Center of Pressure Displacement of Standing Posture during Rapid Movements Is Reorganised Due to Experimental Lower Extremity Muscle Pain.

BACKGROUND: Postural control during rapid movements may be impaired due to musculoskeletal pain. The purpose of this study was to investigate the effect of experimental knee-related muscle pain on the center of pressure (CoP) displacement in a reaction time task condition. METHODS: Nine healthy males performed two reaction time tasks (dominant side shoulder flexion and bilateral heel lift) before, during, and after experimental pain induced in the dominant side vastus medialis or the tibialis anterior muscles by hypertonic saline injections. The CoP displacement was extracted from the ipsilateral and contralateral side by two force plates and the net CoP displacement was calculated. RESULTS: Compared with non-painful sessions, tibialis anterior muscle pain during the peak and peak-to-peak displacement for the CoP during anticipatory postural adjustments (APAs) of the shoulder task reduced the peak-to-peak displacement of the net CoP in the medial-lateral direction (P<0.05). Tibialis anterior and vastus medialis muscle pain during shoulder flexion task reduced the anterior-posterior peak-to-peak displacement in the ipsilateral side (P<0.05). CONCLUSIONS: The central nervous system in healthy individuals was sufficiently robust in maintaining the APA characteristics during pain, although the displacement of net and ipsilateral CoP in the medial-lateral and anterior-posterior directions during unilateral fast shoulder movement was altered.

Reorganised anticipatory postural adjustments due to experimental lower extremity muscle pain.

Automated movements adjusting postural control may be hampered during musculoskeletal pain leaving a risk of incomplete control of balance. This study investigated the effect of experimental muscle pain on anticipatory postural adjustments by reaction task movements. While standing, nine healthy males performed two reaction time tasks (shoulder flexion of dominant side and bilateral heel lift) before, during and after experimental muscle pain. On two different days experimental pain was induced in the m. vastus medialis (VM) or the m. tibialis anterior (TA) of the dominant side by injections of hypertonic saline (1ml, 5.8%). Isotonic saline (1ml, 0.9%) was used as control injection. Electromyography (EMG) was recorded from 13 muscles. EMG onset, EMG amplitude, and kinematic parameters (shoulder and ankle joint) were extracted. During shoulder flexion and VM pain the onset of the ipsilateral biceps femoris was significantly faster than baseline and post injection sessions. During heels lift in the VM and TA pain conditions the onset of the contralateral TA was significantly faster than baseline and post injection sessions in bilateral side. VM pain significantly reduced m. quadriceps femoris activity and TA pain significantly reduced ipsilateral VM activity and TA activity during bilateral heel lift. The EMG reaction time was delayed in bilateral soleus muscles during heels lift with VM and TA pain. The faster onset of postural muscle activity during anticipatory postural adjustments may suggest a compensatory function to maintain postural control whereas the reduced postural muscle activity during APAs may indicate a pain adaptation strategy to avoid secondary damage.

Repeated intramuscular injections of nerve growth factor induced progressive muscle hyperalgesia, facilitated temporal summation, and expanded pain areas.

Intramuscular injection of nerve growth factor (NGF) is known to induce deep-tissue mechanical hyperalgesia. In this study it was hypothesised that daily intramuscular injections of NGF produce a progressive manifestation of soreness, mechanical hyperalgesia, and temporal summation of pain. In a double-blind placebo-controlled design, 12 healthy subjects were injected on 3 days with NGF into the tibialis anterior muscle and with isotonic saline on the contralateral side. Assessments were performed before and after the injections on days 0, 1, and 2, and repeated on days 3, 6, and 10. The self-perceived muscle soreness was assessed on a Likert scale. Computer-controlled pressure algometry was used to assess the pressure pain thresholds (PPTs). Temporal summation of pain after repeated pressure stimulations was assessed by computer-controlled pressure algometry. The pain distribution following painful pressure stimulation was also recorded. Compared with baseline and isotonic saline, the NGF injections caused (P<0.05): (1) progressively increasing soreness scores from 3 hours after the first injection until day 2, after which it remained increased; (2) decreased PPTs at days 1 to 3; (3) facilitated temporal summation of pressure pain at days 1 to 10; and (4) enlarged pressure-induced pain area after the injection on day 1 to day 6. The daily injections of NGF produced a progressive manifestation of muscle soreness, mechanical hyperalgesia, temporal summation of pressure pain, and pressure-induced pain distribution. These data illustrate that the prolonged NGF application affects peripheral and central mechanisms and may reflect process in musculoskeletal pain conditions.

