The machines are used by athletes to build strength and are attached with adhesive electrodes. Johnson said Tuesday the device could have been placed too close to a nerve in Strasburg's right arm.
Johnson says it is too early to determine whether Strasburg will make his next scheduled start on Saturday at Pittsburgh. The manager want to see how the right-hander throws in a bullpen session.
Strasburg frequently shook his right arm while pitching six innings and allowing two runs in the Nationals' 3-2 loss to Atlanta on Monday night.