Effects of Chronic Stretching and Resistance Training on Running Economy in Recreational Runners – A Pilot Study

Introduction & PurposeRunning economy (RE), defined as oxygen consumption at a given submaximal running speed, is a key determinant of endurance performance and can vary substantially even among athletes with similar maximal oxygen uptake (VO2MAX)(Saunders et al., 2004). Research has shown that muscle–tendon unit (MTU) properties, particularly muscle and tendon stiffness, influence RE (Arampatzis et al., 2006). Specifically, lower stiffness of the quadriceps MTU and greater stiffness of the triceps surae MTU have been associated with enhanced RE (Arampatzis et al., 2006). Such differences may be explained by the distinct biomechanical functions of these muscle groups during running. Musculoskeletal analyses of running mechanics have demonstrated that the quadriceps muscle primarily contributes to braking and support during the initial stance phase, whereas the plantar flexors primarily contribute to propulsion and support during the subsequent late stance phase (Hamner et al., 2010). Therefore, it can be hypothesized that different mechanical properties may be advantageous for different MTUs, with a more compliant quadriceps tendon potentially facilitating energy absorption during braking and a stiffer triceps surae MTU potentially enhancing elastic energy recoil during propulsion. As resistance training and stretching interventions can modify MTU properties (Albracht & Arampatzis, 2013; Konrad et al., 2015), they may affect RE by altering the mechanical characteristics of the locomotor system. In line with these mechanisms, resistance training has repeatedly been shown to have beneficial effects on RE (Llanos-Lagos et al., 2024), whereas evidence for the effects of stretching interventions remains limited, especially regarding chronic adaptations (Warneke et al., 2025). Therefore, interventions specifically targeting individual MTUs may represent a more suitable approach than non-specific lower-limb interventions. It has been demonstrated that proprioceptive neuromuscular facilitation (PNF) stretching has the potential to reduce tendon stiffness (Konrad et al., 2015), whereas prolonged high-volume stretching interventions may additionally decrease muscle stiffness (Nakamura et al., 2021). Moreover, isometric strength training has been associated with increased Achilles tendon stiffness and improvements in RE (Albracht & Arampatzis, 2013). Therefore, specifically reducing quadriceps MTU stiffness through PNF stretching while increasing triceps surae MTU stiffness through isometric resistance training may represent a promising, though exploratory, approach to potentially optimize RE. To date, the impact of an isolated quadriceps PNF stretching intervention or its combination with isometric plantar flexor training on RE has not been researched. The purpose of this study is to investigate the isolated and combined effects of chronic PNF stretching of the quadriceps MTU and isometric strength training of the triceps surae MTU on RE in recreational runners. MethodsIn this ongoing randomized controlled trial, recreational runners were recruited and subsequently allocated to one of four groups (resistance training, stretching, combined intervention, or control) using a computer-generated simple randomization. An a priori power analysis indicated a minimum required sample size of 56 participants (f = 0.4, α = 0.05, power = 0.80). The intervention period lasted 14 weeks, with all interventions performed three times per week in addition to participants' habitual endurance training. The stretching intervention consisted of high-volume PNF stretching using a contract–relax protocol targeting the quadriceps. Each set consisted of alternating 12 seconds of passive stretching and 8 seconds of maximal quadriceps contractions, which were repeated three times per leg prior to switching sides. This procedure was repeated for 10 sets per leg.  The resistance intervention consisted of high-intensity isometric plantar flexion training, targeting the triceps surae. Participants were instructed to perform seated isometric calf raises with extended knees at 90% of their maximal voluntary isometric contraction intensity. Each training session consisted of five sets per leg, with each set including four repetitions of three-second contractions, separated by three-second rest intervals, and two-minute rest between sets. Training intensity was regularly adjusted throughout the intervention period. The combined group performed both interventions, whereas the control group maintained their habitual training routines without additional intervention. To determine individual running velocities and ventilatory thresholds, all runners completed an incremental exercise test (IET) on the treadmill. After a recovery period of at least 48 hours, participants completed a RE assessment consisting of a 15-minute run at 10% below the second ventilatory threshold (VT2), followed by a time-to-exhaustion (TtE) run at 10% above VT2 until volitional