Adel Abdullah Alhusaini

Chapter contents

Methodology

All studies were approved through both the Hospital Ethics Committee of The Children’s Hospital at Westmead, Sydney, Australia and the Human Research Ethics Committee of the University of Sydney. Participants were recruited through the Child Assessment Centre in The Children’s Hospital at Westmead and through open advertisement for typically developing children. All participants were aged between 4 and 10 years. Participants were consecutively selected based on inclusion and exclusion criteria, and their willingness to participate. Full details of the participants, including inclusion and exclusion criteria, demographic detail and clinical status are given for each study in the relevant publication.

A specially constructed ankle measurement device, similar to that described by Moseley et al. (2001) was used for this aspect of the studies (Fig. B.1). It consisted of a footplate hinged to a support bracket for the lower leg, with a rotary potentiometer (Model 157, RS Australia, Sydney) aligned with the lateral malleolus. The footplate and axis of rotation were adjustable to match the dimensions of the child. A 450 N load cell (XTRAN S1W, Applied Measurement Australia Pty, Oakleigh) was attached perpendicular to the footplate to measure resistance to movement. A handle was attached to allow manual oscillation of the footplate.

The angle signals were checked and calibrated by holding the footplate in a number of different static positions using rigid struts throughout the arc of motion and recording the electrical signal obtained from the potentiometer. The process was repeated five times. The calibration regression equation was consistent in each repetition and the correlation coefficient for angle to signal was 0.9998. In a similar fashion, the load cell was repeatedly calibrated over its relevant range through the application of a number of different known weights and recording of the output signal. Once again, the linear regression equation was identical in each set of tests and the correlation coefficient for force to signal was 0.9999.

High inter-rater and intra-rater reliability has previously been demonstrated for the measurement of passive torque and ankle displacement using similar techniques (intraclass correlation coefficient (ICC)>0.86) (Chesworth and Vandervoort, 1988) and the procedure has been shown to be highly responsive to change in soft tissue stiffness characteristics (Sinkjaer et al., 1988).

Applied torque values were calculated from the product of applied force and the perpendicular distance from the point of application of the force to the axis of rotation of the footplate. To correct for the effect of the footplate weight, a calibration equation was developed in which the torque due to the footplate was computed as a function of the angle of the footplate and its weight. This torque, which was positive or negative according to the footplate position, was added to the applied torque.

The load transducer measured uniaxial loading. This was consistent with the monoplanar rotation of the ankle and the application of force by means of a rod-like handle minimized any risk of ‘off-perpendicular’ force application. Torque and angle were sampled at 125 Hz.

Each child lay supine with the foot placed in the ankle apparatus and positioned, by visual approximation, such that the point midway between the lateral and medial malleolus in the sagittal plane was aligned with the axis of rotation of the device. The participant’s foot was secured to the footplate with Velcro® straps, the knee was placed in an extended position, and light pressure was applied by the researcher’s hand above the knee over the thigh to ensure that knee position was maintained. The calf was free of contact and clear of all surfaces and structures. In the children with CP, the affected or more affected leg was tested; in typically developing children, the leg tested was randomly selected. The researcher rotated the foot passively in a sinusoidal pattern around the ankle from full plantarflexion to full available dorsiflexion (determined visually by loss of heel contact). All children were instructed to keep their legs relaxed and to avoid assisting or resisting the motion during the sinusoidal rotation.

A warm-up and familiarization process, with two or three repetitions of full range movement of the rig, was applied before recording the data. For each child, a minimum of 10 passive stretch cycles were applied at a target frequency of 0.5 Hz. This sequence was repeated two more times to ensure relaxation and compliance on the part of the participant and to avoid the eliciting of reflex muscle activity. This allowed at least one full cycle of passive stretching and recovery for analysis. Joint displacements into dorsiflexion were taken as positive and those in the plantarflexion direction as negative. An ankle angle of 90° (plantigrade) was considered to be the neutral position and defined as zero.

