the amount of time between the onset of the tone and the onset of muscle contraction in EMG signal

Hey, I have an EMG signal data and i need to determine the amount of time between the onset of the tone and the onset of muscle contraction. I am thinking of using a voltage threshold function to determine when the tone was played and breaks the EMG recorded signal (filtered and half-wave rectified) into epochs when the bicep contracted for each stimulus trial, but i am not sure how to acheive it.

I have this code as a start:

clear all;

Exp1 = load('EMG1.mat'); %load file

Fs = 10000; %sampling rate

Duration = 1.5; %Duration of each epoch

DurSamps = Duration*Fs; %Duration in samples

Reps = 14; %number of epochs

%define blank matrices

Tones(1:Reps,1:DurSamps)=0; EMGRaw(1:Reps,1:DurSamps)=0; EMGFilt(1:Reps,1:DurSamps)=0; EMGFiltRect(1:Reps,1:DurSamps)=0;

%read data from structure

for Rep=1:Reps

Tones(Rep,:) = Exp1.data(Exp1.datastart(1,Rep):Exp1.dataend(1,Rep)); EMGRaw(Rep,:) = Exp1.data(Exp1.datastart(2,Rep):Exp1.dataend(2,Rep)); EMGFilt(Rep,:) = Exp1.data(Exp1.datastart(3,Rep):Exp1.dataend(3,Rep));

end

for i=1:Reps

for j=1:DurSamps

EMGFiltRect(i,j)=EMGFilt(i,j).^2; %half-wave rectification

end

end