Within-session dose–response effectsRange of motion

To examine the within-session dose–response effects, the first sessions in each condition (i.e. EC1 and SS1) were examined. A two-way repeated measures ANOVA revealed a significant interaction effect for ROM (F3.85,65.44 = 6.810, P < 0.001, ηp2 = 0.286). Simple main effects analyses revealed significant increases in ROM (Fig. 3a) after each set in EC1 (3.1°–6.0°, d = 1.04–2.00) and SS1 (1.0°–2.2°, d = 0.54–0.76) when compared to pre-intervention. In EC1, further significant increases in ROM occurred after the 3-5th sets when compared to after the 1st (1.6°–2.9°, d = 0.68–1.01) and 2nd (1.6°–2.9°, d = 0.59–0.77) sets, indicating that improvements occurred after each set. However, in SS1, a further significant increase in ROM was only apparent after the 5th set compared to the 4th (1.0° ± 1.9°, d = 0.53). Despite the significant interaction effect, no significant between-condition difference was detected at any timepoint (0.9°–2.6°, d = 0.01–0.23), indicative of a crossover profile (i.e. mean ROM pre-intervention was lower in the eccentric group but following the final set it was greater). Consequently, data were re-examined using a baseline-adjusted ANCOVA, which revealed significantly greater ROM in EC1 than SS1 after each set (1.9°–3.9°, d = 0.75–1.53).

Fig. 3

Mean (± SD) and individual dorsiflexion range of motion (a), plantarflexor stretch tolerance (b), and muscle–tendon unit (MTU) stiffness (c) data measured pre- and post-intervention to detect acute effects during the first eccentric contraction (EC1) and static stretching (SS1) sessions. Compared to pre-intervention, significant increases were detected in range of motion in both conditions, in stretch tolerance only after EC1, and reductions in MTU stiffness only when data were collapsed. *Significant within-condition difference to pre-intervention, ǂSignificant within-condition difference to sets 2 and 3, †Significant within-condition difference to set 4, #Significant between-condition difference at the same timepoint (using baseline ANCOVA adjusted analyses)

Soleus muscle activity

No significant interaction effect (F2.99,35.91 = 0.710, P = 0.552, ηp2 = 0.056) or main effects of condition (F1,12 = 1.824, P = 0.202, ηp2 = 0.132) or time (F5,60 = 0.535, P = 0.591, ηp2 = 0.032) were detected for soleus muscle activity, with mean activity remaining below 5%MVC.

Peak passive torque

A significant interaction effect was also revealed for peak passive torque (F5,85 = 6.938, P < 0.001, ηp2 = 0.290). Simple main effects analyses revealed significant increases (Fig. 3b) in torque after each set in EC1 (19.3–30.7% [6.9–12.5 Nm], d = 1.16–1.48) when compared to pre-intervention, however no change in torque occurred in SS1 (0.4–5.1% [− 3.5 to 1.7 Nm], d = − 0.15 to 0.23). Despite the significant interaction effect, no between-condition difference was detected at any timepoint (− 8.0 to 22.9% [− 2.4 to 8.4 Nm], d = − 0.38 to 0.21), indicative of a crossover profile. Consequently, data were re-examined using a baseline-adjusted ANCOVA, which revealed significantly greater torque in EC1 than SS1 after each set (7.5–14.1 Nm, d = 1.05–1.49).

Muscle–tendon unit stiffness

No significant interaction effect was detected for MTU stiffness (F5,85 = 1.043, P = 0.398, ηp2 = 0.058), with no main effect of condition (F1,17 = 0.915, P = 0.352, ηp2 = 0.051) but a significant effect of time (F5,85 = 3.614, P = 0.005, ηp2 = 0.175). When compared to pre-intervention (data collapsed across conditions), significantly lower MTU stiffness (Fig. 3c) was detected after each set (EC1 = − 4.6 to − 10.4% [− 0.34 to − 1.12 Nm°−1], d = − 0.44 to − 0.65; SS1 = − 2.8 to − 7.7% [− 0.29 to − 0.91 Nm °−1], d = − 0.28 to − 0.54; collapsed = − 4.7 to − 9.7% [− 0.47 to − 1.00 Nm°−1], d = − 0.47 to − 0.96).

Achilles tendon stiffness

A significant interaction effect was found for Achilles tendon stiffness (F1,17 = 2.520, P = 0.043, ηp2 = 0.220), with simple main effects analyses revealing a significant post-intervention decrease in tendon stiffness (Fig. 4a) in EC1 (− 12.0 ± 9.2% [− 0.91 ± 0.79 Nm mm−1], d = − 1.15) but not SS1 (2.1 ± 11.4% [0.16 ± 1.00 Nm mm−1], d = 0.16). No significant difference was detected between conditions at pre-intervention (2.0 ± 20.0% [0.06 ± 1.32 Nm mm−1], d = 0.04), however significantly lower tendon stiffness was detected in EC1 than SS1 at post-intervention (− 8.4 ± 16.2% [− 0.80 ± 1.37 Nm], d = − 0.59).

