Shinichiro Shiozawa

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

Summary A novel human model is presented where daily nerve growth factor injections produced a progressive muscle soreness, mechanical hyperalgesia, temporal summation of pain, and pressure‐induced pain distribution. Abstract 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.

Impaired anticipatory postural adjustments due to experimental infrapatellar fat pad pain.

Anticipatory postural adjustments (APAs) are motor responses generated to stabilize balance prior to voluntary movement. This study investigated how infrapatellar fat pad pain induces reorganization of APAs during reaction time tasks. It has been hypothesized that knee pain may cause insufficient APAs, thereby impairing the balance.

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.

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.