The effect of static stretching (SS) on the neuromuscular system has been attributed to both central and peripheral mechanisms. These performance (force and power) reductions can originate from neurophysiological (i.e. mechanoreceptors of the skin, muscle and joint proprioception), hormonal, cellular (structural changes such as titin), or mechanical (i.e. stiffness, torque-length characteristics) factors (Avela et al., 2004; Behm et al., 2004; Behm and Chaouachi, 2011; Pacheco et al., 2011; Rubini et al., 2007; Serpa et al., 2014; Wallmann et al., 2005), and in some studies, it might persist for over several hours post-stretch (Brandenburg et al., 2007; Fowles et al., 2000; Haddad et al., 2014; Power et al., 2004).
The majority of these studies focus on how a specific stretching protocol affects the exercised muscle(s). On the other hand, there is evidence that the contralateral untrained limb might be affected by the trained limb, known as cross-over effect (Farthing, 2009; Zhou, 2000). Several articles have reported cross-over effects for different exercise conditions such as fatigue (Doix et al., 2013; Marchetti and Uchida, 2011; Rattey et al., 2006; Regueme et al., 2007; Strang et al., 2009; Todd et al., 2003), and force/power (Carroll et al., 2006; Farthing et al., 2005; Lee and Carroll, 2007; Sariyildiz et al., 2011; Shima et al., 2002), however, few articles have examined the cross-over effect after SS routines (Lima et al., 2014; Nelson et al., 2012). Nelson, et al. (2012) analyzed 10-week stretching program (4 times for 30s, with 30s rest, 3 d x [wk.sup.-1]). The results indicated an increase in strength (1RM) and ROM for both legs (stretched and nonstreched limb), where the strength gain of the nonstretched leg was 56% of the stretched leg. Lima et al. (2014) investigated the acute effects of unilateral ankle plantar flexors SS (6 sets of 45s/15s, 70-90% point of discomfort (POD)) on surface electromyography (sEMG) and the center of pressure (COP) during a single-leg balance task in both lower limbs. The results indicated no differences on non-stretched limb for all dependent variables when compared to stretched limb.
The investigation of cross-over effect is mainly limited to assessing how the stretching protocols of an ipsilateral muscle affects the performance of the contralateral homologous muscle. There is less research investigating possible non-localized effects of upper-body on lower-body performance or vice versa. This type of question addresses whether fatigue is specific to working muscles or is it more of a systemic response. Previous studies have shown evidence for the existence of neural coupling between upper and lower-limb neural networks which may modulate the muscle activation, reflexes and coordination of muscles (Huang and Ferris, 2004; 2009). Considering the neural coupling between upper and lower-limb, some articles have reported this effect for fatigue conditions (Kennedy et al., 2013; Takahashi et al., 2011). Halperin et al. (2014) in two separate papers reported that fatigued elbow flexors impaired force and activation of contralateral knee extensors while in the second paper, knee extension fatigue caused decrements in the ability to perform repeated maximal voluntary contractions of the elbow flexors. Neural coupling could be facilitated by group III and IV muscle afferents (Martin et al., 2008). This feedback loop from muscle afferents can provide an inhibitory effect to the central nervous system leading to systemic decrements in the central drive to exercised and non-exercised muscles (Martin et al., 2008). Gandevia (2001) provides extensive evidence of supraspinal inhibition concluding that muscle feedback concerning biochemical and force-generating status can impair cortical sites. Long loop reflexes from muscle spindle afferents (Marsden et al., 1983) as well as skin and subcutaneous afferents (Corden et al., 2000) can influence cortical activation of the exercised and non-exercised remote muscles (Kagamihara et al., 2003). Hence, it would be important to ascertain the non-local muscle effects of stretching the upper body on lower body performance.
However, there is no scientific evidence for acute stretching-induced crosover or non-local muscle fatigue effects. Therefore, the purpose of the present study was to evaluate the acute effects of an upper limb SS protocol on the maximal concentric jump performance. It was hypothesized that extensive upper body SS would impair lower body jump performance.
Based on a statistical power analysis derived from the maximum jump height data from a pilot study, fifteen subjects would be necessary to achieve an alpha level of 0.05 and a power (1-P) of 0.80 (Eng, 2003). Therefore, we recruited 25 young healthy, resistance trained individuals (stretched group: 15 males, age: 23 [+ or -] 5 years, height: 1.78 [+ or -] 0.08 m, and weight: 74.4 [+ or -] 13kg and control group: 10 males, age: 22 [+ or -] 6 years, height: 1.79 [+ or -] 0.07 m, and weight: 74 [+ or -] 15kg) to participate in this study. Individuals who participated in this study had no previous surgery on both upper and lower extremities, no history of injury with residual symptoms (pain, "giving-away" sensations). All the participants signed an informed consent form after receiving instruction on the experimental protocol of the research. This study was approved by the University research ethics committee (#35/2014).
Prior to the data collection, the subjects were asked to identify the preferred leg for kicking a ball, which was then considered the dominant leg (Maulder and Cronin, 2005), and all subjects were right-arm dominant based on their preferred arm to write. All subjects were right leg and arm dominant. The experimental protocol consisted of: (1) a brief sub-maximal jumping warm-up for 5 minutes (20 jumps, 4 jumps with a rest interval of 45 seconds); (2) concentric jump task familiarization; (3) a pre-stretching evaluation of the upper limb (passive ROM); (4) three trials of maximal concentric jump task; (3) shoulder joint SS protocol; (4) immediately post-stretching evaluation of the upper limb (passive ROM); and (5) three trials of maximal concentric jump task. The control group followed the same procedures as the stretched group, except for the shoulder joint SS protocol.
The control group remained seated during the same time interval that the stretched group performed the stretching protocol. All measures were performed by the same researcher and at the same hour of the day, between 9 and 12 AM.
Concentric jumping task (CJ): The subjects initially stood on the force plate (Biomec410, EMG System do Brasil, Sao Jose dos Campos, Brazil) with knee flexed to 90[degrees], with the hands placed on the hips. The participant then held the...