Critical speed in the rat: implications for hindlimb muscle blood flow distribution and fibre recruitment

Abstract

Critical speed (CS) constitutes an important metabolic and performance demarcator. However, active skeletal muscle blood flow distribution specifically surrounding CS remains unknown. We tested the hypotheses that CS could be accurately determined in the running rat and that measurement of hindlimb inter- and intramuscular blood flow below and above CS would support that the greatest muscle fibre recruitment above, relative to below, CS occurs in the predominantly glycolytic muscles. Seven male Sprague-Dawley rats performed five constant-speed tests to exhaustion at speeds between 95 and 115% of the speed that elicited to determine CS. Subsequent constant-speed tests were performed at speeds incrementally surrounding CS to determine time to exhaustion, V(O2), and hindlimb muscle blood flow distribution. Speed and time to exhaustion conformed to a hyperbolic relationship (r(2) = 0.92 ± 0.03) which corresponded to a linear 1/time function (r(2) = 0.93 ± 0.02) with a CS of 48.6 ± 1.0 m min(-1). Time to exhaustion below CS was ∼ 5× greater (P < 0.01) than that above. Below CS V(O2) stabilized at a submaximal value (58.5 ± 2.5 ml kg(-1) min(-1)) whereas above CS (81.7 ± 2.5 ml kg(-1) min(-1)) increased to (84.0 ± 1.8 ml kg(-1) min(-1), P > 0.05 vs. above CS). The 11 individual muscles or muscle parts that evidenced the greatest blood flow increases above, relative to below, CS were composed of ≥ 69% Type IIb/d/x muscle fibres. Moreover, there was a significant correlation (P < 0.05, r = 0.42) between the increased blood flow above expressed relative to below CS and the percentage Type IIb/d/x fibres found in the individual muscles or muscle parts. These data validate the powerful CS construct in the rat and identify that running above CS, relative to below CS, incurs disproportionate blood flow increases (indicative of recruitment) in predominantly highly glycolytic muscle fibres.

Figure 1. Critical speed (CS) and W′ (model parameter representing a finite work capacity above CS) determined for the same representative rat using three different models

A, hyperbolic model; B, linear distance-time model; C, linear 1/time model.

Figure 2. Individual times to exhaustion during constant speed tests above (56.1 ± 1.1 m min⁻¹) and below (41.3 ± 1.8 m min⁻¹) critical speed (CS)

Inset, average time to exhaustion. *P < 0.05 versus below CS.

Figure 3. Whole body oxygen uptake () measured during pre- and post-protocol maximal exercise tests and constant speed tests at speeds above and below critical speed (CS)

*P < 0.05 versus all other tests.

Figure 4

Blood flows (A) and the relative increases in blood flow (B, calculated as percent of below critical speed (CS), s.e.m. bars for individual muscles removed for clarity) to the total hindlimb musculature and representative individual muscles spanning the range of muscle fibre-type composition (according to Delp & Duan, 1996) measured at speeds above (56.1 ± 1.1 m min⁻¹) and below (41.3 ± 1.8 m min⁻¹) critical speed (CS, 48.5 ± 0.8 m min⁻¹). Vastus R; red portion of the vastus lateralis (65% Type I/IIa), Soleus (91% Type I/IIa), Semimem W; white portion of the semimembranosus (100% Type IIb/d/x), Vastus W; white portion of the vastus lateralis (100% Type IIb/d/x). *P < 0.05 versus below CS. Note disproportionate increase in blood flow in these latter two muscles relative to below CS.

Figure 5. Relationship between the blood flow measured above (56.1 ± 1.1 m min⁻¹) relative to below (41.3 ± 1.8 m min⁻¹) CS and the percentage of Type IIb/d/x fibres found in the individual muscles and muscle parts of the rat hindlimb

Figure 6. Total hindlimb blood flow measured above and below critical speed (CS) in the present study with comparison to an average value from data published at a lower speed (Copp et al. 2010a,;)

Note the disproportionately high blood flow found above CS versus that predicted from the sub-CS measurements. The data from Armstrong & Laughlin (1985) support that the blood flow values above CS are close to maximal for the rat hindlimb.