Neuromuscular Properties of the Human Wrist Flexors as a Function of the Wrist Joint Angle

Abstract

The joint angle dependence of voluntary activation and twitch properties has been investigated for several human skeletal muscles. However, although they play a key role for hand function and possess a unique neural control compared to muscles surrounding other joint complexes, little is known about the wrist flexors innervated by the median nerve. Therefore, isometric voluntary and electrically evoked contractions of the wrist flexors were analyzed at three wrist joint angles (extension: -30°, neutral: 0°, flexion: 30°) to quantify the joint angle dependence of (i) voluntary activation (assessed via peripheral nerve stimulation and electromyography [EMG]), (ii) unpotentiated twitch torques, and (iii) potentiated twitch torques. Maximum voluntary torque was lower in extension compared to neutral and flexion. Although voluntary activation was generally high, data indicate that voluntary activation of the wrist flexors innervated by the median nerve was lower and the antagonist·agonist^-1 EMG ratio was higher with the wrist joint in flexion compared to extension. Peak twitch torque, rate of twitch torque development, and twitch half-relaxation time increased, whereas electromechanical delay decreased from flexion to extension for the unpotentiated twitch torques. Activity-induced potentiation partly abolished these differences and was higher in short than long wrist flexors. Different angle-dependent excitatory and inhibitory inputs to spinal and supraspinal centers might be responsible for the altered activation of the investigated wrist muscles. Potential mechanisms were discussed and might have operated conjointly to increase stiffness of the flexed wrist joint. Differences in twitch torque properties were probably related to angle-dependent alterations in series elastic properties, actin-myosin interaction, Ca²⁺ sensitivity, and phosphorylation of myosin regulatory light chains. The results of the present study provide valuable information about the contribution of neural and muscular properties to changes in strength capabilities of the wrist flexors at different wrist joint angles. These data could help to understand normal wrist function, which is a first step in determining mechanisms underlying musculoskeletal disorders and in giving recommendations for the restoration of musculoskeletal function after traumatic or overuse injuries.

Keywords: activity-dependent potentiation; electrical stimulation; flexor carpi radialis; median nerve; muscle length; post-activation potentiation; twitch; voluntary activation.

Figure 1

Schematic representation of the experimental setup (A) and chronology of the procedures carried out during neuromuscular testing and the extracted parameters (B). Please note that the hand fixation is not displayed and the order of the wrist joint angles was randomized. ECRB, extensor carpi radialis brevis; FCR, flexor carpi radialis; MVC, maximal voluntary contraction; PS100, unpotentiated twitch torques evoked by paired electrical stimuli at 100 Hz [inter-stimulus interval (ISI) 10 ms]; PS100_POT, potentiated twitch torques evoked by paired electrical stimuli at 100 Hz [ISI 10 ms] RMS·M⁻¹, normalized root mean square of the EMG signal; SS, unpotentiated twitch torques evoked by single electrical stimuli; SS_POT, potentiated twitch torques evoked by single electrical stimuli; −30°, extension: long wrist flexor muscle length; 0°, neutral: intermediate wrist flexor muscle length; 30°, flexion: short wrist flexor muscle length.

Figure 2

Torque signals produced by voluntary (A) and electrically evoked contractions (B,C) of one representative participant. PS100, unpotentiated twitch torque evoked by paired electrical stimuli at 100 Hz [inter-stimulus interval (ISI) 10 ms]; PS100_CTT, control twitch torque evoked by paired electrical stimuli at 100 Hz (ISI 10 ms); PS100_ITT, interpolated twitch torque evoked by paired electrical stimuli at 100 Hz (ISI 10 ms); PS100_POT, potentiated twitch torque evoked by paired electrical stimuli at 100 Hz (ISI 10 ms); SS, unpotentiated twitch torque evoked by a single electrical stimulus; SS_POT, potentiated twitch torque evoked by a single electrical stimulus. −30° extension: long wrist flexor muscle length; 0°, neutral: intermediate wrist flexor muscle length; 30°, flexion: short wrist flexor muscle length.

Figure 3

Maximal voluntary torque of the wrist flexors (A), voluntary activation of the wrist flexors (B), root mean square of the EMG signal normalized to the maximal M-wave (RMS·M⁻¹) of the flexor carpi radialis (FCR) during MVC of the wrist flexors (C), RMS·M⁻¹ of the extensor carpi radialis brevis (ECRB) during MVC of the wrist flexors (D), the antagonist·agonist⁻¹ EMG ratio (ECRB·FCR⁻¹ ratio) during MVC of the wrist flexors (E) and the schematic representation of the experimental setup (F). −30°, extension: long wrist flexor muscle length; 0°, neutral: intermediate wrist flexor muscle length; 30°, flexion: short wrist flexor muscle length. *Denotes a significant difference between joint angles (*P ≤ 0.050, **P ≤ 0.010). Values are expressed as means ± standard deviations.

Figure 4

Unpotentiated peak twitch torques evoked by single electrical stimuli [ss] (A), potentiated peak twitch torques evoked by single electrical stimuli [SS_POT] (B), unpotentiated peak twitch torques evoked by paired electrical stimuli at 100 Hz [inter-stimulus interval (ISI) 10 ms, PS100] (C) and potentiated peak twitch torques evoked by paired electrical stimuli at 100 Hz (ISI 10 ms, PS100_POT) (D). −30°, extension: long wrist flexor muscle length; 0°, neutral: intermediate wrist flexor muscle length; 30°, flexion: short wrist flexor muscle length. *Denotes a significant difference between joint angles (**P ≤ 0.010). Values are expressed as means ± standard deviations.

Figure 5

Activity-induced potentiation (%) of peak twitch torques evoked by single (A) and paired electrical stimuli (B). Percentage contribution of a second electrical stimulus to the unpotentiated (C) and the potentiated peak twitch torque production (D). SS, unpotentiated peak twitch torque evoked by a single stimulus; PS100, unpotentiated peak twitch torque evoked by paired stimuli at 100Hz [inter-stimulus interval (ISI) 10 ms]; SS_POT, potentiated peak twitch torque evoked by a single stimulus; PS100_POT, potentiated peak twitch torque evoked by paired stimuli at 100Hz (ISI 10 ms); −30°, extension: long wrist flexor muscle length; 0°, neutral: intermediate wrist flexor muscle length; 30°, flexion: short wrist flexor muscle length. *Denotes a significant difference between joint angles (*P ≤ 0.050, **P ≤ 0.010). Values are expressed as means ± standard deviations.