exhaustion. The RE was determined from oxygen consumption during the final five minutes of the submaximal running stage. Biomechanical running parameters were assessed simultaneously during this period. To investigate potential underlying mechanisms, MTU structure and function were assessed before and after the intervention period. Ultrasound shear-wave elastography (SWE; Aixplorer, SuperSonic Imagine, Aix-en-Provence, France) was used to assess patellar and Achilles tendon stiffness. A linear SL10-2 transducer with a 500 kPa measurement range was used for the patellar tendon, and an SL15-4 transducer with a 1400 kPa range for the Achilles tendon, both with penetration mode enabled. Probe pressure was minimized throughout all measurements. The transducer position was marked on transparent foil and replicated during post-testing, while probe orientation was standardized using a digital angle-measurement application. For both tendons, the region of interest covered the entire tendon thickness in the longitudinal plane, over the full transducer length. Pre-testing inter-day reliability demonstrated good reliability for the patellar tendon (ICC(3,k) = 0.87) and excellent reliability for the Achilles tendon (ICC(3,k) = 0.96; n = 12). Active and passive quadriceps range of motion, ankle dorsiflexion during the wall-push test, knee-to-wall performance, and quadriceps and plantar-flexor strength were additionally assessed using three-dimensional motion capture, functional testing, and dynamometry. At the time of abstract submission, the pre- and post-data were available for five participants, and preliminary analyses were performed. ResultsPreliminary descriptive analyses were conducted on the first five participants with complete pre- and post-intervention data (stretching: n = 4; combined intervention: n = 1). In the stretching group, RE was 198.9 ± 22.3 ml·kg⁻¹·km⁻¹ before and 201.2 ± 28.1 ml·kg⁻¹·km⁻¹ after the intervention. Submaximal oxygen consumption was measured at 37.22 ± 2.55 ml·kg-1·min-1 before and 37.57 ± 2.66 ml·kg-1·min-1 after the intervention. TtE was 617 ± 337 seconds prior to the intervention, and 789 ± 368 seconds post-intervention (Figure 1). Passive knee ROM exhibited an increase from 112.90 ± 6.67° to 127.62 ± 6.17° following the intervention, while ankle dorsiflexion during the wall-push test demonstrated a rise from 97.26 ± 4.03° to 98.51 ± 4.73°. Additionally, knee-to-wall test performance showed an enhancement from 10.56 ± 2.10 cm to 11.38 ± 2.17 cm. The average patellar tendon stiffness decreased from 201.55 ± 30.97 to 163.60 ± 36.35 kPa, whereas the stiffness of the Achilles tendon changed from 383.12 ± 124.75 to 429.20 ± 62.16 kPa following the intervention. Preliminary biomechanical analyses indicated only minor changes in stride characteristics and ground contact time following the intervention period. The single participant allocated to the combined intervention showed a reduction in RE from 219.3 ml·kg-1·km-1 before to 205.6 ml·kg-1·km-1 after the intervention. Submaximal oxygen consumption was recorded at 38.71 ml·kg-1·min-1 and 36.29 ml·kg-1·min-1, respectively, while TtE was 491 and 437 s (Figure 1). Furthermore, patellar tendon stiffness changed from 257.71 to 277.96 kPa, while Achilles tendon stiffness changed from 362.62 to 494.43 kPa. DiscussionAs this trial is still ongoing and based on only five participants with an uneven distribution across groups, no conclusions can yet be drawn regarding the efficacy of the interventions or their underlying mechanisms. The descriptive increase in TtE and passive ROM and the reduction in patellar tendon stiffness observed in the stretching subgroup, as well as the increased Achilles tendon stiffness observed in the single combined-intervention participant, are therefore reported as preliminary, individual-level observations, broadly consistent with the general notion that MTU-specific mechanical properties may relate to distinct biomechanical and physiological determinants of running performance (Arampatzis et al., 2006). ConclusionIt is important to note that this trial is ongoing, and therefore it would be premature to draw any conclusions regarding the efficacy of the interventions or their underlying mechanisms from the current preliminary data. Once the recruitment and testing phases have been completed across all four groups, the results will help clarify whether MTU-specific adaptations, induced by PNF stretching and isometric resistance training, contribute to improvements in running economy in recreational runners.   References Albracht, K., & Arampatzis, A. (2013). Exercise-induced changes in triceps surae tendon stiffness and muscle strength affect running economy in humans. European Journal of Applied Physiology, 113(6), 1605–1615. https://doi.org/10.1007/s00421-012-2585-4 Arampatzis, A., De Monte, G., Karamanidis, K., Morey-Klapsing, G., Stafilidis, S., & Brüggemann, G.-P. (2006). Influence of the muscle-tendon unit’s mechanical and morphological properties on running economy. 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