Contribution of Stretch-Induced Force Enhancement to Increased Performance in Maximal Voluntary and Submaximal Artificially Activated Stretch-Shortening Muscle Action

Abstract

In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°-20° (CON) and 20°-80°-20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.

Keywords: concentric; eccentric; elastic energy; electrical stimulation; force depression; force enhancement; muscular activation.

FIGURE 1

Ultrasound image of the m. vastus lateralis. Fascicle length and pennation angle were determined for three fascicles. Pennation angle was calculated between the muscle fascicle and the deep aponeurosis. Fascicle length was defined as the distance between the intersection of the upper aponeurosis with the muscle fascicle and the intersection of the lower aponeurosis and muscle fascicle. The mean of the three measured values was used for further analysis.

FIGURE 2

Experimental protocol work flow. The test protocol comprised maximal voluntary contractions (100%) and submaximal electrical stimulated contractions at 20%, 35%, and 50% of MVC.

FIGURE 3

Exemplary representation of torque-time and angle-time traces. Vertical lines indicate the time point at the end of stretch (T1), midpoint of shortening (T2), and the steady-state interval where the mean steady-state torque was calculated (T3). Continuous yellow line SSC, dotted green line ISO, and dashed blue line CON.

FIGURE 4

Mean (n = 27, thick horizontal line) (and mean of each individual subject) values of joint torque at the onset of shortening (T1). Squares represent concentric condition (CON), whereas circles represent stretch-shortening conditions (SSC). Braces indicate significant (p < 0.05) differences between CON and SSC at the same intensity level.

FIGURE 5

Mean (n = 27, thick horizontal line) (and mean of each individual subjects) values of mechanical work adjusted to the actual knee rotation during the shortening phase. Squares represent concentric condition (CON), whereas circles represent stretch-shortening conditions (SSC). Braces indicate significant (p < 0.05) differences between CON and SSC at the same intensity level.

FIGURE 6

Mean (n = 27, thick horizonal line) (and mean of each individual subject) values of joint torque in the steady-state after the dynamic phase (T3). Triangles represent purely isometric torque at 20° knee flexion (ISO), squares represent concentric condition (CON), whereas circles represent stretch-shortening conditions (SSC). Braces indicate significant (p < 0.05) differences between the conditions.

FIGURE 7

Mean (±SD, n = 27) values of knee joint angle at different contraction intensities (% of MVC). T1 is the time-point at the onset of shortening, T2 in the middle of the shortening phase, T3 the time-point at steady-state after the dynamic phase. No significant interaction (condition × intensity) was found. Main effect of condition revealed, that knee joint flexion angle was significantly (p < 0.05) higher at the CON compared to the SSC condition at T1.

FIGURE 8

Mean (±SD, n = 27) values of fascicle length at different contraction intensities (% of MVC). T1 is the time-point at the onset of shortening, T2 in the middle of the shortening phase, T3 the time-point at steady-state after the dynamic phase. No significant interaction (condition × intensity) was found. Main effect of condition was also not significant (p > 0.05).

FIGURE 9

Mean (±SD, n = 27) values of pennation angle at different contraction intensities (% of MVC). T1 is the time-point at the onset of shortening, T2 in the middle of the shortening phase, T3 the time-point at steady-state after the dynamic phase. No significant interaction (condition × intensity) was found. Main effect of condition revealed, that the pennation angle was significantly (p < 0.05) higher at the SSC compared to the CON condition at T1.