The neuromuscular system produces an extensive repertoire of force and movement, which ranges from precise movements to powerful movements involving the maximum capacity of the muscle to generate force at maximum velocity (Bawa
2002). Depending on the characteristics of the motor task, multiple features of motor control determine the degree of muscle activation and consequently the magnitude, precision, and speed of force output (Enoka
1988; Bawa
2002; Taylor et al.
2003). For instance, the rate of force development is influenced by the ability of the nervous system to recruit motor units at higher frequencies than needed to achieve full tetanic fusion (Desmedt and Godaux
1977), while the maintenance of stable contractions is more affected by motor unit discharge rate variability (Tracy et al.
2005).
Motor output can be impaired by unaccustomed exercise, particularly by eccentric exercise (Sargeant and Dolan
1987; Saxton et al.
1995; Crameri et al.
2007; Semmler et al.
2007; Meszaros et al.
2010). Maximal force (Prasartwuth et al.
2005; Crameri et al.
2007), power (Sargeant and Dolan
1987; Crameri et al.
2007), and force steadiness (Saxton et al.
1995; Semmler et al.
2007; Meszaros et al.
2010) are reduced following intensive eccentric exercise. These alterations have commonly been associated to morphological alterations of the muscle, such as myofibrillar disruption, disturbance of the extracellular matrix, and an inflammatory reaction, which results in delayed onset muscle soreness (DOMS) and muscle stiffness (Jones et al.
1987; Howell et al.
1993; Yu and Thornell
2002).
The extent to which eccentric exercise and exercise-induced DOMS affects a motor task may vary depending on the characteristics of the task; for example, the rate of force development is more influenced than maximal force by eccentric exercise (Crameri et al.
2007), and during constant force tasks, the force variability increases more at very low force levels (2.5 and 5% MVC) than at moderate force levels (10–30% MVC) (Dartnall et al.
2008). Since different force tasks involve different motor control strategies, the degree of impairment might be determined by changes within the nervous system, which ultimately affect the neural activation of agonist and antagonist muscles (Semmler et al.
2007; Dartnall et al.
2008,
2009; Meszaros et al.
2010; Piitulainen et al.
2010). For example, during maximal isometric contractions, the activity of agonist muscles is depressed for 2 h after eccentric exercise, but recovers following a period of 24 h (Meszaros et al.
2010; Piitulainen et al.
2010), while at submaximal force levels, the agonist and antagonist muscle activities are increased (Semmler et al.
2007; Dartnall et al.
2008,
2009). Nonetheless the results are inconsistent (Leger and Milner
2001; Prasartwuth et al.
2005; Semmler et al.
2007; Paschalis et al.
2007; Meszaros et al.
2010; Piitulainen et al.
2010). For instance, in the 2 h following eccentric exercise of the elbow flexors, the biceps brachii EMG has been shown to decrease (Piitulainen et al.
2010) or to remain unchanged (Semmler et al.
2007; Dundon et al.
2008), while during submaximal contractions, it has been reported to increase (Semmler et al.
2007; Dartnall et al.
2009) or be unchanged for the first 2 h after exercise (Piitulainen et al.
2010), and either return to baseline (Semmler et al.
2007; Dartnall et al.
2009) or decrease (Piitulainen et al.
2010) over the next 24 h in the presence of DOMS. Results for changes in antagonist activity are also inconsistent (Semmler et al.
2007; Turner et al.
2008; Bottas et al.
2009). These differences may result from different protocols used to induce DOMS and to the different severity of impairments induced by the eccentric exercise protocols. As a consequence, the exercise-induced adjustments in neural strategies over a range of force profiles are poorly understood.
Therefore, the aim of the present study was to provide a comprehensive assessment of the effect of eccentric exercise and exercise-induced DOMS on the force profile of the knee extensors and the agonist–antagonist activity during a wide range of force tasks. For this purpose, the subjects performed constant isometric knee extension at target force levels varying between 2.5 and 30% of maximal force, progressive knee extension to maximal force, and explosive isometric knee extension contractions.