Fatigue induced at a specific muscle can contribute to the development of central (neural) fatigue and result in a non-local response (crossover fatigue)(Martin and Rattey, 2007; Rattey et al., 2006). Crossover fatigue is fatigue of a non-exercised muscle following fatiguing contractions of a disparate muscle. Research exploring crossover fatigue appears to be equivocal. Several studies have not shown significant crossover fatigue effects between homologous (Elmer et al., 2013; Grabiner and Owings, 1999; Regueme et al., 2007; Todd et al., 2003; Zijdewind et al., 1998) and heterologous muscles (Decorte et al., 2012; Humphry et al., 2004; Millet and Lepers, 2003; Place et al., 2004; Ross et al., 2007; 2010). In contrast, significant crossover fatigue effects have been found in contralateral homologous (Doix et al., 2013; Martin and Rattey, 2007; Rattey et al., 2006; Triscott et al., 2008) and heterologous muscle groups (Kennedy et al., 2013; Takahashi et al., 2011) following isolated muscular fatigue. Moreover, crossover fatigue can affect single leg landing strategies (McLean and Samorezov, 2009) and postural control (Paillard et al., 2010). Recent research by Amann and colleagues (2013) found that constant load single leg knee-extensor exercise performed to exhaustion resulted in a reduction of endurance time to exhaustion in the consecutively exercised, non-fatigued contralateral leg by ~49%; however, no significant changes were seen in potentiated twitch, maximum voluntary contraction (MVC) force and voluntary muscle activation. Evidently, there is a conflict in the literature regarding the crossover fatigue phenomenon despite studies claiming that subjects were brought to temporary exhaustion. The discrepancy in results may be related to the inconsistency of unilateral fatiguing protocols as several variables differ such as exercise intensities, volumes and types of contraction.
Research examining crossover fatigue following dynamic contractions is sparse. Crossover fatiguing protocols have utilized several contraction types such as isometric, isotonic (dynamic) and isokinetic and varying contraction intensities. Isometric fatiguing protocols have been shown to induce (Doix et al., 2013; Kennedy et al., 2013; Martin and Rattey, 2007; Paillard et al., 2010; Rattey et al., 2006) as well as not induce (Todd et al., 2003; Zijdewind et al. 1998) crossover fatigue. For example, a 100 s knee extensor isometric MVC fatigue protocol reduced voluntary activation of the contralateral non-fatigued knee extensors but did not alter MVC force (Rattey et al., 2006). In contrast, Martin and Rattey (2007) used the same fatiguing protocol and found a reduction in MVC force output from the contralateral non-fatigued homologous muscles, which was accompanied with larger reductions in voluntary activation. Maximal and submaximal bilateral isometric handgrip exercise held until force was reduced to 80% of pre-fatigue values resulted in a decrease in ankle plantar flexion MVC and voluntary activation (Kennedy et al., 2013). Interestingly, the maximal fatiguing protocol was more impactful on reducing ankle plantar flexor MVC and voluntary activation compared to the submaximal fatiguing protocol indicating that central fatigue to an uninvolved lower limb muscle is intensity-specific (Kennedy et al., 2013).
Similarly, fatigue protocols using dynamic contractions also have varying crossover fatigue effects. Isokinetic knee flexion/extension contractions had no effect on the contralateral hamstrings but actually enhanced contralateral quadriceps MVC force (Strang et al., 2009). Dynamic free weight lower body exercise induced crossover fatigue in female athletes (Mclean and Samorezov, 2009). Furthermore, three 5-minute sets of bilateral leg presses performed with 50% of a dynamic MVC was found to depress motor evoked potentials (MEPs) and short interval intracortical inhibition (SICI) in non-exercised upper limb muscles (Takahashi et al., 2011). As the majority of crossover fatigue studies employ isometric contractions, the dearth of literature employing dynamic resistive exercise effects should provide an impetus for further investigations in this area.
