Isometric contractions
Howard and Enoka (1991) identified the presence of the BLD during isometric knee extensions. The study consisted of three groups: untrained individuals, cyclists and weightlifters. Subjects performed maximal one- and two- leg isometric knee extensions at 71°. A BLD was reported in untrained individuals (p=0.015; -9.5%), while no BLD was reported in cyclists (p=0.072; -6.6%) due to the BI not being significantly different to zero. A BLF was also reported in weightlifters (p=0.017; +6.2%). This indicates that athletes who take part in bilateral sports, such as weightlifting are able to produce greater force using two legs, compared to the sum of the left and right leg combined. A BLF has also been documented in rowers (Secher, 1975), however, no BI was reported. This BLF may relate to task familiarity, as a result of weightlifters and rowers being accustomed to bilateral movements.
Additionally, research from Kuruganti, Murphy and Pardy, (2011) investigated the BLD in relation to three joint angles: 0°, 45° and 90°. Recreationally active young subjects were recruited and performed maximal isometric knee extensions. A BLD in peak torque was shown at 45° (-23.4%), however no BLD was shown at 0° (0%) or 90° (0%). It is possible that 45° was an optimal angle for producing maximal force, based on length-tension relationships (Becker and Awiszus, 2001; Kubo et al., 2004). Further research is warranted to examine motor unit recruitment and changes in reflex excitability during isometric knee extensions at varying joint angles.
In contrast to Kuruganti, Murphy and Pardy, (2011), research conducted by Owings and Grabiner’s (1998b) found a BLD at both 45° (-11.1 to -12.9%) and 90° (-6.5 to -8.9%), thus contradicting previous findings. The differences between these studies could be a result of Kuruganti, Murphy and Pardy (2011) recruiting young athletic men and the latter using older participants. In line with the mechanisms previously mentioned, it is possible that older people are less capable in recruiting high threshold motor units due to age related decreases in fast twitch fibre types and strength. However, research conducted by Owings and Grabiner (1998a) investigated the BLD in old adults, during slowly developed maximum voluntary isometric contractions (MVC). The results demonstrated a BLD (p < 0.05; -6.5 to -12.9%), despite the old age of participants. Therefore, this contradicts the suggestion that older people are less capable of recruiting high threshold motor units. In fact, the BI of -6.5 to -12.9% is similar to the BI shown in younger participants (Botton et al., 2013; 2016; Howard and Enoka, 1991; Kuruganti et al., 2011; Owings and Grbaniner, 1998b).
Furthermore, research conducted by Botton et al. (2013) compared the BLD between isometric contractions and concentric contractions of the knee extensors. Physically active males performed isokinetic concentric knee extensions (60°/s) and isometric knee extensions at 60°. Results indicated that both isokinetic and isometric conditions demonstrated superior peak torque production during unilateral performance compared to bilateral (p < 0.05). Also, the BLD between the two conditions were not significantly different (p > 0.05), with a BI of -9.6% for concentric contractions and -9.7% for isometric contractions. This suggests a similar magnitude between isometric and concentric (dynamic) knee extension. However, later research conducted by Botton et al. (2016) identified a BLD during isometric peak torque (-10.5 to -13.8%) but not during dynamic contractions under similar conditions. Despite using males for the first study and females for the latter, results remain consistent, suggesting that gender does not contribute to the presence of the BLD during isometric contractions.
In order to achieve greater consistency, a standardised approach relating to knee angles should be adopted to enhance reliability. Similarly, participants of similar ages, gender and abilities should be compared to further reinforce this. In addition, the majority of literature has investigated the BLD under isometric contractions during concentric knee extension (see table 2). Therefore, further research is warranted to identify is prevalence during different movement types.
