Ground reaction forces, neuromuscular and metabolic responses to combined strength and endurance loading in recreational endurance athletes

Among recreational and elite endurance athletes strength and endurance loadings are often performed concurrently to improve neuromuscular capacity in order to enhance running economy and maximal running velocity (i.e. running performance). Measuring the ground reaction forces provides valuable information about the alteration of running technique. Therefore, this study investigated acute changes in ground reaction forces (GRFs) and running stride variables (RSVs) as well as changes in neuromuscular performance and in metabolic status in responses to a single session combined strength and endurance loading (S+E and E+S). Secondly it studied the order effect of the combined loading. A group of 12 male (38±8 years) and 10 female (34±8 years) recreationally endurance trained subjects participated in the study. All subjects took part in two combined loading sessions; one with E loading followed immediately by S loading (E+S) and one with the opposite order (S+E). Prior to the measurements subjects were tested for their E (VO2max) and S performance (maximal bilateral isometric leg extension force, MVC). The subjects then performed both loadings in a randomized order. E consisted of continuous running for 60 minutes (min) at a given intensity between aerobic and anaerobic thresholds. The S loading (45 min) included both maximal and explosive strength exercises (3 x 8 reps with 75 % of 1 RM and 3 * 10 reps with 40 % of 1 RM with 2 min rest between sets) for leg extensor muscles. Changes in ground reaction forces (in horizontal and vertical direction with impulses) and running stride variables, and neuromuscular (MVC, MVC500, CMJ) responses to combined loadings were measured before (PRE), following the first S or E (MID) and after completing the combined loading (POST) in both S+E and E+S. Metabolic changes in E were measured during the first and last 10 min of the endurance loading and determined as the average of minutes 6–8 and 56–58. The main finding was that running biomechanics were altered significantly only after strength loading preceding endurance loading (S+E) but not after E+S. For men, stride length (p < 0.05), flight time (p < 0.01), vertical active peak force (p < 0.01) and total vertical impact (p < 0.05) decreased, while stride frequency (p < 0.001) increased in response to S+E loading order. There was an order effect in vertical active peak force between the loadings in men. For women, the only significant change was an increase in stride frequency (p < 0.05) in response to S+E loading order. The present study showed that there was an order effect in running biomechanics (GRFs and RSVs) between combined loadings (S+E and E+S). Running biomechanics were altered only after strength loading preceding endurance loading (S+E), suggesting higher fatigue and increased changes in the running stride, after S+E loading compared to E+S loading. The neuromuscular and metabolic responses did not show the order effect and the associations were not unambiguous due to quite high intra- and inter-individual change