Effects of load carriage on biomechanical variables associated with tibial stress fractures in running

Military personnel are required to run while carrying heavy body-borne loads, which is suggested to increase their risk of tibial stress fracture. Research has retrospectively identified biomechanical variables associated with a history of tibial stress fracture in runners, however, the effect that load carriage has on these variables remains unknown.

What are the effects of load carriage on running biomechanical variables associated with a history of tibial stress fracture?

Twenty-one women ran at 3.0 m/s on an instrumented treadmill in four load carriage conditions: 0, 4.5, 11.3, and 22.7 kg. Motion capture and ground reaction force data were collected. Dependent variables included average loading rate, peak absolute free moment, peak hip adduction, peak rearfoot eversion, and stride frequency. Linear mixed models were used to asses the effect of load carriage and body mass on dependent variables.

A load x body mass interaction was observed for stride frequency only (p = 0.017). Stride frequency increased with load carriage of 22.7-kg, but lighter participants illustrated a greater change than heavier participants. Average loading rate (p < 0.001) and peak free moment (p = 0.015) were greater in the 22.7-kg condition, while peak rearfoot eversion (p ≤ 0.023) was greater in the 11.3- and 22.7-kg conditions, compared to the unloaded condition. Load carriage did not affect peak hip adduction (p = 0.67).

Participants adapted to heavy load carriage by increasing stride frequency. This was especially evident in lighter participants who increased stride frequency to a greater extent than heavier participants. Despite this adaptation, running with load carriage of ≥11.3-kg increased variables associated with a history of tibial stress fracture, which may be indicative of elevated stress fracture risk. However, the lack of concomitant change amongst variables as a function of load carriage may highlight the difficulty in assessing injury risk from a single measure of running biomechanics.