It appears that the fatiguing protocol volume is an important factor in the development of crossover fatigue (Doix et al., 2013; Humphry et al., 2004). Performing only one 100-seconds MVC with the knee extensors was insufficient volume to produce crossover fatigue to the contralateral limb while two bouts of 100-seconds MVC was successful (Doix et al., 2013). Additionally, unilateral biceps curls performed with 3.5 kg performed to exhaustion resulted in crossover fatigue effects to the contralateral homologous muscles but failed to do so when only 25% of the volume was performed (Humphry et al., 2004). From the limited research, it appears that fatiguing protocols of higher intensity and larger volumes have greater crossover fatigue effects compared to lower intensity and lower volume protocols.
The objective of the present study was to determine if varying intensities of unilateral dynamic fatiguing resistive exercise would elicit strength and endurance impairments on the contralateral homologous muscle group. Based on the related literature it was hypothesized that a higher intensity of unilateral dynamic exercise would lead to greater crossover fatigue effects, demonstrated by detriments in muscle strength and endurance (time to maintain 70% MVC).
Twelve recreationally trained (at least 2 training sessions a week for the past 6 months) male (height 1.78 [+ or -] 0.05 cm, body mass 84.5 [+ or -] 7.5 kg, age 30.0 [+ or -] 8.5 yrs.) participants were recruited for this study. Eleven of the participants were determined to be right-leg dominant, while one participant was left-leg dominant as assessed with the Edinburgh inventory (Oldfield, 1971) and the leg used to kick a soccer ball. Prior to testing and after a brief explanation of the procedures of the experiment, each participant completed the Physical Activity Readiness Questionnaire-Plus (Canadian Society for Exercise Physiology, 2011) and read and signed a letter of informed consent. Volunteers who reported neurological complications, surgery or injury to knee structures or cardiovascular conditions such as high blood pressure were not allowed to participate in the experiment. To eliminate confounding variables, participants were instructed not to engage in strenuous physical activity and to abstain from alcohol consumption, caffeine or nicotine for the 24-hour period prior to participation. Testing was performed at similar times during the day to avoid diurnal variations. The Health Research Ethics Authority of the Memorial University of Newfoundland approved this research protocol.
A randomized cross over study design was used to examine the acute effects of localized unilateral knee extensor muscle fatigue on the performance of the contralateral homologous muscle (Figure 1). The participants were required to attend the lab on three separate occasions (separated by at least 48 hours) during which muscle performance (force and electromyography (EMG)) data were collected from both dominant and non-dominant knee extensors. Experimental sessions consisted of three testing sessions including control (no intervention), 40% (dominant leg fatigued by a dynamic knee extension protocol using a load equal to 40% of pretest MVC) and 70% (dominant leg fatigued by a dynamic knee extension protocol using a load equal to 70% of pretest MVC). The three experimental protocols were randomly selected for each experimental session. A series of submaximal and maximal isometric knee extensions were performed with the non-dominant and dominant-limb (fatigued limb) before and after the intervention protocols.
Participants were seated in the knee extension machine (Modular Leg Extension, Cybex International, Medway, MA, USA) with the hip and knee were fixed at 90[degrees] and 83[degrees] respectively. The knee angle was measured with a goniometer and was not equal to 90[degrees] because of the angle of the seat pan could not be adjusted. To eliminate upper body involvement, a strap was placed around the waist and participants were instructed to cross their hands across their chest. The dominant--and the non-dominant ankle were then inserted into padded ankle cuffs that were attached to strain gauges (Omega engineering Inc., LCCA 250, Don Mills, Ontario) via taut non-extensible straps. The strain gauge and the straps were secured to the isotonic leg extension machine via a custom-built apparatus (Technical Services Memorial University of Newfoundland) and were adjusted to form a 90[degrees] angle with the subject's lower shin (Figure 2). Differential voltage from the strain gauge, which was sampled at a rate of 2,000-Hz, was amplified (1000X), digitally converted (Biopac Systems Inc. DA 100 and analog to digital converter MP100WSW; Holliston, MA) and monitored on a computer. A commercial software program (AcqKnowledge III, Biopac Systems Inc., Holliston, MA) was used to analyze the digitally converted analog data.
After positioning on the knee extension machine, subjects performed a warm-up including two sets of 10 dynamic bilateral knee extensions with a load equivalent to approximately 30% subject's body mass. In addition, both the left and right legs performed five submaximal unilateral isometric knee extension contractions. Subjects were...