Fig. 4

Mean (± SD) and individual Achilles tendon stiffness (a), gastrocnemius medialis (GM) muscle stiffness (b), and maximal isometric plantarflexor torque (c) data measured pre- and post-intervention to detect acute effects during the first eccentric contraction (EC1) and static stretching (SS1) sessions. Compared to pre-intervention, significant decreases were detected in tendon stiffness only after EC1, and in GM muscle stiffness and torque only when data were collapsed. *Significantly different to pre-intervention, #Significant between-condition difference at the same timepoint

Gastrocnemius medialis muscle stiffness

No significant interaction effect was detected for GM muscle stiffness (F1,17 = 0.028, P = 0.869, ηp2 = 0.002), with no main effect of condition (F1,17 = 2.028, P = 0.173, ηp2 = 0.107) but a significant effect of time (F1,17 = 8.521, P = 0.010, ηp2 = 0.334). A significant post-intervention decrease (Fig. 4b) in muscle stiffness (data collapsed across conditions) was detected (EC1 = − 6.1 ± 22.2% [− 0.15 ± 0.40 Nm mm−1], d = − 0.38; SS1 = − 10.7 ± 14.5% [− 0.17 ± 0.28 Nm mm−1], d = − 0.59; collapsed = − 9.2 ± 15.1% [− 0.16 ± 0.27 Nm mm−1], d = − 0.60).

Maximal isometric plantarflexor torque

No significant interaction effect was detected for maximal isometric plantarflexor torque (F1,17 = 0.281, P = 0.603, ηp2 = 0.016), with no main effect of condition (F1,17 = 2.087, P = 0.167, ηp2 = 0.109) but a significant effect of time (F1,17 = 8.521, P = 0.010, ηp2 = 0.334). A significant reduction (Fig. 4c) in torque (data collapsed across conditions) was detected post-intervention (EC1 = − 4.5 ± 12.1% [− 11.5 ± 22.3 Nm], d = − 0.43; SS1 = − 6.5 ± 14.7% [− 7.8 ± 18.3 Nm], d = − 0.52; collapsed = − 5.7 ± 10.0% [− 9.6 ± 14.0 Nm], d = − 0.69).

Between-session (carry-over) effectsRange of motion

To explore whether one or two sessions of eccentric contractions or static stretches were sufficient to increase ROM at baseline in the subsequent sessions (i.e. between-session carry-over effects), the participants were separated into two groups based on whether they performed EC (n = 9) or SS (n = 9) first. No significant interaction effect was detected for ROM (F1.59,25.38 = 0.550, P = 0.544, ηp2 = 0.033), with no main effect of condition (F1,16 = 1.240, P = 0.282, ηp2 = 0.072) but a significant effect of session (F1.59,25.38 = 13.999, P < 0.001, ηp2 = 0.467). Compared to Session 1 (data collapsed across conditions), no significant ROM increase was detected in Session 2 (collapsed = 2.5° ± 4.5°, d = 0.55; EC = 3.0° ± 4.3°, d = 0.69; SS = 2.0° ± 5.0°, d = 0.40) but a significant increase (Fig. 5a) was detected in Session 3 (collapsed = 5.9° ± 5.9°, d = 1.00; EC = 7.1° ± 4.6°, d = 1.55; SS = 4.7° ± 7.1°, d = 0.67). Thus, a single exposure (training session) was not sufficient to trigger sustained ROM improvement 2–3 days later, but two exposures was sufficient.

Fig. 5

Mean (± SD) and individual dorsiflexion range of motion (a), plantarflexor stretch tolerance (b), and muscle–tendon unit (MTU) stiffness (c) data measured at pre-intervention to detect carry-over effects between Sessions 1, 2, and 3. Compared to Session 1, significant increases were detected in pre-intervention range of motion in Session 3, in stretch tolerance and MTU stiffness in Sessions 2 and 3 when data were collapsed. *Significantly different to pre-intervention in Session 1