Experimental knee pain impairs postural stability during quiet stance but not after perturbations.

The objective of this study was to examine the effect of experimental knee-related pain on postural control. Twelve healthy subjects stood as quietly as possible on a movable force platform (that measured the centre of pressure and provided fast perturbations) before, during, and after experimental knee-related pain. Lower limb electromyographic (EMG) activity and joint angles were measured. Experimental pain was induced by injecting hypertonic saline into the infrapatellar fat pad (unilateral and bilateral) and isotonic saline was used for control sessions. Compared with the baseline and control sessions, unilateral and bilateral knee-related pain during quiet standing evoked (1) an increased sway displacement in the anterior-posterior direction (P < 0.05), (2) larger knee flexion (P < 0.05), and (3) larger EMG changes. Bilateral pain also induced (1) larger medial-lateral sway displacement and speed (P < 0.05) and (2) larger left hip flexion (P < 0.05). During forward perturbations, subjects leaned forward during both painful conditions when compared with baseline (P < 0.05). The additional impairment by bilateral pain suggests that the non-painful limb in unilateral pain conditions compensates for the impaired postural control. These results show that knee-related pain impairs postural stability during quiet standing, indicating the vulnerability of patients with knee pain to falls. This measure could potentially help clinicians who seek to assess how pain responses may contribute to patient's postural control and stability during quiet standing.

Remote facilitation of supraspinal motor excitability depends on the level of effort.

Stretch reflexes and motor-evoked potentials (MEPs) of a muscle are facilitated when performing intensive contraction of muscles located in a different segment (remote effect). We investigated to what extent the remote effect on MEPs in the flexor carpi radialis (FCR) in humans is modulated during sustained maximal and submaximal voluntary contractions of the ipsilateral quadriceps (remote muscle). We found that even when the force of maximal voluntary contraction (MVC) of the remote muscle declined during sustained MVC, the magnitude of the remote effect on MEPs remained constant. Maximal electrical stimulation of the remote muscle and transcranial magnetic stimulation of the corresponding motor cortex revealed that the level of voluntary activation gradually decreased during the sustained MVC. The motor response in the FCR following magnetic stimulation at the level of the foramen magnum, which preferentially elicits muscle response as a direct response of the corticospinal tract, was not modified by the remote effect during the sustained MVC. This finding suggested that the excitability of the spinal motoneuron pool remained constant. In contrast to the sustained MVC, during sustained submaximal contraction of the remote muscle, the magnitude of the remote effect on MEPs gradually increased as muscle fatigue developed. These findings suggest that the remote effect on MEPs was dependent on the level of effort driving the remote muscle, but not on the actual level of force output of the remote muscle, and that the origin of the remote effect was supraspinal, putatively upstream of the primary motor cortex.

[Effectiveness of a community health service program using exercise intervention for elderly people with osteoarthritis of the knees: a randomized controlled trial].

