Kylie Tucker

Moving differently in pain: A new theory to explain the adaptation to pain

People move differently in pain. Although this statement is unquestioned, the underlying mechanisms are surprisingly poorly understood. Existing theories are relatively simplistic, and although their predictions are consistent with a range of experimental and clinical observations, there are many observations that cannot be adequately explained. New theories are required. Here, we seek to consider the motor adaptation to pain from the micro (single motoneuron) to macro (coordination of whole-muscle behaviour) levels and to provide a basis for a new theory to explain the motor changes in pain.

Motor Unit Recruitment Strategies Are Altered during Deep-Tissue Pain

Muscle pain is associated with decreased motor unit discharge rate during constant force contractions. As discharge rate is a determinant of force, other adaptations in strategy must explain force maintenance during pain. Our aim was to determine whether motor unit recruitment strategies are altered during pain to maintain force despite reduced discharge rate. Motor unit discharge behavior was recorded in two muscles, one with (quadriceps) and one without [flexor pollicis longus (FPL)] synergists. Motor units were recruited during matched low-force contractions with and without experimentally induced pain, and at higher force without pain. A total of 52 and 34 units were recorded in quadriceps and FPL, respectively, during low-force contractions with and without pain. Of these, 20 quadriceps and 9 FPL units were identified during both trials. The discharge rate of these units reduced during pain in both muscles [quadriceps: 8.7 (1.5) to 7.5 (1.3) Hz, p < 0.001; FPL: 11.9 (1.5) to 10.0 (1.7) Hz, p < 0.001]. All remaining units discharged only with or without pain, but not in both conditions. Only one-third of the additional units recruited during pain (quadriceps n = 7/19, FPL n = 3/15) were those expected given orderly recruitment of the motor unit pool as determined during higher-force contractions. We conclude that reduced motor unit discharge rate with pain is accompanied by changes in the population of units used to maintain force. The recruitment of new units is partly inconsistent with generalized inhibition of the motoneuron pool predicted by the “pain adaptation” theory, and provides the basis for a new mechanism of motor adaptation with pain.

Elastography for Muscle Biomechanics: Toward the Estimation of Individual Muscle Force

Estimation of individual muscle force remains one of the main challenges in biomechanics. This review presents a series of experiments that used ultrasound shear wave elastography to support the hypothesis that muscle stiffness is linearly related to both active and passive muscle forces. Examples of studies that used measurement of muscle stiffness to estimate changes in muscle force are presented.

Motoneurone recruitment is altered with pain induced in non-muscular tissue

Abstract Motoneurone discharge rate is reduced despite the maintenance of force when pain is induced via injection of hypertonic saline into muscle. Two aspects require consideration. First, hypertonic saline may have direct effects on axons other than small diameter pain fibres including the motoneurones that innervate the painful muscle. Second, it is unclear how force is maintained, when motoneurone discharge rate is decreased. We aimed to determine; (1) if motoneurone discharge rate is reduced during force‐matched tasks when pain is induced in non‐muscle tissue (to exclude direct effects on motoneurones) and (2) if the reduction of discharge rate is associated with additional changes in motoneurone recruitment over multiple muscle regions. Motoneurone discharge was recorded in the quadriceps with eight pairs of fine‐wire electrodes. Seven subjects performed 30‐s low‐level, force‐matched contractions before and during anterior knee pain, which was induced by a bolus (0.25 ml) injection of 5% hypertonic saline into the infra‐patellar fat pad. In total, 119 motor units were identified. Of these, 34 were identified both before and during pain. The discharge rate of these units decreased during pain from 8.9(1.5) to 7.2(1.4) Hz (P < 0.0001). In addition, 31 units were recruited in the no‐pain condition but not during pain, when 53 new units were recruited. These changes coincided with a large variability in gross muscle activity measures between muscle regions. These data confirm that motoneurone recruitment is altered when direct effects of saline on motoneurones are excluded. Recruitment of additional motor units may explain force maintenance despite reduced discharge rate of some units.

Changes in excitability of corticomotor inputs to the trunk muscles during experimentally-induced acute low back pain

Acute low back pain (LBP) is associated with differential changes in motor coordination of deep and superficial trunk muscles. Whether this is related to differential changes in excitability of descending corticomotor inputs remains unclear and was investigated in nine healthy individuals. Fine-wire i.m. electrodes were inserted bilaterally into deep (transversus abdominis (TrA)) and superficial abdominal muscles (obliquus externus abdominis (OE)), and surface electrodes were placed bilaterally over obliquus internus abdominis (OI), rectus abdominis (RA) and lumbar erector spinae (LES) muscles. Corticomotor excitability was assessed as amplitude of motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) at a range of stimulator intensities, at rest and during voluntary abdominal contractions. Pain was induced by injection of hypertonic saline into interspinous ligaments of the lumbar spine. Corticomotor excitability was examined before, during and after the induction of LBP. During pain, amplitude of TrA MEPs to contralateral cortical stimulation was reduced, whereas amplitudes of OE and LES MEPs contralateral and ipsilateral to the stimulated cortex were increased. The findings highlight differential changes in excitability of corticomotor inputs to trunk muscles during acute LBP. Further work is required to reveal whether such changes involve spinal and/or supraspinal centres and their consequence for spine control.

