Your sampling frequency is about 61.6 Hz, so the line frequency certainly could be aliased into your recording. Without knowing your experimental set-up, it would not be possible to identify the cause. I also do not know if your Arduino has an anti-aliasing filter built into it. If not, I suggest that you design and build a Bessel lowpass filter with a cutoff just below the Nyquist frequency (half the sampling frequency), here about 30 Hz. That will eliminate aliasing, and will also eliminate all power frequencies (that I am aware of).
You can eliminate the baseline wander and some of the high-frequency noise with this:
F = openfig('BaselineWanderBefore.fig','invisible');
f = gca;
EKG = findobj(f, 'Type','line');
t = EKG.XData; % Recover Data
s = EKG.YData;
L = numel(t); % Signal Length
Ts = mean(diff(t));
St = std(diff(t)); % Test For Uniform Sampling
tr = linspace(min(t), max(t), L); % New, Regularly-Sampled Time Vector
Tsr = mean(diff(tr)) * 1E-3; % Convert To Seconds
sr = resample(s, tr); % Resample To Regularly-Sampled Signal
Fsr = 1/Tsr; % Sampling Frequency (Hz)
Fnr = Fsr/2;
figure
plot(tr, sr)
grid
FTsr = fft(sr-mean(sr))/L; % Fourier Transform Of Mean-Corrected Signal
Fv = linspace(0, 1, fix(L/2)+1)*Fnr;
Iv = 1:numel(Fv);
figure
plot(Fv, abs(FTsr(Iv))*2)
grid
Fs = Fsr; % Sampling Frequency (Hz)
Fn = Fs/2; % Nyquist Frequency (Hz)
Ws = [6.0 7.0]/Fn; % Passband Frequency Vector (Normalised)
Wp = [2.0 14.0]/Fn; % Stopband Frequency Vector (Normalised)
Rp = 1; % Passband Ripple (dB)
Rs = 150; % Stopband Attenuation (dB)
[n,Wp] = ellipord(Wp,Ws,Rp,Rs); % Calculate Filter Order
[z,p,k] = ellip(n,Rp,Rs,Wp,'bandpass'); % Default Here Is A Lowpass Filter
[sos,g] = zp2sos(z,p,k); % Use Second-Order-Section Implementation For Stability
sr_filtered = filtfilt(sos,g,sr); % Filter Signal (Here: ‘sr’)
figure
freqz(sos, 2^14, Fs) % Bode Plot Of Filter
set(subplot(2,1,1), 'XLim',[0 Fn]) % Optional, Change Limits As Necessary
set(subplot(2,1,2), 'XLim',[0 Fn]) % Optional, Change Limits As Necessary
figure
plot(tr, sr_filtered)
grid
Feel free to experiment with the filter passbands if you want a different result.