OBJECTIVE: The objective of this study was to evaluate the effectiveness of an exercise program conducted as part of community health services to improve pain and physical function in elderly people with osteoarthritis of the knee (knee OA). METHODS; The subjects were 88 (12 males aged 77.8 +/- 5.4 years and 76 females aged 73.2 +/- 5.3 years) community-dwelling independent elderly people with knee OA who participated voluntarily in exercise classes sponsored by Musashino city of Tokyo. They were allocated randomly to the intervention group (n = 44) and the control group (n = 44). For the intervention group, exercise classes of 90 min duration were held 8 times over 3 months. The exercise program comprised flexibility exercises (stretching of knee and ankle joints), resistance exercises (strengthening of quadriceps, extension and flexion of the knee joint with an elastic band), and movement exercises (turning over, getting up, standing up). The subjects were instructed to perform these exercises at home every day. Knee pain scores (Western Ontario and McMaster Universities OA Index; WOMAC), peak torque of joint during knee flexion and extension, range of motion (ROM) of the knee joint, and functional fitness (standing and walking ability) were evaluated before and after the intervention period. RESULTS: Significant inter-group differences were observed for peak torque during knee flexion and standing and walking ability of females. Assessment of interactions between time and group by repeated measure ANOVA adjusted for age and sex revealed significance differences for the WOMAC score (P = 0.031), the peak torque during knee extension (P = 0.016) and knee flexion (P = 0.000), ROM (P = 0.037), standing ability (P = 0.000)and walking ability (P = 0.000). The effect of the intervention was 0.44 for WOMAC score, 0.23 for peak torque during knee extension, 0.64 for knee flexion, 0.32 for ROM, 0.81 for standing ability, and 1.13 for walking ability. CONCLUSIONS: These results suggested that the exercise program for elderly people with knee OA improved knee pain and physical function.

Voluntary changes in leg cadence modulate arm cadence during simultaneous arm and leg cycling.

Although there is some evidence showing that neural coupling plays an important role in regulating coordination between the upper and lower limbs during walking, it is unclear how tightly the upper and lower limbs are linked during rhythmic movements in humans. The present study was conducted to investigate how coupling of both limbs is coordinated during independent rhythmic movement of the upper and lower limbs. Ten subjects performed simultaneous arm and leg cycling (AL cycling) at their preferred cadences without feedback for 10 s, and then were asked to voluntarily change the cadence (increase, decrease, or stop) of arm or leg cycling. Leg cycling cadence was not affected by voluntary changes in arm cadence. By contrast, arm cycling cadence was significantly altered when leg cycling cadence was changed. These results suggest the existence of a predominant lumbocervical influence of leg cycling on arm cycling during AL cycling.

Modulations of interlimb and intralimb cutaneous reflexes during simultaneous arm and leg cycling in humans.

OBJECTIVE: We investigated to what extent intralimb and interlimb cutaneous reflexes are altered while simultaneously performing arm and leg cycling (AL cycling) under different kinematic and postural conditions. METHODS: Eleven subjects performed AL cycling under conditions in which the arm and leg crank ipsilateral to the stimulation side were moved synchronously (in-phase cycling) or asynchronously (anti-phase cycling) while sitting or standing. Cutaneous reflexes following superficial radial or superficial peroneal nerve stimulation (2.0-2.5 times radiating threshold, 5 pulses at 333 Hz) were recorded at 4 different pedal positions from 12 muscles in the upper and lower limbs. Cutaneous reflexes with a peak latency of 80-120 ms were then analyzed. RESULTS: The magnitude of interlimb and intralimb cutaneous reflexes in the arm and leg muscles was significantly modulated depending on the crank position for the relevant limb (phase-dependent modulation). A significant correlation between the magnitude of the cutaneous reflex and background EMG was observed in the majority of muscles during static contraction, but not during AL cycling (task-dependent modulation). No significant difference was found in comparisons of the magnitude of intralimb and interlimb cutaneous reflexes obtained during in- and anti-phase AL cycling. Qualitatively, the same results were obtained during AL cycling while sitting or standing. In addition, the modulation of cutaneous reflexes in arm muscles was identical among in-phase, anti-phase and isolated arm cycling. Results were the same for leg muscles. CONCLUSIONS: Cutaneous reflexes in arm muscles are little influenced by rhythmic movement of the legs and vice versa during AL cycling. It is likely that neural components that control interlimb reflexes are loosely coupled during AL cycling while sitting or standing. SIGNIFICANCE: Our results provide a better understanding of the coordination between the upper and lower limbs during rhythmic movement.