the tension in a particular muscle depends upon how many motor units are contracted at one time. in muscles involved in maintaining posture, only a few motor units contract at any one time, producing a continuous low tension, eg in the leg muscles when standing still. when greater force is required, eg for running, most of the motor units in the appropriate leg muscles will contract at the same time.
from this it can be seen that whole muscles contract in a graded manner, producing a range of tensions. in contrast, both individual muscle fibres and single motor units contract in an all or nothing manner-they either contract fully or not at all. this is similar to the response of nerve cells.