4.2 Dynamic contractions
The BLD has been reported consistently in dynamic contractions (concentric, eccentric and isokinetic) (See table 3). Research conducted by Dickin and Too (2006) examined the effects of six movement velocities (30, 60, 90, 120, 150 and 180°/seconds(s) and maximal concentric and eccentric actions on the BLD. A significant difference (p<0.025) was reported between bilateral actions and the sum of unilateral contractions during concentric and eccentric knee extensions, at all velocities. Post-hoc analysis highlighted that increased velocity was associated with a larger BLD, which increased from -18 to -25%. Similarly, Vandervoort et al. (1984) recruited nine physical education students and examined the magnitude of the BLD over ten movement velocities (0°/s to 424°/s). Subjects performed an isokinetic combined hip and knee extension (leg press) with peak torque being used to calculate the BLD. Results showed a BLD deficit of -9% during 0°/sec, which rose to -49% during 424°/sec. These studies suggest that velocity influences the magnitude of the BLD.
However, research relating to the BLD and velocity has shown conflicting results. Brown et al. (1994) investigated the effect of velocity on the BLD in untrained women. Subjects performed unilateral and bilateral concentric knee extension and flexion repetitions at isokinetic speeds of 60, 120, 180, 240 and 360°/s. A BLD was reported for knee extension peak torque (-1 to -12%) at speeds of 60°/s to 240°/s, but not at 360°/s. Similarly, a BLD was present in flexion peak torque at speeds of 60°/s to 240°/s, but not at 360°/s, indicating greater peak torque outputs during bilateral performance. Based on these results, it has been suggested that the BLD may occur as a result of reduced slow twitch muscle fibre activation (Brown, et al., 1994). However, it is worth noting that the velocities used to conduct these experiments could be regarded as slow when compared to sport specific movements such as sprinting, which require far greater levels of movement velocity. Therefore, further research should investigate the BLD during sport-specific movements.
Furthermore, the BLD has been consistently observed during dynamic concentric knee extension. Botton et al. (2013) identified a BLD of -9.6% as mentioned previously. To support this, Costa et al. (2015) also reported a BLD of -11% during one repetition max knee extensions comparing bilateral and unilateral (right + left) contractions.
However, Janzen et al., (2006) was unable to report a BLD during concentric knee extension in post-menopausal women. Subjects performed the leg press, knee extension, and lat pull-down. A BLD was reported in the leg press (p<0.001; -12.7%) and lat pull down (p<0.001; -8.8%), but not during the knee extension, with participants displaying a BLD that was not significantly different from zero (p=0.095). It has been suggested that this may occur as a result of differences in postural stability. To support this notion, Magnus and Farthing (2008) investigated the contribution of postural stability requirements in relation to the BLD. They compared a multi-joint movement (leg press) and a single-joint movement (handgrip), which are exercises with big and small postural stability requirements. Results indicated that the BLD only occurred during the leg press (-12.1%), but not the handgrip. The leg extension is an exercise that is also categorised as an exercise with lower postural stability requirements (Škarabot, Strojnik and Avela, 2016), thus supporting the hypothesis that postural stability may influence the magnitude of BLD. Therefore, it is essential that all future research should try to minimise postural stabilisation and also report the exact position of participants, to ensure similar methodologies are being conducted. Another confounding factor that may influence the expression of the BLD is the type of dynamometer used. If the dynamometer allows counterbalancing of movement to occur, it is possible that torque in other parts of the body not being studied could potentially impact the net torque of the body part being research (Simoneau-Buessinger at al., 2015). Therefore, the use of a dynamometer that restricts body movement may be useful and positively impact the consistency and reliability of research.
Furthermore, research has demonstrated a BLD during dynamic upper-body contractions. Taniguchi (1997) recruited male and student females to perform isokinetic (80°/s) hip and knee extensions and arm extensions and were reassessed following a training intervention. A BLD was reported for the hip and knee extension (-(6.5 to -18.6%), as well as arm extension (-7.2 to -9.6%). Under similar conditions, later research conducted by Taniguchi (1998) reported a BLD for the hip and knee extension (-0.5 to -15.3%), as well as arm extension (-3.7 to -11.8%). These BLD percentages are also similar to arm flexion, as reported by Janzen et al. (2006). Based on the research provided, the BLD is generally much less in the upper body, in comparison to the lower body (see table 3).
Overall, the BLD has been reported consistently during dynamic contractions during concentric and eccentric contractions. The available literature also demonstrates a higher BLD during isokinetic contractions, particularly at higher velocities (See table 3), though inconsistencies are present (Brown et al., 1994). In order to establish clarity amongst the current literature, future research should aim to conduct research using a variety of upper body and lower body movements. Future research should also aim to include a larger sample size, with participants of a similar age and ability. Additionally, in order to enhance reliability, factors such as equipment, positioning and movement compensations should be reported and considered.
