ECSS Paris 2023: OP-BM10
INTRODUCTION: Skeletal muscle efficiency – the ability to convert chemical energy into mechanical work – peaks at around 25-30%, but the ratio between mechanical output and metabolic input exceeds these values (e.g. up to 60% in hopping and 80% in running) during human movements involving the stretch-shortening cycle (SSC); this ratio is thus called “apparent” efficiency (AE). The reasons AE is so elevated in SSC remain a matter of debate, but may be attributed either to tendon mechanical capacity (stiffness or elastic recoil) or to the active (muscle) MTU component behaviours (i.e. fascicle behaviours). In this study we combined metabolic, kinematic, kinetic, and ultrasound data to explore the determinants of AE during a SSC task. METHODS: Fifteen healthy subjects (8M/7F) (age: 27.6 ± 4.6 y; body mass: 63.4 ± 11.6 kg; height: 1.69 ± 0.08 m) performed bilateral hops both unloaded and with added mass of 15% or 30% of body mass (BM), to manipulate this task’s mechanical and metabolic demands (and hence AE). During each trial, whole-body kinematics, plantarflexor EMG activity (soleus, gastrocnemius medialis and lateralis), ground reaction forces, and ultrasound images of the medial gastrocnemius fascicles and the Achilles tendon (AT) were collected using a 3D motion-capture, electromyography (EMG), force platform, and ultrasonography. Oxygen uptake was measured breath-by-breath using a metabolimeter. Total mechanical power at the whole-body level (Ptot) was calculated from kinetic data. Net metabolic power (Pmet) was computed from net oxygen uptake, and AE was calculated as Ptot / Pmet. Fascicle shortening velocity (Vfas) was determined during the stance phase by tracking ultrasound images. AT mechanical power (Pten) and stiffness were derived from the tendon force‒elongation relationship. Plantarflexor cumulative muscle activation (ACT) was also calculated from EMG signals. Changes in AE across loading conditions were analysed using multivariate regression combined with dominance analysis to quantify the relative contribution of each parameter to the observed changes in AE. RESULTS: Ptot and AE decreased with increasing load, whereas Pmet increased. Vfas and Pten decreased as load increased, while AT stiffness and ACT increased. The model explained 77% of the variance in AE across loading conditions. Reductions in Pten accounted for 35% and 38% of the AE changes at 15% and 30% BM, respectively, whereas increases in AT stiffness accounted for 18% and 20%. Vfas contributed 10% and 12% of the AE variance, while ACT accounted for the remaining variance (14% and 7%). CONCLUSION: Changes in the AT mechanical capacity explain about 55% of the changes in AE with increasing load, while the activity of the contractile components accounts for 20%. These data suggest that the decrease in AE observed in obese individuals results from an impairment of tendon elasticity rather than of the active (muscle) components of the MTU.
Read CV Andrea MonteECSS Paris 2023: OP-BM10
INTRODUCTION: The strain of a tendon under load is a key determinant of its mechanical loading. Strains between 4.5% and 6.5% appear to effectively promote tendon adaptation [1], whereas strains > 9% combined with high loading volumes may represent a risk factor for overuse injuries [2]. However, the actual strain of tendons in vivo across various exercise are widely unkown. Therefore, the present study investigated patellar tendon strain as an indicator of its mechanical demand in vivo during exercise variations on a knee extension machine and a leg press. METHODS: Using ultrasound imaging, patellar tendon strain was measured in 22 trained individuals during unilateral exercise execution in both machines at 40%, 60%, and 80% of the concentric one-repetition maximum (1RM), as well as isolated loading during the flexion phase (i.e., predominantly eccentric contraction) at 100% and 120% 1RM. Additionally, the electromyographic (EMG) activity of the knee extensors and antagonistic musculature was recorded. Data analysis was performed using linear mixed models. This included testing the feasibility of predicting the resulting strain based on the load and the tendon strain measured during a maximal fixed-end contraction (εmax). RESULTS: A significant effect of load and movement direction was observed for both machines and exercise variations, with higher strains at higher loads (e.g. in the leg press 3.2% and 5.5% at 40% and 80% 1RM, respectively) and during extension compared to flexion (e.g. 6.0 vs. 4.9% at 80% 1RM). Similar patterns were observed regarding agonistic and antagonistic EMG activity. However, individual prediction of strain using εmax was not possible with high accuracy (R2 ≤ 0.51). CONCLUSION: To achieve patellar tendon strains of 4.5% to 6.5% effective for adaptive responses, unilateral exercises on the knee extension machine and leg press require loads of approximately 70% and 80% 1RM, respectively. In this context, strain during knee extension tends to be in the upper part of the target range, while strain during knee flexion is in the lower part, as higher resultant moments are required to overcome the same load during knee extension. Furthermore, antagonist muscle activation increases the moments which the knee extensors need to generate for a given resultant joint moment. The higher antagonistic muscle activation, additionally contributes to the higher strains observed during extension and complicates the predictability of strain using εmax. 1. Arampatzis et al., J Biomech, 43:3073-79, 2010. 2. Mersmann et al., Sports Med Open, 9:83, 2023.
