Cannot convert Serial String into char
You try to convert the whole command string everytime a space character appears. This works good for the first occurence, but then the string will become longer and longer and the converted integer will be garbage. You should reset the string with command = "" everytime you have processed a number, or just using the current part of it with the substring member function.
Currently you echo the converted integer with Serial.write(). With this, the pure byte code is send over the serial. Your terminal program will interpret the bytes as characters, so you may get human readable numbers. Try the function Serial.println(), which converts the number to a human readable form and prints a newline after it as a delimiter.
You don't need to write to your outputs on every loop cycle. Especially the Serial prints will make the debugging more difficult. I think you should only write a new value to your outputs, when you received new values over Serial.
Am I right, that your protocol looks something like this: "0 255 1 100 1 150 0 80" ?
In your current code, if not the whole string is received in one loop cycle (maybe if there was some delay when sending from the computer) the rest of the string is than written to the first motor values. You can use a packet delimiter character like "\n" after a full packet. Then you can collect all received characters in your command string until you receive the "\n", which marks the end of the current package. Then you would process the packet as a whole.