Where should the (linear position transducer) LPT be attached to the barbell and will the resistance of the spring return affect the athlete’s movements?
The attachment of the LPT to the barbell should not cause any problems for the athlete even when using a very lightweight carbon fibre or wooden pole if the attachment is to the centre and the LPT is suspended above the athlete. This is the best solution. If the LPT has to be positioned on the ground then it must be placed to one side so as not to interfere with the athlete movement. When there is some minimal weight being lifted this does not create any problems because the resistance of the LPT is so small relative to the load being moved. We find that even a standard 20 kg Olympic barbell can be used with the LPT attached just lateral to the left or right hand without any interference to normal jumping, squatting, overhead lifts and weightlifting movements. For any weight less than this the LPT really needs to be mounted centrally and above the athlete.
For any movement where the athlete and resistance do not contact any surface if is preferable to use the force plate with all measures derived from the vertical ground reaction force. In this case there is no requirement to attach the LPT at all. This allows very free movement of the athlete in activities such as jumping, squatting, overhead lifts and any weightlifting movements from the hang position. This circumvents the issue of LPT placement and also makes consideration of horizontal movement irrelevant as only vertical force, impulse, power, velocity and displacement are being measured without contamination from movement in the horizontal plane.
If it is required to track barbell displacement using the LPT and horizontal movement is a concern then multiple displacement transducers can be used and the two or even three-dimensional position of the barbell determined using triangulation. In my experience this is not worth the additional effort as the error introduced due to horizontal movement of the barbell for most movements is insignificant.