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  • Author or Editor: James Wakeling x
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Abstract

Vertebrate skeletal muscles act across joints and produce segmental accelerations and therefore animal movement when they contract. Different muscles and different motor units vary in their mechanical contractile properties. Early studies on motor unit recruitment demonstrated orderly recruitment of motor units from the slowest to the fastest during a graded contraction. However, many subsequent studies illustrate conditions when alternative recruitment strategies may exist. Motor unit recruitment during locomotion is thus multifactorial and more complex than typically thought.Different types of motor unit vary in their mechanical properties, including rates of force activation and deactivation, maximum unloaded shortening velocities and the shortening velocities at which maximum mechanical power output and maximum mechanical efficiency occur. In short, it would make mechanical sense to perform fast activities with the faster motor units and slow activities with the slower motor units. However, determining patterns of motor unit recruitment during locomotion has presented experimental challenges.Comparisons between distinct muscles have shown that fast fish swimming and the cat paw shake are activities which employ predominantly the fast and not the slower muscle. Glycogen depletion studies have showed that jumping in the bushbaby uses fast without slow motor units within the vastus lateralis and gastrocnemius muscles. Studies in man show that differential recruitment of the different types of muscle fibre occurs at different times within each running stride. It is suggested that vertebrates may have a strategy of recruiting the motor units that are most mechanically suited for the different locomotor demands. However, we have much to learn about motor recruitment patterns during locomotion.

In: Animal Biology

Abstract

This study aimed to determine whether training aids (side reins and a Pessoa) increased the use of the longissimus dorsi when horses are being lunged. Horses were lunged on a circle under four different conditions on the left and right reins in walk and trot, and electromyographic (EMG) and speed measurements were taken using surface EMG at T16 and GPS, respectively. The EMG intensity was substantially greater for the longissimus dorsi on the inside of the circle. Differences occurred in both the timing and the intensity of the EMG between the conditions. At walk, the EMG intensity was the greatest for the control condition and at trot the EMG intensity was the greatest for the control and Pessoa control conditions. It is concluded that the training aids of side reins and a Pessoa do not increase the use of the longissimus dorsi to stabilise the back.

In: Comparative Exercise Physiology

Abstract

Abstract Muscle function depends in part on the interplay between its activity and its length within the stretch-shortening cycle. The longissimus dorsi is a large epaxial muscle running along the thoracic and lumbar regions of the equine back. Due to its anatomical positioning, the longissimus dorsi has the capability of contributing to many functions: developing bending moments in the dorsoventral and lateral (coupled to axial rotation) directions and also providing stiffness to limit motion in these directions. We hypothesize that the exact function of the longissimus dorsi will vary along the back and between gaits as the relation between activity and motion of the back changes. Electromyograms (EMG) were recorded at walk (inclined and level) and trot (on the level) on a treadmill from the longissimus dorsi at muscle segments T14, T16, T18 and L2. Back motion was additionally measured using a fibre-optic goniometer. Co-contractions of the muscle between its left and right sides were quantified using correlation analysis. A greater dominance of unilateral activity was found at more cranial segments and for level walking, suggesting a greater role of the longissimus dorsi in developing lateral bending moments. Timing of the EMG varied between muscle segments relative to the gait cycle, the locomotor condition tested and the flexion-extension cycle of the back. This supports the hypothesis that the function of the longissimus dorsi changes along the back and between gaits.

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In: Comparative Exercise Physiology