Blood Flow Response to Muscle Contractions Not Determined by Work Performed

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The magnitude of the blood flow response to exercise has been linked to both the work performed and the metabolic cost of the activity. Although these two factors are generally related, they may be dissociated under certain conditions. For example, when total contraction time is kept constant, trains of short contractions have a higher energy cost than trains of longer contractions. PURPOSE: This study examined the blood flow response to trains of contractions when contraction duration was manipulated under conditions of similar tension-time indices (isometric analog of work). The hypothesis was that the blood flow response would be greater to a train of short duration contractions than a train of long duration contractions. METHODS: Canine gastrocnemius-plantaris muscle (n = 8) was isolated and blood flow assessed with an ultrasound flow probe placed around the popliteal artery. The sciatic nerve was stimulated to produce two contraction protocols that resulted in similar contraction/rest ratios: short duration: 0.25 s/0.75 s vs. long duration: 1 s/3 s. RESULTS: In accord with the design of the experiment, the tension-time indices were identical for the two contraction protocols (short: 15.3 ± 0.8 vs long: 15.3 ± 0.8 kN • s). Steady-state VO2 was greater in the short duration contractions (17.2 ± 0.9 ml/100g/min) than in the long duration contractions (11.7 ± 0.7 ml/100g/min). Similarly, the steady-state blood flow was greater in contractions of short duration (124 ± 9 ml/min) compared to long duration contractions (93 ± 8 ml/min). CONCLUSIONS: Contractions of short duration resulted in significantly higher oxygen consumptions and blood flows compared to contractions of long duration despite the same total work. The blood flow response to muscle contraction appears to be more closely associated with energy expenditure than work performed. Supported by NHLBI and VA.