A treadmill has a base with a vertically displaceable platform. A mechanism is provided that resists the downward movement of the platform in response to a load applied to the platform. The mechanism also rebounds the platform upwardly with a force applied to the platform in response to a decrease of a load on the platform. A support structure enables stable vertical positioning of the platform with respect to the base.

A fitness machine operable by a user. The fitness machine includes a base that extends in a length direction, a width direction, and a height direction that are perpendicular to each other. A mobile portion is configured to support the user during operation of the fitness machine. A resilient body is configured to provide a resistance against the mobile portion moving towards the base in the height direction. A frame is moveable in the length direction relative to the base to adjust the resistance provided by the resilient body. A fastener assembly moveably couples the frame to the base, where the fastener assembly is configured to expand in the width direction to prevent the frame from moving in the width direction relative to the base during operation of the fitness machine.

A fitness machine operable by a user. The fitness machine has a base that extends in a length direction, a width direction, and a height direction that are perpendicular to each other. A mobile portion is configured to support the user during operation of the fitness machine. A resilient body is supported by a frame and configured to provide a resistance against the mobile portion moving towards the base in the height direction. A fastener assembly couples the frame to the base, wherein the fastener assembly is configured to expand to prevent the frame from moving relative to the base during operation of the fitness machine.

A system for adjusting a stiffness of a fitness machine having a base and a mobile portion that moves relative to the base. A resilient body that resists movement of the mobile portion towards the base in a height direction. The resilient body has a length defined in a length direction perpendicular to the height direction, which increases when the mobile portion moves towards the base. An end stop prevents the length from increasing beyond a set maximum, the end stop being fixed relative to the base in the length direction. An actuator is operable to adjust the set maximum at which the end stop prevents the length of the resilient body from increasing. Resistance provided by the resilient body to resist movement of the mobile portion towards the base is unaffected by operation of the actuator when the length of the resilient body is less than the set maximum.