This study investigated how joint angle influences fascicle shortening dynamics of gastrocnemius medialis (GM) during explosive contractions, and the resulting impact on rate of torque development (RTD). Sixteen participants performed six sets of five maximal explosive voluntary isometric plantar flexions at -20°, -10°, 0° (neutral position), 10°, 20° and 30° of ankle angle, and five no-load ballistic plantar flexions. RTD assessed over all time windows (from 0 to 200 ms) was significantly lower in extreme plantar flexed (≥20°) and dorsiflexed (- 20°) positions compared to -10, 0° (475±105 Nm.s-1) and 10°. At these neutral positions, RTD was maximal and muscle fascicles mainly operated over the plateau of the force-length relationship. At 0°, fascicle shortening velocity peaked at 9.26±2.85 cm.s-1 (i.e., 28.2% of maximal shortening velocity measured during no-load ballistic condition). At 112 ms after RTD onset, fascicle force reached 208 ± 78 N (i.e., 85.6% of the theoretical maximum force at the corresponding shortening velocity) and was thereafter comprised within the 95% confidence interval of the force-velocity curve. This clearly indicates that muscle force reached the maximal force that accounts for the fascicle shortening velocity. These findings suggest that the dynamic behavior of muscle fascicles, and the associated fascicle shortening velocity, may influence the rapid force-generating capacity mainly from 100 ms of RTD onset. The present study provides important information to better understand the determinants of human muscle performance during explosive tasks.

Background The relationship between the diaphragm thickening fraction and the transdiaphragmatic pressure, the reference method to evaluate the diaphragm function, has not been clearly established. This study investigated the global and intraindividual relationship between the thickening fraction of the diaphragm and the transdiaphragmatic pressure. The authors hypothesized that the diaphragm thickening fraction would be positively and significantly correlated to the transdiaphragmatic pressure, in both healthy participants and ventilated patients. Methods Fourteen healthy individuals and 25 mechanically ventilated patients (enrolled in two previous physiologic investigations) participated in the current study. The zone of apposition of the right hemidiaphragm was imaged simultaneously to transdiaphragmatic pressure recording within different breathing conditions, i.e., external inspiratory threshold loading in healthy individuals and various pressure support settings in patients. A blinded offline breath-by-breath analysis synchronously computed the changes in transdiaphragmatic pressure, the diaphragm pressure-time product, and diaphragm thickening fraction. Global and intraindividual relationships between variables were assessed. Results In healthy subjects, both changes in transdiaphragmatic pressure and diaphragm pressure-time product were moderately correlated to diaphragm thickening fraction (repeated measures correlation = 0.40, P < 0.0001; and repeated measures correlation = 0.38, P < 0.0001, respectively). In mechanically ventilated patients, changes in transdiaphragmatic pressure and thickening fraction were weakly correlated (repeated measures correlation = 0.11, P = 0.008), while diaphragm pressure-time product and thickening fraction were not (repeated measures correlation = 0.04, P = 0.396). Individually, changes in transdiaphragmatic pressure and thickening fraction were significantly correlated in 8 of 14 healthy subjects (ρ = 0.30 to 0.85, all P < 0.05) and in 2 of 25 mechanically ventilated patients (ρ = 0.47 to 0.64, all P < 0.05). Diaphragm pressure-time product and thickening fraction correlated in 8 of 14 healthy subjects (ρ = 0.41 to 0.82, all P < 0.02) and in 2 of 25 mechanically ventilated patients (ρ = 0.63 to 0.66, all P < 0.01). Conclusions Overall, diaphragm function as assessed with transdiaphragmatic pressure was weakly related to diaphragm thickening fraction. The diaphragm thickening fraction should not be used in healthy subjects or ventilated patients when changes in diaphragm function are evaluated. Editor’s Perspective What We Already Know about This Topic What This Article Tells Us That Is New