Skeletal muscle elastic modulus in marathon distance runners

Abstract

Skeletal muscle shear elastic modulus is a non-invasive surrogate for early detection of muscle damage and soreness consequent to unaccustomed eccentric muscle work. We investigated the influence of marathon distance running on skeletal muscle shear elastic modulus. Shear modulus of the rectus femoris was measured via ultrasound shear wave elastography in 80 participants (30 female, 50 male) before and after running a World Marathon Major. Experience level, muscle soreness and run readiness were surveyed. Pre-marathon shear elastic modulus was lower in competitive vs recreational runners (13.0 ± 4.6 vs 15.6 ± 5.6 kPa; P = 0.0014), lower for fastest vs slowest finish times (11.6 ± 3.0 vs 16.9 ± 6.5 kPa; P = < 0.0001) and associated with marathon finish time (r = - 0.40; P < 0.0003). Marathon running increased shear modulus (~ 23%), irrespective of experience, sex or course, but was blunted in runners wearing highly cushioned footwear with plates who had matched finish times (Other = ~ 31% vs Vaporfly = ~ 17%). Muscle soreness was strongly associated with run readiness (R² = 0.995; P = 0.0026), and marathon recovery time was longer in runners with greater increases in marathon-mediated shear modulus. Skeletal muscle elastic modulus may reflect both short- and long-term muscle adaptation as a function of marathon running, or factors occurring before or after a marathon itself, such as exercise capacity or recovery time. These data are consistent with marathon-mediated muscle damage and soreness, but equally highlight a possibility to monitor and modulate outcomes in favor of a runner.

Keywords: Long distance running; Muscle stiffness; Shear wave elastography.

Fig. 1

Illustration showing ultrasonography of the quadriceps site on the rectus femoris (RF) with participant seated at 90° hip angle and 110° knee angle (A), the overlay of the shear wave elastography (SWE) application on the ultrasound image coupled to the measurement region of interest (ROI) (B), and example images with text-based description for improved visual context (C). VI = vastus intermedius

Fig. 2

Individual (circles) and average (bars) pre-marathon shear elastic modulus (kPa) in recreationally and competitively experienced marathoners (A), shear elastic modulus expressed as the line slope for the ratio between normalized strain and stress estimates for recreationally and competitively experienced marathoners (B), individual (circles) and average (bars) pre-marathon shear elastic modulus for auto-generated quartiles of average marathon speed presented from Slowest to Fastest (C), the correlation between marathon finish time and pre-marathon shear elastic modulus for all individuals (D). * and ** and *** all indicate significance at P < 0.05 but highlight the different group comparison. Table 1 contains N and characteristics for all participants/groupings

Fig. 3

Individual (circles) and average (bars) pre-marathon and post-marathon shear elastic modulus (kPa) for the entire cohort (A), recreationally and competitively experienced marathoners (B), Other and Vaporfly footwear groups across all runners (D), and Other and Vaporfly footwear groups matched for marathon finish time (E). Shear elastic modulus expressed as the line slope for the ratio between normalized strain and stress estimates for pre-marathon and post-marathon (C) and for post-marathon Other and post-marathon Vaporfly (F). ns = non-significant at P > 0.05. * indicate significance at P < 0.05 from pre-marathon to post-marathon. Table 1 contains N and characteristics for all participants/groupings

Fig. 4

Individual (circles) and average (bars) marathon recovery indices of muscle soreness (A) and run readiness (B) are plotted before (Pre) and 24-, 48-, and 72-h after marathon running. Average values for run readiness were correlated with muscle soreness via non-linear exponential regression (R² = 0.996) (C). Absolute individual (circles) and average (bars) changes in shear elastic modulus for each return-to-baseline recovery timepoint of 24-, 48-, or 72-h post marathon (D). * and ** indicate significance at P < 0.05 but highlight the different group comparison. Table 1 contains N and characteristics for all participants/groupings for A–C. Participant counts in D are as follows: N = 4/80 for 24 h, N = 7/80 for 48 h, and N = 13/80 for 72 h, equating to a total N = 24/80

Fig. 5

Summary graphic depicting the cycle of change in skeletal muscle shear elastic modulus to marathon distance running. The upper left quadrant depicts baseline difference in muscle shear modulus as function of pre-marathon exercise performance capacity, presumably from some combination of experience and exercise training. The upper right quadrant depicts how an eccentric dominant exercise insult or unaccustomed mechanical stress beyond present day tolerance (i.e. such as in a marathon race) is expected to acutely increase shear elastic modulus. The bottom right quadrant depicts how highly cushioned footwear with plates can blunt marathon-induced rises in muscle stiffness (an index of muscle damage). The bottom left quadrant depicts how muscle soreness recovery time is longer in marathoners who have greater changes in muscle stiffness the preceding running exercise bout, and that this has associative influence on the time a person can run again. Hypothetically, the cycle could continue with extended time and long-term adaptation, where muscle shear elastic modulus returns to baseline or continues to progress lower in kPa, depending on the future magnitude and frequency of insult (i.e. repeated endurance exercise training)