Effects of NMES-elicited versus voluntary low-level conditioning contractions on explosive knee extensions

Abstract

Objectives: Electrically-induced or voluntary conditioning-contractions (CC) can be used to affect contractile properties of a subsequent explosive contraction (EC). Here, we aimed at comparing the effect of neuromuscular-electrical-stimulation (NMES) vs voluntary CC performed prior to explosive contractions of the knee extensors.

Methods: A 10 sec NMES CC (100Hz, 1000μs, 10% MVC), or a voluntary contraction (VOL CC) mimicking the NMES CC, preceded an isometric EC of the knee extensors. Explosive contraction was performed with the goal to reach the target (70% MVC) as quickly as possible.

Results: All the parameters related with the explosive contractions' muscle-output were similar between protocols (difference ranging from 0.23%, Mean Torque; to 5.8%, Time to Target), except for the Time to Peak Torque, which was lower when preceded by NMES (11.1%, p=0.019). Interestingly, the RTD 0-50 ms_EC was 37.3% higher after the NMES compared with the VOL CC protocol.

Conclusion: Explosive contraction was potentiated by an NMES CC as compared with a voluntary CC. This may be due to a reduction in descending drive following VOL CC, which has been shown to occur even with low-level voluntary efforts. These findings could be used to improve rehabilitation or training protocols that include conditioning contractions.

Keywords: Afferent pathways; Electrical Stimulation; Explosive contraction; Rate of Torque development.

Figure 1

Schematic representation of the experimental protocol (A-B). Maximal Voluntary Contraction (MVC), Neuromuscular Electrical stimulation (NMES) recruitment curve and the NMES Conditioning Contractions protocol were performed on Day 1 (A). MVC and the Voluntary Conditioning Contractions protocol were performed on Day 2 (B).

Figure 2

Muscle mechanical output for one representative subject. Maximal Voluntary Contraction (MVC) (A) was first performed to identify the 10% MVC to be elicited during the Neuromuscular Electrical stimulation (NMES) recruitment phase (B) and during the NMES-Conditioning Contractions protocol. Explosive Voluntary contractions of knee extensors were performed at 70% MVC during the NMES-elicited Conditioning Contractions protocol and the Voluntary one (D). Muscle contractile properties of knee extensors were evaluated during explosive contractions in the first phase after contraction on-set (E).