Iso = Same, metric = length (here it means Same length of a Muscle tissue). To strengthen a Muscle, you should probably have to increase the length of it using various modalities, such as weights, resistance, etc. Next question comes immediately to most of the Physiotherapists, then how come we can improve the Knee range for Post op patients, just by giving ISO METRIC Quad Ex's?. The Answer is really simple. For Post op patients Quadriceps muscle goes for INHIBITION. By giving Isometric Quads ex's what we actually do is MOTIVATE the muscle, there by inhibition breaking mechanism occurs thereby ROM is improved.
Hence a Muscle CAN NEVER be Strengthened using ISO METRIC EXERCISES.