frequency is low at 25%, suggesting synchronization of MUs. In ramp contractions, amplitude and frequency increase with torque. In ramp contractions, amplitude and frequency increase with torque.

EMG Amplitude in Ramp & Ballistic Contractions

EMG Frequency in Ramp & Ballistic Contractions