Peak passive torque

No significant interaction effect was detected for pre-intervention peak passive torque (F1.50,24.06 = 1.354, P = 0.271, ηp2 = 0.078), with no main effect of condition (F1,16 = 1.135, P = 0.302, ηp2 = 0.066) but a significant effect of session (F1.50,24.06 = 12.849, P < 0.001, ηp2 = 0.445). Compared to Session 1 (data collapsed across conditions), significant increases in torque (Fig. 5b) were detected at Session 2 (EC = 34.7 ± 29.3% [14.5 ± 15.2 Nm], d = 0.96; SS = 16.4 ± 43.4% [5.4 ± 13.9 Nm], d = 0.39; collapsed = 25.4 ± 38.2% [10.0 ± 14.9 Nm, d = 0.67) and Session 3 (EC = 62.2 ± 63.4% [29.1 ± 38.9 Nm], d = 0.75; SS = 13.9 ± 87.3% [12.3 ± 22.1 Nm], d = 0.56; collapsed = 38.0 ± 78.1% [20.7 ± 31.9 Nm], d = 0.65). Thus, a single exposure was sufficient to produce a sustained increase in torque 2–3 days later.

Muscle–tendon unit stiffness

No significant interaction effect was detected for pre-intervention MTU stiffness (F2,16 = 0.012, P = 1.847, ηp2 = 0.174), with no main effect of condition (F1,16 = 1.183, P = 0.293, ηp2 = 0.069) but a significant effect of session (F2,32 = 8.353, P = 0.001, ηp2 = 0.343). Compared to Session 1 (data collapsed across conditions), significant increases in MTU stiffness (Fig. 5c) were detected at Session 2 (EC = 42.4 ± 46.6% [3.3 ± 3.1 Nm °], d = 1.07; SS = 10.8 ± 30.8% [1.0 ± 2.7 Nm °], d = 0.37; collapsed = 26.6 ± 41.6% [2.2 ± 3.1 Nm °], d = 0.71) and Session 3 (EC = 43.2 ± 58.2% [3.1 ± 3.8 Nm °], d = 0.80; SS = 40.2 ± 84.4% [1.7 ± 3.3 Nm °], d = 0.50; collapsed = 41.7 ± 70.3% [2.4 ± 3.5 Nm °], d = 0.66). Thus, a single exposure was sufficient to produce a sustained increase in MTU stiffness 2–3 days later.

Achilles tendon stiffness

No significant interaction effect (F2,32 = 3.124, P = 0.058, ηp2 = 0.163) or main effects of condition (F1,16 = 0.243, P = 0.629, ηp2 = 0.015) or session (F2,32 = 0.535, P = 0.591, ηp2 = 0.032) were detected for Achilles tendon stiffness (Fig. 6a).

Fig. 6

Mean (± SD) and individual Achilles tendon stiffness (a), gastrocnemius medialis (GM) muscle stiffness (b), and maximal isometric plantarflexor torque (c) data measured at pre-intervention to detect carry-over effects between Sessions 1, 2, and 3. No significant changes in pre-intervention Achilles tendon stiffness, GM muscle stiffness, or torque were detected after one or two sessions of eccentric contraction or static stretch sessions

Gastrocnemius medialis muscle stiffness

No significant interaction effect (F2,32 = 0.798, P = 0.459, ηp2 = 0.047) or main effects of condition (F1,16 = 0.462, P = 0.506, ηp2 = 0.028) or session (F2,32 = 1.451, P = 0.249, ηp2 = 0.083) were detected for GM muscle stiffness (Fig. 6b).

Maximal isometric plantarflexor torque

No significant interaction effect (F1.59,25.46 = 3.033, P = 0.076, ηp2 = 0.159) or main effects of condition (F1,16 = 0.418, P = 0.527, ηp2 = 0.025) or session (F1.59,25.46 = 3.178, P = 0.069, ηp2 = 0.166) were detected for maximal isometric plantarflexor torque (Fig. 6c).

Correlations

During SS1, no significant within-session correlations were detected between the absolute change in ROM and absolute changes in MTU stiffness (rs = 0.25), peak passive torque (rs = 0.43), muscle stiffness (rs = − 0.20) or tendon stiffness (rs = − 0.33), while a significant correlation was detected between changes in MTU stiffness and peak passive torque (rs = 0.57). However, during EC1 a significant within-session correlation was detected between changes in ROM and peak passive torque (rs = 0.66), with no significant within-session correlations detected between the changes in ROM and any other variable (rs = − 0.16 to 0.24).

For the baseline assessment across sessions, significant between-session correlations were detected between the changes in ROM and both peak passive torque (rs = 0.81–0.85) and MTU stiffness (rs = − 0.73–0.80) after one and two sessions of static stretching. However, significant between-session correlations were only found for the changes in ROM and peak passive torque (rs = 0.70–0.78) after one and two sessions of eccentric contractions. No significant between-session correlations were detected between the changes in ROM and any other variable after one or two sessions of stretches or eccentric contractions.