The effects of asymmetric length trajectories on the initial mechanical efficiency of mouse soleus muscles

Abstract

SUMMARYAsymmetric cycles with more than half of the cycle spent shortening enhance the mechanical power output of muscle during flight and vocalisation. However, strategies that enhance muscle mechanical power output often compromise efficiency. In order to establish whether a trade-off necessarily exists between power and efficiency, we investigated the effects of asymmetric muscle length trajectories on the maximal mechanical cycle-average power output and initial mechanical efficiency (E i ). Work and heat were measured in vitro in a mouse soleus muscle undergoing contraction cycles with 25% (Saw25%), 50% (Saw50%) and 75% (Saw75%) of the cycles spent shortening. Cycle-average power output tended to increase with the proportion of the cycle spent shortening at a given frequency. Maximum cycle-average power output was 102.9±7.6Wkg -1 for Saw75% cycles at 5Hz. E i was very similar for Saw50% and Saw75% cycles at all frequencies (approximately 0.27 at 5Hz). Saw25% cycles had E i values similar to those of Saw50% and Saw75% cycles at 1Hz (approximately 0.20), but were much less efficient at 5Hz (0.08±0.03). The lower initial mechanical efficiency of Saw25% cycles at higher frequencies suggests that initial mechanical efficiency is reduced if the time available for force generation and relaxation during shortening is insufficient. The similar initial mechanical efficiency of Saw50% and Saw75% cycles at all frequencies shows that increasing the proportion of the contraction cycle spent shortening is a strategy that allows an animal to increase muscle mechanical power output without compromising initial mechanical efficiency.

The energetic benefits of tendon springs in running: is the reduction of muscle work important?

“…The use of regional heat measurements to estimate whole muscle energy consumption (e.g. Hill, 1949;Woledge et al, 1985;Barclay et al, 1993;Barclay and Weber, 2004;Holt and Askew, 2012) is dependent on the assumption of uniform heat production throughout the muscle. It has been hypothesized that non-uniform heat production could occur because of non-uniformity in sarcomere strain arising during active stretch and/or relaxation (Hill, 1949;Linari et al, 2003;Woledge et al, 1985) and uneven distribution of strain across entire muscles (Hill, 1949;Ahn et al, 2003;Carr et al, 2011).…”

“…These procedures have been reported in greater detail previously (Holt and Askew, 2012). Briefly, muscles were suspended in a Perspex flow-through muscle chamber circulated with oxygenated Ringer's solution at 20°C.…”

“…Procedures for myothermic measurements have been reported previously in greater detail (Holt and Askew, 2012). Briefly, muscles were placed in a thermopile chamber (20°C) and positioned so they were sitting over the active region of an antimony-bismuth thermopile (4 mm long, containing 16 thermocouples with a Seebeck coefficient of 56.8 μV K −1 couple −1 ).…”

Stuck in gear: age-related loss of variable gearing in skeletal muscle

“…An isometric tetanus was performed to determine peak isometric force (F 0 ) (Holt and Azizi, 2014). After-loaded isotonic tetanic contractions (Askew and Marsh, 1997;Holt and Askew, 2012) (Fig. 2) were performed at a range of forces (0.1 to 0.9 F 0 ) to determine the relationships between force and muscle velocity, force and fiber velocity, and force and gear ratio (muscle velocity/ fiber velocity) (Azizi et al, 2008).…”

Muscle function during takeoff and landing flight in the pigeon (Columba livia)

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