Time course of muscular, neural and tendinous adaptations to 23 day unilateral lower-limb suspension in young men

Abstract

Muscles and tendons are highly adaptive to changes in chronic loading, though little is known about the adaptative time course. We tested the hypothesis that, in response to unilateral lower limb suspension (ULLS), the magnitude of tendon mechanical adaptations would match or exceed those of skeletal muscle. Seventeen men (1.79 +/- 0.05 m, 76.6 +/- 10.3 kg, 22.3 +/- 3.8 years) underwent ULLS for 23 days (n = 9) or acted as controls (n = 8). Knee extensor (KE) torque, voluntary activation (VA), cross-sectional area (CSA) (by magnetic resonance imaging), vastus lateralis fascicle length (L(f)) and pennation angle (), patellar tendon stiffness and Young's modulus (by ultrasonography) were measured before, during and at the end of ULLS. After 14 and 23 days (i) KE torque decreased by 14.8 +/- 5.5% (P < 0.001) and 21.0 +/- 7.1% (P < 0.001), respectively; (ii) VA did not change; (iii) KE CSA decreased by 5.2 +/- 0.7% (P < 0.001) and 10.0 +/- 2.0% (P < 0.001), respectively; L(f) decreased by 5.9% (n.s.) and 7.7% (P < 0.05), respectively, and by 3.2% (P < 0.05) and 7.6% (P < 0.01); (iv) tendon stiffness decreased by 9.8 +/- 8.2% (P < 0.05) and 29.3 +/- 11.5% (P < 0.005), respectively, and Young's modulus by 9.2 +/- 8.2% (P < 0.05) and 30.1 +/- 11.9% (P < 0.01), respectively, with no changes in the controls. Hence, ULLS induces rapid losses of KE muscle size, architecture and function, but not in neural drive. Significant deterioration in tendon mechanical properties also occurs within 2 weeks, exacerbating in the third week of ULLS. Rehabilitation to limit muscle and tendon deterioration should probably start within 2 weeks of unloading.

Figure 1

Typical knee extensor torque–patellar tendon elongation relationship for one individual at baseline (•), 14 days (□) and 23 days (○) of suspension

Figure 2

Relative decrease (%) at 14 and 23 days of suspension for m. quadriceps femoris ACSA (s), patellar tendon stiffness (•), patellar tendon Young's modulus (○) and torque (♦) Data are displayed as means ±s.e.m. *Significantly different from baseline (P < 0.05); **significantly different from baseline (P < 0.01); †significantly different from 14 days (P < 0.05); ††significantly different from 14 days (P < 0.01). Lines connecting the tendon stiffness and Young's modulus data points are added for clarity.

Figure 3

VL muscle fascicle length and pennation angle at baseline (filled bars), and 14 days (dashed bars) and 23 days (open bars) of suspension *Significantly different from baseline (P < 0.05); **significantly different from baseline (P < 0.01).

Figure 4

Patellar tendon force–elongation relationships at baseline (•), and after 14 days (□) and 23 days (○) of suspension Data are displayed as mean and s.e.m.

Figure 5

Patellar tendon stress–strain relationships at baseline (•) and after 14 days (□) and 23 days (○) of suspension Data are displayed as means and s.e.m.