Changes in motor unit recruitment strategy during pain alters force direction

Motor unit (MU) recruitment is altered (decreased discharge rate and cessation of discharge in some units, and recruitment of new units) in force‐matched contractions during pain compared to contractions performed before pain. As MUs within a motoneurone pool have different force direction properties we hypothesised that altered MU recruitment during experimental knee pain would change the force vector (total force (FT): amplitude and angle) generated by the quadriceps. Force was produced at two levels during 1 × 60‐s and 3 × 10‐s isometric contractions of knee extensors, and recorded by two force transducers at right angles. This enabled calculation of both FE (extension force) and FT. MU recruitment was recorded from the medial and lateral vastii with four fine‐wire electrodes. Pain was induced by hypertonic saline injection in the infra‐patella fat pad. Nine subjects matched FE and six subjects also matched both medial and lateral forces (FT) before and during pain. Changes in MU discharge pattern (decreased discharge rate (P < 0.001), complete cessation of firing, and recruitment of new units) during pain were associated with a ∼5° change in absolute force angle. As force angle changed in both directions (left/right) for individual subjects with pain there was no change in average FT amplitude between conditions. When both medial and lateral forces were matched MU discharge rate decreased (P < 0.001) with pain, but, fewer units ceased firing or were newly recruited during pain. Change in motoneurone recruitment during pain alters direction of muscle force. This may be a strategy to avoid pain or protect the painful part.

Standardization of H-reflex analyses

Variability in the H-reflex can make it difficult to identify significant changes using traditional pooled analysis techniques. This study was undertaken to introduce a normalisation approach to calculate both the relative size and the relative stimulus intensity required to elicit the H-reflex response so that comparisons can be made not only with results obtained during different experimental session but also between different subjects. This normalisation process fits the size of the measured M-responses and H-reflexes over the entire stimulus range with model curves to better facilitate the calculation of important parameters. This approach allows normalisation of not only the size of the response but also the relative stimulus intensity required to elicit the response. This eases the comparison of the reflex responses under various situations, and is capable of bringing out any genuine differences in the reflex in a reliable manner not previously possible. This study illustrates that comparison of the reflex between days is problematic, even in the same subject, as both the reflex size and the relative stimulus intensity required to obtain this reflex changed in all subjects. We suggest that H-reflex studies need to use normalisation not only for size of the reflex but also for the stimulus intensity, and also that all experiments for a single subject should be performed in the same session or during the same day using some level of background muscle activity in the muscle concerned as the variability of the muscle at rest was found to be larger.

Muscle spindle feedback differs between the soleus and gastrocnemius in humans.

The Hoffmann (H) reflex and motor (M) response were studied in soleus and gastrocnemius during voluntary contraction in eight male volunteers. Aims: To determine if the strength of spindle input to the muscles is the same. To assess if the M response size changes during contraction. Results: The size of the maximum M response (M max) changed during contraction in each subject. Hence, all H reflex measurements were normalized to the M max at each level of contraction for each subject. The largest H/M max was bigger in soleus than gastrocnemius at every contraction level. The overall largest H/M max for soleus (97%) and gastrocnemius (55%) were achieved at 40 and 100% maximum voluntary contraction (MVC), respectively. Conclusion: Soleus receives greater spindle feedback than the gastrocnemius both at rest and during voluntary contraction.

Experimentally-induced low back pain from hypertonic saline injections into lumbar interspinous ligament and erector spinae muscle

&NA; Injection of hypertonic saline into back muscles or ligaments can induce acute low back pain (LBP). However, no study has systematically investigated pain characteristics from these structures. Further, induced muscle pain can change with stretching and contraction, which is problematic for studies into the effect of pain on sensorimotor control. However, it is unclear whether this occurs with experimental ligament pain. In separate sessions, 10 healthy volunteers received a single bolus injection of hypertonic (0.2 ml, 5% NaCl) or isotonic saline (0.3 ml, 0.9% NaCl) into L4/5 interspinous ligament, or hypertonic saline into the left paraspinal muscle. Pain intensity, size and duration were recorded, and a body chart was completed for each injection. Changes in pain intensity and size with stretching or back muscle contractions were also assessed during muscle and ligament pain. Injection of hypertonic saline into the interspinous ligament produced central LBP that was longer in duration and greater in intensity and size compared to hypertonic saline injection into lumbar paraspinal muscles. Isotonic saline injection into the interspinous ligament yielded mild pain that was short‐lasting (<2 min). Intensity and size of muscle pain reduced with stretching and contraction, whereas these tasks did not affect ligament pain. Surprisingly, some participants pointed to a location of pain that was 1–2 segments above or below the injected level. The results highlight that injection into the interspinous ligament elicits consistent pain that is not influenced by trunk movements. These findings support the implementation of this experimental ligament pain model in research.

A new method to estimate signal cancellation in the human maximal M-wave

A new method is introduced that estimates EMG signal cancellation in surface recorded investigations. Its usefulness is demonstrated when determining changes in the maximal motor response (M-wave) magnitude during rest and voluntary contraction. The accuracy of recording and analysis methods and the reliability of the maximal M-wave were assessed in the human gastrocnemius and soleus. The maximal M-wave was recorded by bipolar surface electrodes placed 2 cm, 3 cm and 4 cm apart, and by monopolar (one active and one indifferent reference) surface electrodes. Up to 85% of the maximal M-wave was lost due to signal cancellation during bipolar recording. The maximal M-wave magnitude decreased consistently and significantly during triceps surae contraction compared to rest when recorded by monopolar electrodes, but not when recorded by bipolar electrodes. Area and peak-to-peak (PTP) amplitude analysis methods provided similar results when determining the magnitude of the maximal M-wave. This provides evidence that monopolar recording is superior to bipolar recording as it removes the signal cancellation error and allows the genuine changes in maximal M-wave magnitude to be observed.