Electromyographic (EMG) activity of the soleus, medial gastrocnemius and tibialis anterior muscles was recorded simultaneously with ankle rotation, using a telemetric 16-channel EMG unit (Telemyo 2400 R G2 system, Noraxon, Arizona) with a sampling rate of 3000 Hz. We used disposable, self-adhesive Ag/AgCl bipolar surface electrodes (Kendall Medi-Trace Mini 130 Foam ECG Electrodes, Neurotronics, Randwick, NSW), placing pairs of electrodes parallel with the muscle fibre direction with minimal inter-electrode distance. Before electrode placement, we cleaned the skin with isopropyl alcohol. The location of electrodes was based on contemporary recommendations (Hermens et al., 2000). EMG acquisition enabled monitoring of muscle activity during the test.

We collected force and angle data simultaneously at a frequency of 125 Hz using a 16-bit analogue-to-digital converter (DAQCard-6036E, National Instruments, Austin, Texas). The application software (PhysioDAQXS version 3.0, the University of Sydney) consisted of a graphical user interface designed using Borland C++builder. Access to the data was gained by using National Instruments’ call-back functions to retrieve data collected by the data acquisition card. The graphical user interface supports the collection, display and storage of data in real time. The repeatability and linearity of the force and angle signals were confirmed before data collection.

The child’s compliance with the instruction to keep the leg relaxed and to avoid assisting or resisting the motion was confirmed by the absence of EMG activity (Spike2 software version 2.09, Cambridge Electronic Design, Cambridge). During analysis of data in those studies in which passive mechanics were investigated, one complete loading and unloading cycle without evidence of EMG activity was chosen for analysis, ensuring that there was no reflex contribution to total stiffness. In the event of more than one cycle meeting this criterion, selection was random. Specific details of analytic conventions are given for each study in the relevant publication.

Torque values were extracted from the data at predetermined dorsiflexion angles (0°, 5° and 10°; Fig. B.2, points A, B and C, respectively). These provided an indication of the torque necessary to displace the ankle to comparable positions for each group tested. Stiffness was computed as the slope of the line between 0° and 5° of dorsiflexion. Hysteresis, expressing the energy absorbed by the muscle–tendon unit during the loading/unloading cycle, was calculated as the area enclosed between the loading and unloading curves of applied torque against ankle angle.

Study 1: Non-reflex stiffness in children with CP

This study revealed that the plantarflexors of the children with CP were much less mechanically compliant than an age-matched group of typically developing children (Alhusaini et al., 2010a). Torques at predetermined joint angles, ankle stiffness (slope of curve) and hysteresis were almost three times as great in the children with CP compared with the typically developing group (p<0.001).

These findings are important, particularly as we know that non-reflex stiffness in children with CP has not been described or quantified, nor have the effects been recognized of adaptive changes such as decreased muscle extensibility on motor development (Lambertz et al., 2003). This knowledge, therefore, has the potential to improve clinical practice by furthering what is known about the changes in the muscle’s intrinsic properties and broadening physiotherapists’ understanding about the effects of such changes in these children.

The attempt was made to expose the calf muscle to conditions similar to those found during normal gait by using a sinusoidal movement that was similar in angular velocity and range of motion to that encountered during the stance phase of walking (Ada et al., 1998; Becher et al., 1998), while also avoiding stimulation of reflex hyperactivity in the calf muscles. Although this criterion was not always easy to achieve, and the responses of the children did vary in their ability to relax, it was possible to produce at least one complete cycle in each test during which EMG activity was silent. This situation, therefore, fulfilled the requirement for measurement of the passive myotendinous characteristics. Torque–angle curves at velocities up to 70 degrees/second show the characteristic shape of tissue stretch only (van der Salm et al., 2005). In a study of adults exhibiting spastic hypertonia, it was reported that, even at stretch velocities of 120 degrees/second, only four of 15 people demonstrated reflex responses in the triceps surae (Rabita et al., 2005). The sinusoidal movement speeds used in the experiments were felt to be adequate to measure the passive myotendinous stiffness parameters while not eliciting simultaneous reflex activity. Although the oscillation frequency was significantly different between groups, the children with CP, who were more likely to respond with hyper-reflexia, were stretched at the slightly lower rate, consistent with the subjective need to maintain a relaxed, passive movement.