Fast Running Does Not Contribute More to Cumulative Load than Slow Running.

PURPOSE: As running speed increases there are concomitant changes in loads associated with tibial stress fracture risk. Runners often include multiple speeds in their training, but the effect of speed distribution on load accumulation is unknown. We studied how running at different proportions of speed within a given running distance affects the cumulative loading of the vertical average loading rate, cumulative peak absolute tibial free moment, and cumulative peak axial tibial load. These loads were compared between two proportions of speed: running all distance at normal self-selected speed, and running the same distance at a combination of slow/fast speeds with the same average speed as normal. Also, the contributions of slow and fast running to the combined condition were compared.

METHODS: Forty-three recreational runners (age, 18-49 yr; 29 female, 14 male) ran around a 50-m indoor track for three laps each at self-selected slow, normal, and fast speeds. Per-step peak loads and cumulative loads per kilometer were calculated at each speed and for each speed distribution, respectively.

RESULTS: Only cumulative vertical average loading rate was lower at normal speed compared with the slow/fast speed combination. The contribution of fast speed running to cumulative tibial load was less than the contribution of slow speed running.

CONCLUSIONS: Running at a combination of slow and fast speeds, rather than a single moderate speed, increased cumulative vertical average loading rate but not cumulative tibial load or free moment. Fast running can be included in a training program without necessarily increasing the cumulative load. Total distance and average speed may not be sufficient information to estimate cumulative load from running training.