Targeted muscle stretching to improve running economy

Running economy is a crucial variable in endurance performance, and can be quantified as energy utilization at a given submaximal exercise intensity. In sports practice, stretching exercises are commonly used as part of a warm-up routine prior to running events. However, there have been conflicting reports in the literature about the effects of a single stretching exercise on running economy and running performance. One possible reason for these conflicting reports might be the approach to stretching multiple muscles prior to a running event. Whilst a compliant tendon of the quadriceps is favorable for running economy/performance, for other muscle-tendon units (MTUs), such as plantar flexors and hamstrings, stiff tendons are advantageous. To date, most of the studies have applied static stretching (=holding the stretch statically in a stretching position) prior to a running event. However, static stretching applies only slight tension to the MTU, in contrast to proprioceptive neuromuscular facilitation (PNF) stretching (=similar to static stretching but additionally it includes contractions of e.g. the stretched muscle), which is able to reduce tendon stiffness. Since it is known that a single stretching exercises for more than 60s likely decreases force production, a further reason for the conflicting reports might be the excessive stretching durations (up to 360s/muscle) applied in most of the studies. However, to counteract such detrimental effects in force production dynamic activities following stretching can be applied. According to the aforementioned explanations, we expect that stretching only the quadriceps muscles, applying the PNF stretching technique for a moderate duration (i.e. 415s) including dynamic activities, will increase running economy, while stretching only the plantar flexors or hamstrings will have a negative effect on running economy. To test these hypotheses, 24 male trained endurance athletes will be randomly assigned to quadriceps stretch, plantar flexor stretch, or control condition. After a standardized warm-up, the flexibility of the quadriceps and plantar flexors will be assessed. Depending on the group allocation, the quadriceps, plantar flexors, or no muscle (control) will be stretched with the PNF technique for 415s, and then the flexibility assessment will be repeated. Moreover, running economy and the related biomechanical variables (i.e. ground contact time) will be assessed at a velocity related to 70% of maximum oxygen uptake (assessed in an incremental test) in a 15-min run on a treadmill. The current project is the first which will investigate the effects of a single PNF stretching exercise with moderate duration of a targeted muscle group (i.e quadriceps) on running economy.

Running economy is a crucial variable in endurance performance, and can be quantified as energy utilization at a given submaximal exercise intensity. In sports practice, stretching exercises are commonly used as part of a warm-up routine prior to running events. However, there have been conflicting reports in the literature about the effects of a single stretching exercise on running economy and running performance. Potential reasons for those conflicting reports might be the approach to stretching multiple muscles prior to a running event. However, it is known that a more compliant tendon of the quadriceps as well as a stiffer Achilles tendon are advantageous for running economy. Additionally, previous studies applied excessive stretching durations and mainly used an isolated static stretching approach, which will likely cause a decrease in force production. According to that, we expected that stretching only the quadriceps muscles, applying the proprioceptive neuromuscular facilitation (PNF) stretching technique for a moderate duration (i.e., 415s) including dynamic activities, will increase running economy, while stretching only the plantar flexors will have a negative effect on running economy. Contrary to our hypotheses our results showed neither a difference in running economy nor a change in any variable of the running biomechanics during the steady-state run between the tested groups. Regarding the mechanism in the plantar flexor stretching group, we found as expected a decrease in Achilles tendon stiffness. However, in the quadriceps stretching group, a decrease in the overall quadriceps muscle stiffness and in vastus medialis stiffness was seen with no changes in the patellar tendon stiffness. Additionally, both groups showed no changes in counter movement jump performance, whilst there was a time effect in drop jump performance indicating a decrease in jump height as well as in reactive strength index in both groups. Hence, according to our findings athletes can perform moderate duration PNF stretching of the quadriceps or plantar flexors including dynamic activities prior to a running event, without affecting running economy. Based on this data we expect that a single stretching exercise might not be enough stimulus to improve running economy and consequently training studies where the muscle-tendon unit adapts chronically might be the better approach.