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Twisting and Bending: The Functional Role of Salamander Lateral Hypaxial Musculature During Locomotion

Wallace O. Bennett, Rachel S. Simons^* and Elizabeth L. Brainerd

Department of Biology and Graduate Program in Organismic and Evolutionary Biology, University of Massachusetts Amherst, 611 North Pleasant Road, Amherst, MA 01003-9297, USA
^* Present address: Department of Biology, Monroe Community College, Rochester, NY 14623, USA

View larger version (56K): [in a new window]   Fig. 1. Left lateral view of the hypaxial musculature of an adult Ambystoma tigrinum. Muscle fibers in the lateral pair (OES and OEP) slope from craniodorsal towards caudoventral, whereas muscle fibers in the medial pair (OI and TA) slope from cranioventral towards caudodorsal. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis; RA, m. rectus abdominis. The figure is taken from Simons and Brainerd (Simons and Brainerd, 1999) (with permission). Copyright 1999 Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.

View larger version (23K): [in a new window]   Fig. 2. Schematic representation of two hypothetical functions of the lateral hypaxial muscles during locomotion (after Carrier, 1993). The first hypothesis is that the hypaxial muscles function to bend the body during swimming (A) and walking (B) through the synchronous action of the lateral and medial pairs of hypaxial musculature. A second hypothesis (C) is that the hypaxial muscles function to counteract long-axis torsion of the body during walking. The torsion hypothesis predicts that the lateral and medial pairs of hypaxial muscles will be active asynchronously on one side of the body during walking. Filled bars represent bursts of muscle activity recorded from the lateral and medial hypaxial muscle pairs on one side of the body. Darkened feet in (C) mark the limbs generating the torsional moments countered by the indicated muscle activity. *Denotes the side of the body with hypothetical muscle activity.

View larger version (11K): [in a new window]   Fig. 3. Torsion control during terrestrial locomotion in salamanders. During walking, the ground reaction forces directed through the diagonally opposed limbs cause the pectoral and pelvic girdles to rotate (curved arrows over the trunk). The oblique orientations of the lateral hypaxial muscles are hypothesized to prevent torsion caused by the limbs. In this example, the medial muscles (open arrow, m. obliquus internus and m. transversus abdominis) on the salamander’s right side are active to counteract torsion caused by the hindlimb, while the lateral pair (filled arrow, m. obliquus externus superficialis and m. obliquus externus profundus) of hypaxial muscles on the left side of the trunk is active to counteract the torsion caused by the forelimb (figure modified from Carrier, 1993).

View larger version (32K): [in a new window]   Fig. 4. Representative electromyographic (EMG) activity from the lateral hypaxial muscles of Ambystoma tigrinum during swimming. (A) An example of locomotor strides with only the -bursts present in the TA, OEP and OES muscles. (B) Electromyographic activity recorded from the same individual as A with - and ß-bursts present in the OI and OES. A new locomotor cycle begins at each arrow. Filled and open bars represent our scoring of the - and ß-bursts, respectively, for the corresponding traces. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis.

View larger version (31K): [in a new window]   Fig. 5. Representative electromyographic (EMG) activity from the lateral hypaxial muscles of Ambystoma tigrinum during walking. (A) EMG activity in which - and ß-bursts occur together within a single walking stride. (B) EMG activity recorded from the same individual in which - and ß-bursts are not separated by silent periods during two strides (continuous bursts). A new locomotor cycle begins at each arrow. Filled and open bars represent our scoring of the of - and ß-bursts, respectively, for the corresponding traces. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis.

View larger version (27K): [in a new window]   Fig. 6. Bar chart of - and ß-burst intensities from the lateral hypaxial muscles of one tiger salamander (individual A) during walking and swimming. For this individual, the intensity of the -bursts is significantly higher than the intensity of the ß-bursts in all four lateral hypaxial muscles during both swimming and walking (paired t-test, P<0.0001). The muscle burst intensity was measured as the rectified integrated area divided by the duration of the burst. Filled and open columns represent the mean burst intensity + S.E.M. of the - and ß-bursts, respectively.

View larger version (21K): [in a new window]   Fig. 7. Summary diagram showing the onset and duration of lateral hypaxial muscle EMG activity relative to maximum body bending in two Ambystoma tigrinum during swimming. Values are means ± S.D. for -burst (filled bars) and ß-burst (open bars) onset and duration times, which are calculated from values normalized to locomotor cycle duration. -bursts occur during every swimming cycle (obligate), whereas ß-bursts are more variable and do not necessarily occur in every swimming cycle (facultative). The S.D. of the normalized time of maximum opposite bending is plotted on a point in the center of the body. Sample sizes (N): individual A, TA=52, TAß=20, OI=52, OIß=40, OEP=52, OEPß=26, OES=52, OESß=3; individual B, TA=21, TAß=21, OI=20, OEP=21, OEPß=4, OES=21, OESß=8. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis. *Denotes the side of the body on which the electrodes were implanted.

View larger version (27K): [in a new window]   Fig. 8. Summary diagram showing the onset and duration of lateral hypaxial muscle activity relative to maximum body bending and footfall pattern in three Ambystoma tigrinum during walking. Means and standard deviations for -bursts (filled bars), ß-bursts (open bars) and limb onset and offset times are calculated from values normalized to stride duration. -bursts occur during every locomotor cycle, whereas ß-bursts are facultative. The S.D. of the normalized time of maximum opposite bending is plotted on a point in the center of the body. LH, left hindlimb; LF, left forelimb; RH, right hindlimb; RF, right forelimb. Sample sizes (N): individual A, TA=28, TAß=25, OI=27, OIß=27, OEP=28, OEPß=23, OES=26, OESß=26; individual B, TA=30, TAß=28, OI=31, OIß=31, OEP=21, OEPß=27, OES=31, OESß=31; Individual C, TA=37, TAß=6, OI=38, OIß=28, OES=38, OESß=10. OES, m. obliquus externus superficialis; OEP, m. obliquus externus profundus; OI, m. obliquus internus; TA, m. transversus abdominis. *Denotes the side of the body on which the electrodes were implanted.

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