Do changes in neuromuscular activation contribute to the knee extensor angle–torque relationship?

Abstract

What is the central question of the study? Do changes in neuromuscular activation contribute to the knee extensor angle-torque relationship? What is the main finding and its importance? Both agonist (quadriceps) and antagonist coactivation (hamstrings) differed with knee joint angle during maximal isometric knee extensions and thus both are likely to contribute to the angle-torque relationship. Specifically, two independent measurement techniques showed quadriceps activation to be lower at more extended positions. These effects might influence the capacity for neural changes in response to training and rehabilitation at different knee joint angles. The influence of joint angle on knee extensor neuromuscular activation is unclear, owing in part to the diversity of surface electromyography (sEMG) and/or interpolated twitch technique (ITT) methods used. The aim of the study was to compare neuromuscular activation, using rigorous contemporary sEMG and ITT procedures, during isometric maximal voluntary contractions (iMVCs) of the quadriceps femoris at different knee joint angles and examine whether activation contributes to the angle-torque relationship. Sixteen healthy active men completed two familiarization sessions and two experimental sessions of isometric knee extension and knee flexion contractions. The experimental sessions included the following at each of four joint angles (25, 50, 80 and 106 deg): iMVCs (with and without superimposed evoked doublets); submaximal contractions with superimposed doublets; and evoked twitch and doublet contractions whilst voluntarily passive, and knee flexion iMVC at the same knee joint positions. The absolute quadriceps femoris EMG was normalized to the peak-to-peak amplitude of an evoked maximal M-wave, and the doublet-voluntary torque relationship was used to calculate activation with the ITT. Agonist activation, assessed with both normalized EMG and the ITT, was reduced at the more extended compared with the more flexed positions (25 and 50 versus 80 and 106 deg; P ≤ 0.016), whereas antagonist coactivation was greatest in the most flexed compared with the extended positions (106 versus 25 and 50 deg; P ≤ 0.02). In conclusion, both agonist and antagonist activation differed with knee joint angle during knee extension iMVCs, and thus both are likely to contribute to the knee extensor angle-torque relationship.

Is the joint-angle specificity of isometric resistance training real? And if so, does it have a neural basis?

“…Thus, the adaptations of the training intervention at the present study could have occurred at supraspinal level at the trained angle and reflected as well in the other knee-joint angles. Additionally, hamstrings coactivation changes across knee-joint angles (Kubo et al 2004;Lanza et al 2017) and may have altered after the training intervention and reflected in our results. Moreover, mechanically, the cross-brigde binding is not optimall at more extended or flexed positions (Rassier et al 1999), which may also influence the adpations after training with higher responses where the binding is optimal.…”

“…Therefore, whether increases in agonist neuromuscular activation underpin joint angle specific strength increases after short-term SCT remains unclear. These contrasting findings may be due to a lack of contemporary sEMG methods such as joint angle specific M-wave normalization (Lanza et al 2017), duplicate sensors on each superficial constituent muscle (Balshaw et al 2017), and duplicate measurement sessions at each timepoint (i.e. pre and post training) to enhance measurement reliability.…”

An Electromyographic Analysis of Lateral Raise Variations and Frontal Raise in Competitive Bodybuilders

“…The internal or external rotation of the humerus generates an increment in moment arm for posterior and anterior deltoid, respectively, thus increasing their activity [ 5 ]. Additionally, bi-articular muscles show greater sEMG RMS at long compared with short muscle length [ 27 , 28 ], possibly because of changes in moment arm and muscle architecture [ 29 ], modifications in both spinal and supraspinal mechanisms [ 30 ], or also changes in the characteristics of motoneurons pool when a muscle is lengthened [ 28 ]. Interestingly, although a computational study found that the humerus rotation could have led to negligible differences, here, the present competitive bodybuilders might have been able to increase the muscle activation for each exercise [ 3 ].…”

Reference Values for Isometric, Dynamic, and Asymmetry Leg Extension Strength in Patients with Multiple Sclerosis

“…In the evaluation of the performance of a particular muscle group, it is fundamental, especially in the measurement of isometric strength, to standardize the joint angle on which this group acts, as well as that of the joints on which the agonist groups in that exercise are operating [36][37][38]. In general it is considered that the angle of knee flexion which produces the greatest isometric torque is between 80 and 90 • [36,39]; however, the angle of the patient's hip in the evaluation position also modifies the result of the force expressed. Therefore, the standardization of angles is necessary to be able to establish valid comparisons of results obtained in evaluations sequenced over time and for comparison with data from other studies.…”