The use of linear position transducers (LPTs) are gaining popularity amongst S&C coaches to either track measures of power or to use as a daily or weekly athlete monitoring tool (for information on the use of an LPT as a monitoring tool, check out the interview with Kristie Taylor from the ACT Academy of Sport). I have been using GymAware for almost seven years now for tracking player strength and power. The purpose of this series of articles is to provide a few examples of how I use LPT’s in my S&C coaching and program design. But first, a bit of a background in the basics of an LPT.
LPT’s come in two main types: an optical rotary encoder or voltage potentiometer. The encoder uses a light beam passed through, or reflected off, a slotted rotating disc to determine distance, while the potentiometer uses a voltage signal that changes with the length of the cable. The main difference is that the voltage potentiometer requires calibration before every use whereas the rotary encoder does not. In the gym, time is precious, especially when you have up to 30 players ready to go, so not having to calibrate is advantageous.
There have been several investigations which have found the use of LPT’s to measure performance in the gym, to be valid and reliable (in bench press  and jumps [2-4]. The table below provides a summary of reliability literature. The ICC (intraclass correlation) refers to the relatedness of repeated measures (similar to a correlation, the closer to 1.0 the stronger the relationship) and the CV (coefficient of variation) refers to the repeatability of the measurement in repeated trials (technically under 10% is good, but the lower the better, so under 5% is excellent). High reliability is critical to determine changes in athletic capacity .
From a scientific perspective, not every exercise has been investigated for reliability which does not mean that it is not reliable for an exercise not listed, it just means that no one has investigated it yet.
I (and my athletes) can use the GymAware system very easily to capture a high volume of important training data every session.
The key to assuring reliability on the gym floor is to maintain strict protocols as to how you use the device such as:
- The placement of the PowerTool in the most vertical position to the lift – See article on compensation for angle of lift.
- Attaching it as close to the centre of the barbell as possible (which can be difficult in exercises such as squats or bench press). In those cases, choose one side of the bar and keep it on that side for all testing.
- Observe the “zero displacement and angle” function prior to use.
What Does GymAware Measure?
It is important to also understand what an LPT measures. The LPT measures distance (and time in the Power Tool) – it does not measure force, it does not measure power – just displacement. It knows how much cable has been pulled out, or pulled in. When combined with time, you can calculate velocity and then acceleration by differentiation and double differentiation respectively. Position and time are the only true kinetic variables measured. All equations after this, such as force and power are calculated against the system mass. By system mass I mean the mass being moved by the athlete. In jumps for example, the system mass is the sum of the body mass and the bar weight.
For a great article on this process, check out Harris et al . Studies have demonstrated that the values obtained are very reliable and comparable to a force plate (and your athletes probably won’t worry either), but it is just important to realise this.
Furthermore, although I use GymAware to track strength, it does not measure strength. However, since an LPT system requires the input of the external resistance, over time, you can use this data to track changes in strength. It is just important to remember that:
A LPT measures displacement,
- It calculates force and power, see this fact sheet
- It records bar load (it does not measure strength).
In Part 2 of this introduction, I will discuss the performance variables that I believe are most useful, the exercises I use for routine monitoring, and the analyses I perform on the results. I will also cover the importance of athlete “buy in” and offer some strategies that will motivate your athletes.
Cormack SJ, Newton, R.U., McGuigian, M.R. and Doyle, T.L.A. Reliability of Measures Obtained During Single Repeated Countermovement Jumps. International Journal of Sports Physiology and Performance. 2008;3(131-144).
Taylor K-L, Cronin, J., Gill, N.D., Chapman, D.W. and Sheppard, J. Source of variability in iso-inertial jump assessments. International Journal of Sports Physiology and Performance. 2010;5:546-58.
Hori N, and Andrews, W.A. Reliability of Velocity, Force and Power Obtained from the GymAware Optical Encoder During Countermovement Jump With and Without External Load. Journal of Australian Strength and Conditioning. 2009;17(1):12-7.
Cronin JB, Hing, R.D. and McNair, P.J. Reliability and Validity of a Linear Position Transducer for Measuring Jump Performance. Journal of Strength and Conditioning Research. 2004;18(3):590-3.
Drinkwater EJ, Galna, B., McKenna, M.J., Hunt, P.H. and Pynes, D.B. Validation of an Optical Encoder During Free Weight Resistance Movements and Analysis of Bench Press Sticking Point Power During Fatigue. Journal of Strength and Conditioning Research. 2007;21(2):510-7.
Harris NK, Cronin, J.B., Taylor, K-L., Boris, J., and Sheppard, J. Understanding position transducer technology for Strength and Conditioning practioners. Strength and Conditioning Journal. 2010;32(4):66-79.