Reduced Maximal Force during Acute Anterior Knee Pain Is Associated with Deficits in Voluntary Muscle Activation

Abstract

Although maximal voluntary contraction (MVC) force is reduced during pain, studies using interpolated twitch show no consistent reduction of voluntary muscle drive. The present study aimed to test if the reduction in MVC force during acute experimental pain could be explained by increased activation of antagonist muscles, weak voluntary activation at baseline, or changes in force direction. Twenty-two healthy volunteers performed maximal voluntary isometric knee extensions before, during, and after the effects of hypertonic (pain) and isotonic (control) saline injections into the infrapatellar fat pad. The MVC force, voluntary activation, electromyographic (EMG) activity of agonist, antagonist, and auxiliary (hip) muscles, and pain cognition and anxiety scores were recorded. MVC force was 9.3% lower during pain than baseline (p < 0.001), but there was no systematic change in voluntary activation. Reduced MVC force during pain was variable between participants (SD: 14%), and was correlated with reduced voluntary activation (r = 0.90), baseline voluntary activation (r = − 0.62), and reduced EMG amplitude of agonist and antagonist muscles (all r > 0.52), but not with changes in force direction, pain or anxiety scores. Hence, reduced MVC force during acute pain was mainly explained by deficits in maximal voluntary drive.

Early detection of exercise-induced muscle damage using elastography

“…In the present study, we considered the decrease in peak torque measured at 48 H during maximal voluntary isometric contractions as a surrogate of exercise-induced myofibrillar disruptions ). This is questionable as muscle damage is also associated with delayed soreness that might peak at 48 H. As experimental pain alone (i.e., without associated tissue damage) may be associated with reduced maximal voluntary contraction torque to an extent that can vary between individuals (Salomoni et al 2016), soreness may have acted as a cofounding factor in our experiment. In addition, shear modulus was not measured in all the synergist muscles, with only ~80% (knee extension) and ~87% (elbow flexion) of the physiological cross sectional area considered (Kawakami et al 1994;Farahmand et al 1998).…”

Motor Strategies Learned during Pain Are Sustained upon Pain-free Reexposure to Task

“…Reduced muscle activity during pain is generally believed to be part of a protective adaptation aimed at reducing the stress in the painful area and protect from further pain (2). As the peripheral properties of the neuromuscular system are not affected by injections of hypertonic saline (24), reduced EMG has been attributed to reduced central drive to the muscles (7,25). This reduction in descending drive is believed to originate, at least in part, at the cortical level, as acute experimental pain reduces the amplitude of motor-evoked potentials (MEPs)…”

“…In contrast, during high-intensity tasks, such as fast arm-reaching movements, reduced activation of the painful muscle can significantly impact task performance, e.g. reducing force magnitude and movement acceleration (6,7).…”

Individuals With Recurrent Low Back Pain Exhibit Significant Changes in Paraspinal Muscle Strength After Intramuscular Fine Wire Electrode Insertion

“…Interestingly, whether the IFWE remained within the lumbar multifidus muscle made no significant difference --the process of IFWE insertion alone was enough to cause diminished torque in those with RLBP, perhaps due to pain or microtrauma associated with the insertion of guide needle. Experimentally induced pain has been shown to reduce maximal force in various muscle groups [33,34,35]. Puta hyperalgesia to punctate mechanical pinprick stimuli, a sensation similar to needle insertion [36].…”

“…In the current study, a reduction in maximal torque was observed in individuals with RLBP, however, we observed no significant difference between groups in muscle endurance. This may be due to that during the endurance task the paraspinal muscles were only submaximally activated [40], requiring only ~50% paraspinal muscle activation [35] compared to the near 100% activation required during strength testing.…”