Try this:
D = load('100m.mat');
EKG = D.val; % Signal
L = numel(EKG); % Signal Length (Samples)
Fs = 360; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
t = linspace(0, L, L)/Fs; % Time Vector
b = fir1(12, (35/Fn)); % Order 12 FIR LPF Fco = 35 Hz
EKGfilt = filtfilt(b,1,EKG); % Filter Signal
figure
freqz(b,1, 2^12, Fs) % Display Filter Bode Plot
figure
plot(t, EKG)
hold on
plot(t, EKGfilt)
hold off
grid
The filter does not appear to be very effective, however it is what you requested, and from the freqz plot, has approximately 6 dB attenuation at 35 Hz.
EDIT — (29 Nov 2019 at 13:54)
Changed filter to filtfilt, since there appears to be significant phase delay usiung filter, even thouigh this is a FIR filter.
