Tests of running performance do not predict subsequent spontaneous running in rats

Rats of similar mass and genetic stock have up to a 50-fold difference in spontaneous daily running distance. However, the reasons for this large variability in spontaneous running distance are not known. This study examined whether tests of running performance predict subsequent spontaneous running distance in rats housed in individual running wheel cages. Long-Evans rats (n = 56) were randomly assigned to either a sedentary control group (C) or a group housed in specially designed wheel cages in which they were able to exercise spontaneously (ES). They then underwent a high-intensity running test (MAX), during which oxygen consumption was measured at a submaximal (VO2 submax) and maximal workload (VO2 max). The rats' submaximal running endurance (END) and maximal sprinting speed (SPRINT) were also tested on the treadmill. After 8 weeks the average spontaneous running distance of ES was 29.7 +/- 3.7 km.wk-1 (mean +/- SE), but ranged from 1.4 to 71.1 km.wk-1. Tests of running performance and oxygen consumption were repeated in both groups, followed by in situ measurements of muscle contractile properties and of citrate synthase activity in the skeletal muscle. None of the measurements of running performance or oxygen consumption during the initial tests conducted at the start of the experiment was related to subsequent average spontaneous running distance. After 8 weeks, the mean peak force generated by the electrically stimulated gastrocnemius/plantaris muscles was greater in ES than in C (746 +/- 89 vs. 455 +/- 28 mg, p < 0.005), but this difference was not related to spontaneous running distance. Conversely, citrate synthase activity of the soleus muscle after training was related to average spontaneous running distance (r = 0.92, p < 0.0004). Average spontaneous running distance was also related to MAX (r = 0.65, p < 0.002), END (r = 0.59, p < 0.0009), and SPRINT (r = 0.61, p < 0.0005) and was inversely related to running intensity (r = -0.66, p < 0.002) after 8 weeks of training. It can be concluded from this study that 1) spontaneous running distance in rats cannot be predicted by pretraining tests of running performance. Hence, low levels of spontaneous running activity in some rats are not explained by skeletal muscular and cardiovascular factors thought to determine running capacity, and 2) posttraining tests of running performance were proportionally related to total spontaneous running distance and muscle oxidative enzyme changes but not to the in situ contractile properties of the muscles.