Perhaps the results of the Sparx trial give us a hint as to what might have gone wrong in the CYCLE trial.

In the Sparx trial [1] participants were assigned to one of three groups: control, moderate-intensity treadmill exercise (60 to 65 percent maximum heart rate) or high-intensity treadmill exercise (80 to 85 percent maximum heart rate). After six months researchers found that the motor symptoms of only the high-intensity group had not progressed.

In the CYCLE trial [2] and the earlier (smaller) trial [3], both the VE and the FE groups were instructed to maintain their heart rates within the range of 60 - 80 percent of HRmax.

Hypothesis: Perhaps in the CYCLE trial the average heart rate for both groups was up near 80. Perhaps in the earlier trial the average heart rate for only the FE group was up near 80 while the average heart rate for the VE group was down near 60.

Unfortunately, references [2] and [3] do not contain the data needed to test this hypothesis.

Bottom line: We now know (from the results of the Sparx trial) that a heart-rate range of 60 - 80 percent of HRmax is just too large.

[1] Effect of High-Intensity Treadmill Exercise on Motor Symptoms in Patients With De Novo Parkinson Disease: A Phase 2 Randomized Clinical Trial, Margaret Schenkman et al., JAMA Neurology: 75(2) 2018.

[2] Results from the Randomized Controlled Trial Cyclical Lower Extremity Exercise (CYCLE) Trial for Parkinson’s disease, Jay Alberts et al., Medicine & Science in Sports & Exercise: May 2018.

[3] Forced, Not Voluntary, Exercise Improves Motor Function in Parkinson's Disease Patients, Angela Ridgel et al., Neurorehabilitation and Neural Repair: 23(6) 2009.