A movable user support assembly for supporting a user in a seated or kneeling position while performing an exercise on an exercise machine, comprising: a base; a two-directional pivoting user support associated with the base and including a user support platform configured to support a user in an exercise position while performing an exercise, the user support being rotatable about a first pivot axis during the exercise; a pivotal connection between the base and the user support defining at least one second pivot axis for free rotational movement of the user support about said at least one second pivot axis in at least two opposite directions between first and second end positions, the user support having a centered support position between said end positions; and a locking device which is configured to selectively lock the user support against rotation about the second pivot axis, the locking device having a first, locking position defining a stable support mode in which the user support rotates about the first pivot axis only and is locked against rotation about the second pivot axis, and a second, released position defining an unstable support mode in which the user support is free to rotate about said second pivot axis between said end positions; wherein the at least one second pivot axis is parallel to the user support platform, whereby said user support platform is configured for free tilting movement in opposite directions about said second pivot axis in the unstable support mode; wherein the locking device comprises a first lock part having an opening and a second part comprising a releasable locking pin movable between a locked position engaging the opening to prevent rotation of the user support about the second pivot axis and an unlocked position retracted from the opening, one of the lock parts being associated with the user support and the other lock part being associated with the base; and wherein the second part of the locking device further comprises a lock pin housing secured to the user support, the locking pin being slidably mounted in the lock pin housing for movement between the extended locked position engaging the opening and a retracted, unlocked position, and a manually operable lock release tab mounted for rotation relative to the housing between first and second end positions corresponding to the locked and unlocked positions of the locking pin. a base; a two-directional pivoting user support associated with the base and including a user support platform configured to support a user in an exercise position while performing an exercise, the user support being rotatable about a first pivot axis during the exercise; a pivotal connection between the base and the user support defining at least one second pivot axis for free rotational movement of the user support about said at least one second pivot axis in at least two opposite directions between first and second end positions, the user support having a centered support position between said end positions; and a locking device which is configured to selectively lock the user support against rotation about the second pivot axis, the locking device having a first, locking position defining a stable support mode in which the user support rotates about the first pivot axis only and is locked against rotation about the second pivot axis, and a second, released position defining an unstable support mode in which the user support is free to rotate about said second pivot axis between said end positions; wherein the at least one second pivot axis is parallel to the user support platform, whereby said user support platform is configured for free tilting movement in opposite directions about said second pivot axis in the unstable support mode; wherein the locking device comprises a first lock part having an opening and a second part comprising a releasable locking pin movable between a locked position engaging the opening to prevent rotation of the user support about the second pivot axis and an unlocked position retracted from the opening, one of the lock parts being associated with the user support and the other lock part being associated with the base; and wherein the second part of the locking device further comprises a lock pin housing secured to the user support, the locking pin being slidably mounted in the lock pin housing for movement between the extended locked position engaging the opening and a retracted, unlocked position, and a manually operable lock release tab mounted for rotation relative to the housing between first and second end positions corresponding to the locked and unlocked positions of the locking pin.