Either of the Cosmed or Cortex systems are ideal for free movement metabolic testing. I have used both and personally prefer the Cortex software. However, i do find the cortex systems a little easy to damage and obviously expensive to repair. You mention jumping as one of your activities, i would worry about doing this too vigorously with either system. Just make sure they are well strapped on and the cable and sample lines are free. 

We have found with the cortex system that when doing lots of testing sample lines do require changing and drying (hanging) for some time. 

Cosmed system like k4b2 have some problems with connection and HR data collection. It use older version of HR belt (plastic one). You need to replace O2 sensor it's about 1000euro+ service. The same situation is in Cortex system but there You could replace the sensor by your selw. In both systems You need to replace nafion/permapure lines. I observed that it schould be done every 2-3 tests when they are done continously. Cosmed recommend to replace the sampling line every 100 tests. During the test with low external temperature (lower than 10C) I observed some problems with O2 sensor temperature so I need to cover k4 with DIY bag.

In my opinion both systems are fine, there are some technical differences like:

calibration: Cosmed use large tank, some problems with transport but you might do calibration more frequently. I rather recommend to do it before each test as the response time and pressures change after the test or when you change sampling line.

O2 sensor lifetime and cost

Now there is new Cosmd k5 but I heard that there are quite big differences when it work on breath by breath mode or micro mixing chamber. So there might be some accuracy problems. Also there is new software system Omnia 1.6, which is different than the older one Suite 10d and more friendly. I use it with the Cosmed Quark CPET stationary system. Big minus of this system is that it lost sometimes connection and then test is gone compleatly.... There is no way to reconnect. In portable devices there is internal memory so it's OK.

Dear Alan and Szczepan:

Thank you so much for your answer.

Best,

Jose Heredia-Jimenez

Dear Dr Heredia-Jimenez,

Thank you for this post. It has been some years since you asked about these devices. Any updates? Which device did you try out and how was your experience with the tests? I am very interested in knowing how often connections are lost (tests data lost) when using the devices mentioned.

Thanks!

Kiran