If you sampled the signal at 200Hz it is useless adopt a low-passfilter at 100Hz since 100Hz is already the maximum bandwidth that you get, see Nyqyust frequency for more detauls. For antialiasing purpose you MUST use analogic filters, not digital filters. So I did not get the reason why you want to filter the signal at 100Hz.
Anyhow, this could be a possible solution.
Fs = 200; % Sampling frequency in Hz
fcutlow = 20; % low cut frequency in Hz
Order = 4;
[b,a] = butter(Order,fcutlow/(Fs/2)); % Get filter coeff of butter filter
Filt_filt_signal = filtfilt(b,a, rawSignal ); % zero phase delay low-pass filtering
As usual, if that helps you, please consider to accept the answer.
Thanks and best regards, A
