The movement of animals has always been of interest to researchers. Movement may be defined by description, based upon observation and dissection, and investigations into the function of structures observed. Domestic turkeys (Meleagris gallapavo) are one of a number of species of birds that develop extensive mineralisation primarily within their crural pelvic limb tendons. This circumstance provides a novel opportunity to study the in vivo force dynamics behind bipedal gait.
Turkeys underwent surgery for implantation of strain gauges onto selected mineralised tendons in the crural region (fibularis longus, medial and lateral heads of gastrocnemius, tibialis cranialis). Post-operatively, the birds were videoed (from a lateral view to the direction of travel) moving on the treadmill at predetermined speeds (between 0.26 and 2.9 m.s-1), under both level and inclined conditions. Simultaneously, in vivo strain data was collected. Upon completion of the experiment, birds were euthanased and pelvic limbs were removed at the hip joint for morphological and morphometrical analysis.
All morphological findings were in agreement with previous literature. A complete data set on the morphometrical measures of twelve crural turkey pelvic limb muscle-tendon complexes was collated, and related to body mass. Muscle masses scaled isometrically with body mass (M)0.93, however bone limb lengths scaled with negative allometry from (M)0.108 to (M)0.126 Muscle fibre lengths showed no correlation to body mass. The deposition of mineral within the tendon, may limits the extensibility of the tendon. Despite the increase in length seen in tendon with age (and increasing body mass), the increased tendon stiffness may offset the increase in length, resulting in a similar extension under a given load irrespective of body mass. Muscle area scaled to a greater exponent than for geometric similarity ((M)0.87), while tendon area scaled at (M)0.40 Uneven development of the muscle-tendon system occurs with growth resulting in high cross-sectional area ratios and low safety factors. It was thought that mineralisation levels may affect this, however a similar relationship was found. This may result in higher tendon strain levels (from greater muscle stress), causing adaptation of the mechanical properties of the tendon.
Changes in the mechanical properties of tendon were observed with mineralised tendons becoming stiffer (increased Young's modulus from 0.09 - 7.80 GPa), as their densities increased from 1.07 g.cm-3 to 1.86 g.cm-3. Ultimate strength values for mineralised tendon were not significantly different to non-mineralised tendon.
The kinematics of steady state turkey locomotion were analysed for joints below the knee. The parameters of duty factor, stride length, step length, stride frequency, stride duration, stance duration and swing duration were measured. Duty factor decreased (from 0.73 to 0.55) in a linear fashion with increasing speed (from 0.59 to 2.4 m.s-1). This was reflected by a decreasing stride and stance duration, while the swing duration remained fairly constant over the speed range, suggesting that the swing phase is performed at an optimal efficiency level, independent of speed. Turkeys increase their speed of locomotion by moving their limbs faster, and farther for speeds below 2.5m.s-1 , but not with greater excursion of joints of the distal limb.
Kinetic measures revealed that gastrocnemius (medial and lateral heads) and fibularis longus are ankle extensors (plantarflexors) (active throughout stance), whilst tibialis cranialis showed force levels during swing when ankle flexion is required. No relationship was observed between speed and tendon strain (~force) level over horizontal terrain. At the maximal speed observed, the magnitude of force was only 20% of the maximal isometric muscle force. This suggests that low levels of muscle contraction are required to support high speed steady state locomotion, allowing the animal to utilise the large amount of potential muscle power for highly accelerative movements (e.g. for take off, or acceleration phases in running). Force-sharing was shown to occur between the ankle extensors during stance.
The only kinematic parameter observed to be different, due to locomotion on a 10 degree incline, was relative stride frequency, where turkeys on an incline achieve increases in speed by increasing their stride frequency at a greater rate than their increases in stride length. Kinetic analysis revealed that the strain levels did not increase substantially. This is thought to be due to a change in the contraction type of the muscle (from isometric to concentric contraction).
This study helps to detail the locomotor behaviour of the cursorial bird, Meleagris gallapavo, and delves further into the dynamics behind bipedal gait.