This is when the muscle contracts at a constant speed and max effort through the full ROM. This can be achieved by a therapist when either assessing or treating by applying an active resistance to the patient/client but allowing them to overcome this at a certain speed through the entire range. A KIN-COM machine achieves the same effect, permitting the muscle to contract whilst keeping the same max force/rate movement through range (therefore building strength through range).
The force/tone remains constant BUT speed can vary. This speed can vary based on lever length.
Disadvantage is isotonic training is that because there's that potential for variability muscles can develop predominantly in only the area where the most force is applied.