In a pioneering study Henry and Smith (1961) found that the total force generated in a maximal bilateral hand grip test was significantly reduced compared to the sum of the right and left maximal unilateral hand grip strength. In the last decades force differences between one- and two limb exercises have been reported for various maximal strength tasks (isometric elbow flexion and extension, isometric knee extension, leg press and vertical jumps) (Oda and Moritani, 1995; Koh et al.,1993; Schantz et al., 1989). This phenomenon is usually called bilateral force deficit (BFD) and defined as a reduction in the amount of force produced from bilateral movements of homonymous limbs compared to the sum of forces produced by the left and right limbs when acting alone (Sale, 1992). Depending on the exercise mode (static vs. dynamic, isolated vs. complex tasks), the extent of the BFD was reported to range between 7 to 25 % (Owings and Grabiner, 1998, Jakobi and Chilibeck, 2001). These varying magnitudes of the BFD are considered to be caused by methodological restrictions (randomization and testing procedures), sport-specific stroke patterns (bilateral or unilateral limb movements), training-induced changes of muscle length, muscle activation, muscle fiber types, the type of exercises (multi joint vs. single joint, dynamic vs. static) and motivational prerequisites (Howard and Enoka, 1991).
However, the underlying central as well as peripheral neuromuscular mechanisms leading to lower bilateral strength are still not fully understood. It has been proven that decreased activation of fast motor units (Koh et al., 1993; Secher et al., 1978), reciprocal inhibitions on spinal level and inter-hemisphere inhibition on supra-spinal level (Ohtsuki, 1994; Taniguchi, 1998) may account for the BFD. Interestingly, also volitional modulations have been hypothesized (Koh et al., 1993; Secher et al., 1988).
In this regard, Jakobi and Chilibeck (2001) as well as Secher et al. (Secher et al., 1988) discussed whether a priori achieved information on the theoretical basis of the BFD may influence its occurrence and magnitude. From a physiological point of view, expectations of the force output according to previously achieved information were reported to influence maximal voluntary force and force development (Sahaly et al., 2001). In this regard, sensorimotor feed-forwards related to the expected forceoutput have been frequently discussed (Blakemore et al., 1998; Diedrichsen et al., 2007). Thus, internal force prediction during strength tasks might rely on the provided visual information, afferent information and expectations. In summary, motor commands appear to be interfered with knowledge and imagery (Lorenzo et al., 2003).
As a consequence, the present study was conducted to examine a sample of non-specifically trained young adults in order to differentiate whether an absent, false and correct pre-information prior to force testing potentially affects the occurrence and magnitude of the BFD during a combined maximal isometric hip- and leg-extension (leg press). Based on the assumption that visual afferent information may provoke expectations of force outputs, we hypothesized that an inverse (false) instruction of the theoretical basis of the BFD might lower the extent of the BFD compared to experiments performed with correct pre-information. From a practical relevance view-point, a verification of this assumption would emphasize (a) the importance of correct instructions due to expected force-outputs (b) to consider the manipulative impact of acute instruction-based placebo/nocebo effects during strength testings.
Twenty trained young male adults (age: 24.5 [+ or -] 1.7 years; weight: 77.5 [+ or -] 7.1 kg; height: 1.81 [+ or -] 0.05 m, practiced sports disciplines: soccer, n=8; boxing, n=3; power-lifting, volleyball, basketball, swimming, tennis, gymnastics, track and field, each with n = 1; all-round sport, n = 2) were initially examined on a leg press slide (Figure 1) (Ertelt and Blickhan, 2009). None of the participants reported any medication intake and internal as well as orthopedic health impairments (hypertension, knee or hip injuries) that could affect maximal isometric strength testing. The local ethical committee of the University of Jena previously approved this strength measuring approach. No additional approval was needed concerning the present study. Furthermore, the study complied with the declaration of Helsinki and all participants signed an informed written consent prior to the start of the study.
The present study was conducted as a semi-randomized controlled cross-over trial. All participants were examined on three days within one week on Monday, Wednesday and Friday. Testing was intra-individually performed at the same time of day. On the first testing day, no...