Multiscale hamstring muscle adaptations following 9 weeks of eccentric training

Abstract

Background: Eccentric training, such as Nordic hamstring exercise (NHE) training, is commonly used as a preventive measure for hamstring strains. Eccentric training is believed to induce lengthening of muscle fascicles and to be associated with the addition of sarcomeres in series within muscle fibers. However, the difficulty in measuring sarcomere adaptation in human muscles has severely limited information about the precise mechanisms of adaptation. This study addressed this limitation by measuring the multiscale hamstring muscle adaptations in response to 9 weeks of NHE training and 3 weeks of detraining.

Methods: Twelve participants completed 9 weeks of supervised NHE training, followed by a 3-week detraining period. We assessed biceps femoris long-head (BFlh) muscle fascicle length, sarcomere length, and serial sarcomere number in the central and distal regions of the muscle. Additionally, we measured muscle volume and eccentric strength at baseline, post-training, and post-detraining.

Results: NHE training over 9 weeks induced significant architectural and strength adaptations in the BFlh muscle. Fascicle length increased by 19% in the central muscle region and 33% in the distal muscle region. NHE also induced increases in serial sarcomere number (25% in the central region and 49% in the distal region). BFlh muscle volume increased by 8%, and knee flexion strength increased by 40% with training. Following 3 weeks of detraining, fascicle length decreased by 12% in the central region and 16% in the distal region along with reductions in serial sarcomere number.

Conclusion: Nine weeks of NHE training produced substantial, region-specific increases in BFlh muscle fascicle length, muscle volume, and force generation. The direct measurement of sarcomere lengths revealed that the increased fascicle length was accompanied by the addition of sarcomeres in series within the muscle fascicles.

Keywords: Adaptation; Muscle architecture; Nordic hamstring exercise.

Graphical abstract

Fig. 1

Timeline of the study. Illustration of the key measurement phases and the 9-week Nordic hamstring exercise (NHE) training program, followed by a 3-week detraining period. reps = repetitions.

Fig. 2

Multiscale assessment of BFlh muscle fascicle length and sarcomere length. (A) Experimental configuration for BFlh fascicle and sarcomere assessment. The scanning region for 3D ultrasound fascicle imaging of the BFlh covers 20%–80% of the distance between the fibular head and the ischial tuberosity, with 30% and 50% landmarks denoting the insertion sites for sarcomere imaging. (B) BFlh architecture extracted from a 3D ultrasound scan. Solid lines with 6 points along each fascicle represent fascicle length measures in the central and distal regions. Sarcomere insertion sites at 30% and 50% are indicated by the needle probe icon. (C) Sarcomere image collected with second harmonic generation microendoscopy.^, The boxed area represents the region of interest where Fourier transform analysis was applied to estimate sarcomere lengths. BFlh = biceps femoris long-head.

Fig. 3

(A) BFlh muscle fascicle length substantially increased after NHE training, later decreasing during detraining but remaining longer than pre-training (F_,415 = 442, ${\eta }_{\mathrm{p}}^2$ = 0.68, p < 0.001). Fascicle length was longer in the central region compared to the distal region (F_1,415 = 871, ${\eta }_{\mathrm{p}}^2$ = 0.68, p < 0.001). (B) Temporal progression of fascicle length revealed distinct region-specific adaptations (F_2,415 = 10, ${\eta }_{\mathrm{p}}^2$ = 0.05, p < 0.001) with no significant changes in the central region during the initial 3 weeks of training, followed by a substantial increase throughout the training period, whereas the distal region consistently displayed an increase in fascicle length throughout NHE training. (C) Passive sarcomere length shows no significant changes post-training and conservation after detraining (F_1,254 = 3, ${\eta }_{\mathrm{p}}^2$ = 0.01, p = 0.083). Inherent nonuniformity among sarcomere lengths was present (F_1,257 = 5, p = 0.027, ${\eta }_{\mathrm{p}}^2$ = 0.02), with varied adaptations between regions with training (F_1,256 = 6, ${\eta }_{\mathrm{p}}^2$ = 0.02, p = 0.017). (D) NHE training program stimulated sarcomerogenesis (F_1,19 = 105, ${\eta }_{\mathrm{p}}^2$ = 0.85, p <0.001) with a slight decrease in sarcomere number after detraining. There were more serial sarcomeres in the central region (F_1,19 = 56, ${\eta }_{\mathrm{p}}^2$ = 0.75, p < 0.001) but no significant interaction effect (F_1,19 = 1, ${\eta }_{\mathrm{p}}^2$ = 0.05, p = 0.339). (E) BFlh hypertrophy with NHE training (F_2,20 = 20, ${\eta }_{\mathrm{p}}^2$ = 0.66, p < 0.001), maintained during detraining. Muscle volume has been normalized to height squared. (F) Increased eccentric strength occurred with training (F_2,129 = 113, ${\eta }_{\mathrm{p}}^2$ = 0.64, p <0.001), remaining higher than pre-training levels even with detraining. Knee flexor torque was normalized to body mass. The main effects summarized in brackets include F-statistic values, degrees of freedom, p values, and effect size (${\eta }_{\mathrm{p}}^2$) categorized as small (0.01), medium (0.06), and large (0.14). ** p < 0.01, *** p < 0.001 indicate significant differences between pre-training and post-training. ^##p < 0.01, ^###p < 0.001 indicate significant differences between post-training and detraining. Colored dots represent individual participant averages. BFlh = biceps femoris long-head; NHE = Nordic hamstring exercise.