improving poor blob contours

edit savecancel

// blur like hell !
int B=8;
blur(ocv,ocv,Size(B,B));

//then threshold again:
ocv = (ocv > 30);

Another alternative. This one, just a little upgrade to @berak 's answer - using closing and opening morphological operations after the blur, you can get one final and single contour:

Mat image = imread("hand.jpg", 0);
blur(image, image, Size(8, 8));
image = image > 30;
morphologyEx(image, image, MORPH_CLOSE, Mat(25, 2, CV_8U));
morphologyEx(image, image, MORPH_OPEN, Mat(10, 10, CV_8U));

vector<vector<Point>> contours;
findContours(image, contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
Mat image_contour(image.rows, image.cols, CV_8UC1, Scalar(0));
drawContours(image_contour, contours, 0, Scalar(255));

And so we get this (image on the left, image_contour on the right):

also it seems that your image is not exactly binary so if you just apply some thresholding or some floodfill() you can get the whole arm easily:

Mat img = imread("img13.jpg", 0);

Mat bin;
threshold(img, bin, 0, 255, THRESH_BINARY);
imshow("bin", bin);

cv::Mat mask = cv::Mat::zeros(img.rows + 2, img.cols + 2, CV_8U); // A image with size greater than the present object is created, needed from floodfil

cv::floodFill(img_c, mask, cv::Point(0,0), 255, 0, cv::Scalar(), cv::Scalar(), 4 + (255 << 8) + cv::FLOODFILL_MASK_ONLY);

imshow("mask", ~mask);