for(i in 1: length[ , 5]-9) {
 data$Mean [i] <- mean (data[i,5]:data[(i+9),5])
}

for(i in 1: length(data[ , 5]-9)) {
    data$Mean [i] <- mean(data[i:min(i+9, nrow(data)),5])
}

mydata <- data.frame(col_1 = rnorm(100),
                     col_2 = rnorm(100),
                     col_3 = rnorm(100),
                     col_4 = rnorm(100),
                     col_5 = rnorm(100))


for(i in 1: length(mydata[ , 5]-9)) {
    mydata$Mean [i] <- mean(mydata[i:min(i+9, nrow(mydata)),5])
}