Read CV Falk MersmannECSS Paris 2023: OP-BM10
INTRODUCTION: Explosive strength is the neuromuscular capacity to produce joint torque as rapidly as possible. This quality is critical for sport performance, physical activity, and clinical function. When measured as an absolute value (Nm·s⁻¹), explosive strength depends on athletic background (e.g., endurance vs. power), sex, and fatigability status. However, these differences almost completely disappear as soon as explosive strength is normalised to maximal strength (peak torque per second; PT·s⁻¹), indicating the existence of an upper limit of explosive strength imposed by maximal torque capacity. By combining experimental measures and computational modelling, we tested the existence of such a limit, and checked whether it can be reached for physiological ranges of motor unit (MU) recruitment rates. METHODS: Isometric ankle dorsiflexion torques were recorded in 13 participants during trains of supramaximal electrical stimuli delivered to the common peroneal nerve at 10 and 100 Hz, including catch-like contractions. Torque traces were used to extract parameters for a model of electrically evoked contractions, and to derive mean MU force. The discharge rate of MU was modelled using a bi-exponential system to account for spike-frequency adaptation (Spielmann et al., 1993), from experimental discharge rates of 158 MUs identified in the tibialis anterior during explosive contractions (Grootenhuis et al., 2025). Simulations included 150 motor units, consistent with anatomical estimates for the tibialis anterior (Debenham et al., 2025). Explosive isometric contractions were simulated across physiological MU recruitment rates, from 200 to 12000 MU·s⁻¹ (Del Vecchio et al., 2022). Explosive strength was quantified as the peak value of the rate of torque development function (RTD), computed with progressively increasing time windows (by 1 ms) from torque onset. RESULTS: The effect of MU recruitment rate on RTD was non-linear. Increasing recruitment rate from 200 to 4000 MU·s⁻¹ produced a disproportionate rise in RTD from 1.1 to 5.0 PT·s⁻¹. Beyond this range, progressively larger increases in recruitment rate yielded only marginal RTD gains. At high but physiologically plausible MU recruitment rates (> 6000 MU·s⁻¹), RTD approached a plateau of ~6 PT·s⁻¹, consistent with maximal values reported in the literature for the ankle dorsiflexors. CONCLUSION: These findings support the existence of an upper physiological limit to explosive strength determined by maximal torque capacity and provide an explanation for similar normalized explosive strength across populations and conditions. Although maximal strength constrains explosive performance, its precise role as a confounder, mediator, or collider needs still to be determined. References: Del Vecchio et al., 2022, J Appl Physiol, 132(1): 84–94. Debenham et al., 2025, J Electromyogr Kinesiol, EPub ahead of print. Grootenhuis et al., 2025, Scand J Med Sci Sports, 35(5) ; e70065. Spielmann et al., 1993, J Physiol, 464(1):75-120.
Read CV Luca RuggieroECSS Paris 2023: OP-BM10