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Context:Quadriceps dysfunction is a common consequence of knee joint injury and disease, yet its causes remain elusive.Objective:To determine the effects of pain on quadriceps strength and activation and to learn if simultaneous pain and knee joint effusion affect the magnitude of quadriceps dysfunction.Design:Crossover study.Setting:University research laboratory.Patients or Other Participants:Fourteen (8 men, 6 women; age = 23.6 ± 4.8 years, height = 170.3 ± 9.16 cm, mass = 72.9 ± 11.84 kg) healthy volunteers.Intervention(s):All participants were tested under 4 randomized conditions: normal knee, effused knee, painful knee, and effused and painful knee.Main Outcome Measure(s):Quadriceps strength (Nm/kg) and activation (central activation ratio) were assessed after each condition was induced.Results:Quadriceps strength and activation were highest under the normal knee condition and differed from the 3 experimental knee conditions (P < .05). No differences were noted among the 3 experimental knee conditions for either variable (P > .05).Conclusions:Both pain and effusion led to quadriceps dysfunction, but the interaction of the 2 stimuli did not increase the magnitude of the strength or activation deficits. Therefore, pain and effusion can be considered equally potent in eliciting quadriceps inhibition. Given that pain and effusion accompany numerous knee conditions, the prevalence of quadriceps dysfunction is likely high.

Context:Quadriceps and hamstrings weakness occurs frequently after anterior cruciate ligament (ACL) injury and reconstruction. Evidence suggests that knee injury may precipitate hip and ankle muscle weakness, but few data support this contention after ACL injury and reconstruction.Objective:To determine if hip, knee, and ankle muscle weakness present after ACL injury and after rehabilitation for ACL reconstruction.Design:Case-control study.Setting:University research laboratory.Patients or Other Participants:Fifteen individuals with ACL injury (8 males, 7 females; age = 20.27 ± 5.38 years, height = 1.75 ± 0.10 m, mass = 74.39 ± 13.26 kg) and 15 control individuals (7 men, 8 women; age = 24.73 ± 3.37 years, height = 1.75 ± 0.09 m, mass = 73.25 ± 13.48 kg).Intervention(s):Bilateral concentric strength was assessed at 60°/s on an isokinetic dynamometer. The participants with ACL injury were tested preoperatively and 6 months postoperatively. Control participants were tested on 1 occasion.Main Outcome Measures:Hip-flexor, -extensor, -abductor, and -adductor; knee-extensor and -flexor; and ankle–plantar-flexor and -dorsiflexor strength (Nm/kg).Results:The ACL-injured participants demonstrated greater hip-extensor (percentage difference = 19.7, F1,14 = 7.28, P = .02) and -adductor (percentage difference = 16.3, F1,14 = 6.15, P = .03) weakness preoperatively than postoperatively, regardless of limb, and greater postoperative hip-adductor strength (percentage difference = 29.0, F1,28 = 10.66, P = .003) than control participants. Knee-extensor and -flexor strength were lower in the injured than in the uninjured limb preoperatively and postoperatively (extensor percentage difference = 34.6 preoperatively and 32.6 postoperatively, t14 range = −4.59 to −4.23, P ≤ .001; flexor percentage difference = 30.6 preoperatively and 10.6 postoperatively, t14 range = −6.05 to −3.24, P < .05) with greater knee-flexor (percentage difference = 25.3, t14 = −4.65, P < .001) weakness preoperatively in the injured limb of ACL-injured participants. The ACL-injured participants had less injured limb knee-extensor (percentage difference = 32.0, t28 = −2.84, P = .008) and -flexor (percentage difference = 24.0, t28 = −2.44, P = .02) strength preoperatively but not postoperatively (extensor: t28 = −1.79, P = .08; flexor: t28 = 0.57, P = .58) than control participants. Ankle–plantar-flexor weakness was greater preoperatively than postoperatively in the ACL-injured limb (percentage difference = 31.9, t14 = −3.20, P = .006).Conclusions:The ACL-injured participants presented with hip-extensor, -adductor, and ankle–plantar-flexor weakness that appeared to be countered during postoperative rehabilitation. Our results confirmed previous findings suggesting greater knee-extensor and -flexor weakness postoperatively in the injured limb than the uninjured limb. The knee extensors and flexors are important dynamic stabilizers; weakness in these muscles could impair knee joint stability. Improving rehabilitation strategies to better target this lingering weakness seems imperative.