Context: Quadriceps activation failure has been observed following various pathological conditions in a knee joint such as knee surgery, pain, effusion in knee, and osteoarthritis also could be aging matter. Those patients are unable to attain maximal quadriceps strength for a long period of time although their quadriceps itself is not damaged. This impairment is termed arthrogenic muscle inhibition (AMI). AMI has been of concern to clinicians because this weakness hinders the rehabilitation process considerably and delays recovery because strengthening protocols for the AMI could be largely ineffective. Clinically, it is important to understand neurophysiological mechanisms of the AMI to treat patients with the impairment. Objectives: This is a narrative review of the literature. The purpose of this review is to understand the following: (1) Why investigations of only peripheral spinal reflexive pathways are not enough for elucidation of the mechanisms of the AMI? (2) What we know about the role of the gamma spindle system in AMI so far? (3) Could a dysfunctional gamma spindle system contribute to AMI lead neural changes in upper central nervous system? and (4) Concerns that a clinician should take into consideration when deciding whether to apply therapeutic interventions for AMI. Data Sources: The databases PubMed, MEDLINE, SPORTDiscus, and CINAHL were searched with the terms arthrogenic muscle inhibition (AMI), reflex inhibition, joint mechanoreceptor, gamma loop, corticospinal pathway, spinal reflex, effusion, and joint injury. The remaining citations were collected from references of similar papers. Conclusions: AMI is a limiting factor in the rehabilitation of joint injury. Motor unit recruitment could be hindered in patients with AMI as a result of a dysfunctional gamma spindle system. Clinicians should understand the mechanism of AMI well in order to establish effective rehabilitation programs for AMI. Indeed, AMI is not caused by a single factor, but rather, multiple neural factors can change over time following the appearance of AMI. Therefore, multiple interventions targeting different neural pathways should be combined to achieve the ideal therapeutic goal for the treatment of AMI.

Circumference measurements have been used to estimate muscle cross-sectional area (CSA) in clinical settings. Measurements of thigh circumference are affected by muscle and subcutaneous fat (SF). In fact, SF could increase over a short period. Therefore, clarifying the relationship between thigh circumference and muscle and SF following ACL reconstruction is important. This study’s primary purpose was to examine pre- and post-operative changes in thigh circumference, thigh muscles and SF CSAs in both legs. Secondary, the relationship between thigh circumference and muscle and SF CSAs was examined to demonstrate that circumference measurements could be used to detect atrophy. Quadriceps, hamstrings, and SF CSAs at 15, 10, and 5 cm proximal to the patella were measured by MRI pre- and 4 weeks postoperatively to examine how reconstruction affected those tissues in the thighs. The results showed increases in SF CSA (r=0.72 at 10 cm, r=0.67 at 15 cm) greatly affected thigh circumference in females on the surgical side. In males, increases in SF CSA (r=0.83) at 15- and 5-cm and decreases in quadriceps muscle CSA (r=0.73) at 5 cm affected thigh circumference on the surgical side. Thigh circumference measurements might not reflect actual muscle CSA in ACL patients.

Purpose: Arthrogenic muscle inhibition(AMI) is induced by pathological knee conditions. The present study aimed to investigate the effect of tactile stimulation on reflex changes induced by simulated AMI during unpredictable landing performances. Methods: Twenty participants performed six unilateral landing tasks: 15cm normal landing (15NL), 30cm normal landing(30NL), surprise landing(SL), 30cm normal landing following vibration (30NLV), surprise landing following vibration(SLV), and surprise landing following vibration with Kinesiology-tape(SLK). For the surprise landing tasks, the solid landing platform(15cm) was removed and replaced by a false plate platform. Since the false plate dislodged easily under load, participants unpredictably fell through the plate to the actual landing surface 15cm below. After completing 15NL, 30NL, SL, vibration stimulation was applied to participants' knees to induce neurological changes similar to AMI. After vibration stimulation, participants performed 30NLV, SLV and SLK in a random order. EMG in the post-landing short latency(31-60 ms) and medium latency(61-90 ms) periods were examined. EMG signals from the vastus lateralis(VL), vastus medialis(VM), and lateral hamstrings(LH) were recorded and compared between tasks. Results: EMG of all muscles were significantly enhanced in the medium latency period. Enhanced EMG were suppressed by vibration stimulation in the VL, but the suppressed EMG were restored after cutaneous stimulation(p<0.01). Conclusion: Our findings suggest that AMI could alter motor control patterns during unpredictable landing, and that tactile stimulation could restore the altered motor control to a normal state. The use of tactile stimulation may help maintain normal motor control patterns in injured individuals.