A device for resistance exercise of muscle groups that cross two or more joints such as biceps group, triceps group, quadriceps group, and hamstring group. The device comprises a primary cam or gear fixed non-rotatably to the frame of an appropriate exercise machine for working the desired body part, and a secondary cam or gear connected only to the primary cam in such a way which, when rotated, tracks around the primary cam or gear. Resistance is provided with a cable, belt, chain, or other means to the axis of the secondary cam only. When properly positioned user applies force to the contact point connected to the secondary cam and secondary axis, the device moves both desired joints simultaneously through a range of motion while under a predictable resistance. The device provides an improved range of movement while under resistance, thereby supplying a greater extension and contraction of the desired muscle or muscle group than prior art devices.