An adjustable dumbbell system, comprising: a handle assembly ; a plurality of weights selectively fixedly connectable to the handle assembly ; at least one sensor positioned on the handle assembly , the at least one sensor configured to detect a handle assembly attribute indicative of whether the plurality of weights are fixedly connected to the handle assembly ; and a computing device in communication with the at least one sensor and configured to receive information regarding the handle assembly attribute from the at least one sensor, characterized in that : the handle assembly includes a disc that is arranged for rotation to selectively fixedly connect a different combination of the plurality of weights to the handle assembly in each of the rotational positions of a set of discrete rotational positions of the disc; the at least one sensor is configured to detect the rotational position of the disc , and the computing device is configured to determine which of the plurality of weights are fixedly connected to the handle assembly based on the rotational position detected by the at least one sensor; and the at least one sensor includes at least one of the following: an optical sensor, a reflective sensor, a mechanical sensor, an inductive sensor, a capacitive sensor, a potentiometer, an accelerometer, or a magnetometer. a handle assembly ; a plurality of weights selectively fixedly connectable to the handle assembly ; at least one sensor positioned on the handle assembly , the at least one sensor configured to detect a handle assembly attribute indicative of whether the plurality of weights are fixedly connected to the handle assembly ; and a computing device in communication with the at least one sensor and configured to receive information regarding the handle assembly attribute from the at least one sensor, characterized in that : the handle assembly includes a disc that is arranged for rotation to selectively fixedly connect a different combination of the plurality of weights to the handle assembly in each of the rotational positions of a set of discrete rotational positions of the disc; the at least one sensor is configured to detect the rotational position of the disc , and the computing device is configured to determine which of the plurality of weights are fixedly connected to the handle assembly based on the rotational position detected by the at least one sensor; and the at least one sensor includes at least one of the following: an optical sensor, a reflective sensor, a mechanical sensor, an inductive sensor, a capacitive sensor, a potentiometer, an accelerometer, or a magnetometer. the handle assembly includes a disc that is arranged for rotation to selectively fixedly connect a different combination of the plurality of weights to the handle assembly in each of the rotational positions of a set of discrete rotational positions of the disc; the at least one sensor is configured to detect the rotational position of the disc , and the computing device is configured to determine which of the plurality of weights are fixedly connected to the handle assembly based on the rotational position detected by the at least one sensor; and the at least one sensor includes at least one of the following: an optical sensor, a reflective sensor, a mechanical sensor, an inductive sensor, a capacitive sensor, a potentiometer, an accelerometer, or a magnetometer. <EOS>