Musculotendinous adaptations to improve running economy

Running economy (RE) is a crucial variable in endurance performance, and can be quantified as energy utilization at a given submaximal exercise intensity. Correlation analyses have shown that stiff triceps surae (TS) tendon and muscles and also compliant quadriceps (QU) tendon and muscles are associated with better RE. Isometric strength training can increase tendon stiffness, and some but not all studies have shown that long-term isometric strength training of the TS can lead to improvements in RE. Although it is known that proprioceptive neuromuscular facilitation (PNF) stretching training can decrease tendon stiffness, to date, no study has explored the effects of long-term PNF stretching training on the QU, to test if the decrease in tendon stiffness can increase RE. Thus, this project will be the first to investigate the isolated and combined effects of 14 weeks of PNF stretching training of the QU, as well as isometric strength training of the TS, on RE. We hypothesize that 14 weeks of both PNF stretch training of the QU and isometric strength training of the TS will result in improvements in RE, which can be explained by the changes in tissue stiffness (decrease in QU stiffness, increase in TS stiffness). Moreover, we assume that the combination of these two training strategies (QU stretch; TS strength) will result in a superior effect on RE, compared to the individual treatments. To test these hypotheses, a total of 72 male and 72 female endurance athletes will be randomly assigned to either: 1) 14 weeks of PNF QU stretch training; 2) 14 weeks of isometric strength training of the TS; 3) a combination of 1) and 2); or 4) a control group. All the training sessions will be performed three times a week, and will be supervised by the investigators. Following a familiarization session, each participant will be tested for maximal oxygen consumption (VO2max) during running. In the next session, they will be tested for RE (70% vVO2max) and running performance (90% vVO2max), as well as running biomechanics (e.g., stride length). In the subsequent session, each participants muscle- tendon function (e.g., flexibility) and muscle-tendon structure (e.g., tendon stiffness, muscle stiffness) will be tested. After completing the respective training regimes, all the parameters will be reassessed. This project will be the first to investigate the long-term training effects of stretch or strength training (or the combination of both) of specific body regions (TS or QU), to induce changes in stiffness, and hence likely related changes in RE.