function [sumWalk, siRunning, nonSumWalk, nsiRunning] = findFracWalking(cutoff, summit, nonsummit, si, nsi)

    [sumWalk, sumNans] = deal(nan(size(summit,1),length(summit)));
    [nonSumWalk, nonSumNans] = deal(nan(size(nonsummit,1),length(nonsummit)));
    for n=1:si
        sumWalk(n,:) = summit(n,:)>cutoff;
        sumNans(n,:) = isnan(summit(n,:));
    end
    for n=1:nsi
        nonSumWalk(n,:) = nonsummit(n,:)>cutoff;
        nonSumNans(n,:) = isnan(nonsummit(n,:));
    end

    % Determine number of flies being tracked (i.e. not showing NaN x,y
    % values) at any given time in experiment
    siRunning = repmat(si,1,length(sumNans))-nansum(sumNans);
    nsiRunning = repmat(nsi,1,length(nonSumNans))-nansum(nonSumNans);

end