Individual adaptation to experimental changes in running speed and step rate during treadmill running in injury-free runners: Adaptation til ændringer I løbehastighed og kadence

Introduction Running is a popular sport (1) and important in health promotion (2). The incidence of running related injures however is up to 85% (3) and running patterns have been hypothesized to influence the injury mechanisms by means of reaction forces influencing the load of the foot and lower extremity. These mechanisms are complex and the load in the individual joints is a result of the interaction between several parameters (4). The peak force however is important in evaluation of the all-overload of the lower extremity. Increased step rate during constant velocity has recently showed to result in decreased load of the knee and hip joints. However, based on clinical observations, most runners simultaneously changes the velocity when forced to change the step rate. The relation between reaction force, step rate and velocity is therefore explored in this multi-condition study of recreational runners. Hypotheses i. Increased velocity will result in decreased duration of the stance phase and increased peak reaction force and step length, independently of the step rate ii. Decreased velocity will result in increased duration of the stance phase and decreased peak reaction force and step length, independently of the step rate Methods 10 participants (8 male) aged 27,6 years (23-40), height 179,25 cm (160-189), weight 74.7 kg (56-94), running experience 9.9 years (2-30), training sessions/week 2.4 (1-6), amount of training 15,9 km/week (5-50) participated in an experimental study of peak reaction force, step length and duration of stance phase of the stride during treadmill running with self-selected velocity and step rate and manipulation of the velocity to 90% and 110% of the self-selected with respectively self-selected step rate and 105% step rate during each velocity. All measurements were conducted as 30 seconds intervals after the participants ran 5 minutes before first measurement and after manipulation of velocity and step rate, the participants continued running to stable step rate. Reaction force was measured by Zebris FDM-T 2-d pressure plates integrated in a HP Quasar treadmill and peak reaction force and temporal parameters were calculated as mean of respectively left and right side during the 30 seconds recording. All measurements were calculated as differences from values from running with self-selected velocity and step rate and the 5 conditions (90% and 110% velocity with self-selected and 105% step rate and 100% velocity with 105% step rate) and 2 sides (left and right) were analyzed by RM- ANOVA with a Greenhouse-Geisser correction and significance level 0.05 in IBM SPSS version 22.0 Results A 2-way RM-ANOVA including sides (left and right) and conditions (5 different changes in velocity and step rate from self-selected) showed: 1. no significant interaction between sides and conditions in step length RM-ANOVA(F(4,36)=1.97, P = 0.172 with significant differences between conditions RM-ANOVA(F(4,36)=665.30, P < 0.001 where 10% decreased velocity with normal and 5% increased step rate and normal velocity with 5% increased step rate resulted in decreased step length with respectively 8.6, 13.1 and 5.1 % in left (P<0.001) and 7.5, 12.1 and 4.8% in right side (P<0.001) ii) no significant interaction between sides and conditions in the percentage duration of the stance phase from the stride RM-ANOVA(F(4,36)=0.45, P = 0.701 with significant differences between conditions RM-ANOVA(F(4,36)=665.30, P < 0.001 where 10% decreased velocity with normal and 5% increased step rate and normal velocity with 5% increased step rate resulted in increased duration of stance phase with respectively 5.5, 6.8 and 2.1 % in left (P<0.007) and 5.3, 6.6 and 2.1% in right side (P<0.001) ii) no significant interaction between sides and conditions in the peak reaction force RM-ANOVA(F(4,36)= 1.22, P = 0.315 and no significant differences between conditions RM-ANOVA(F(4,36)= 1.69, P = 0.207 ii) no significant interaction between sides and conditions in the peak reaction force RM-ANOVA(F(4,36)= 1.22, P = 0.315 and no significant differences between conditions RM-ANOVA(F(4,36)= 1.69, P = 0.207 Conclusion Changes in step rate or velocity from self-selected step rate and velocity resulted in significant changes in mean step length and length of the stance and swing-phase during treadmill running without significant changes in peak reaction force Discussion Analysis of the peak reaction force of the individual participants showed that in 5 of the 10 participants the peak force was decreased bilaterally during all or at least 4 of the conditions, while the last 5 participants showed variable effects of the manipulations between conditions and even larger differences between left and right side. This could indicate that the individual participants use individual strategies when challenged in velocity and / or step rate and further research is needed to explore the underlying mechanisms and the force reaction patterns during running.