4.3 Explosive/Ballistic contractions
The BLD has also been shown in ballistic actions, such as jumping. The general consensus is that the force production of the left and right leg combined are often superior to two-legged performance (Bračič et al., 2010; Buckthorpe et al., 2013; Pain, 2014; Rejc et al., 2010, 2015; Samozino et al., 2014; Veligekas and Bogdanis, 2013).
Research conducted by Bračič et al. (2010) investigated the BLD in elite sprinters and examined the relationship between the BLD during the countermovement jump (CMJ) and sprint start performance. With the use of force plate, participants were required to perform bilateral and unilateral CMJs, in addition to sprint starts off of blocks. Results showed a BLD average of -19% during the CMJ. Results indicated that a lower BLD in the CMJ related to higher peak force production (r=-.630; p=.000) and higher total impulse of force on the blocks (r=-.550; p=.000). Based on these moderate relationships, the CMJ is arguably a good indicator of a lower performance in the sprint start. However, further research is warranted in order to identify the best method of measuring the BLD and its subsequent impact on performance.
In contrast to this, Veligekas and Bogdanis (2013) investigated BLD differences in pre-pubertal males and females (aged 10 to 12), during the CMJ. Participants performed unilateral and bilateral CMJs without arm swing on a contact mat. Results reported a BLD that was not significantly to zero in females (-1.4%), indicating no clear difference between the sum of single-legged jumps and two-legged jumps. However, a BLF (7 to 13%) was observed in males, indicating greater two-legged jumps, compared to single legged jumps combined. However, differences between the aforementioned study and this are likely to have occurred as a result of pre-pubertal children having a reduced capacity to recruit motor units, in line with the proposed mechanism. These findings suggest that the BLD may be age dependant. Therefore, it may be more applicable to test young and older adults.
In addition, the BLD has also been observed during the drop jump. Pain (2014) examined the BLD by measuring peak concentric force and peak power in elite power and endurance athletes. Participants performed single and double leg drop jumps from a range of heights (15cm, 30cm and 60cm). Results demonstrated a range in BLD scores, with athletes showing a BI ranging from -16.8 to -35.5. It was also reported that power athletes had a significantly greater (p<0.05) BLD for jump height and peak power, compared to endurance athletes. It is possible that this difference is due to power athletes being better able to recruit fast twitch muscle fibres. Also, it is likely that power athletes more accustomed to performing high intensity plyometric tasks compared to endurance athletes, despite both groups being familiarised with the drop jumps prior to participation.
Furthermore, Buckthorpe et al. (2013) investigated the BLD during explosive contractions of the knee extensors. Physically active participants performed maximal explosive knee extensions bilaterally and unilaterally. Rate of force development (RFD) and explosive force were monitored over various time points (0–50, 50–100 and 100–150 ms). Results indicated a BLD for explosive force at 100 ms (p=0.007; 11.2%). A BLD in RFD was also reported at 50–100 ms (p = 0.004; 14.9%), but not for other time points. It has been suggested that the BLD may have occurred as a result of differences in neuromuscular activation of agonist, antagonist and stabiliser muscles, during bilateral and unilateral explosive contractions (Buckthorpe et al., 2013; Simoneau-Buessinger et al., 2015). Further research should aim to quantify how much variance in the BLD can be attributed to activation of muscle groups not being assessed.
Lastly, research conducted by Rejc et al. (2010), Rejc et al. (2015) and Samozino et al. (2014) investigated the presence of the BLD in force during maximal explosive hip and knee extension on a sledge ergometer. Rejc et al., (2010) reported a BLD of -30.5%, Rejc et al. (2015) reported a BLD of -18.1% and Samozino et al. (2014) reported a BLD of -36.7%. These results indicate that the BLD is higher in explosive hip and knee extension, compared to other movements shown in the literature (See table 4). However, the reason for this is unclear. It could be suggested that task-related mechanisms and individual factors may contribute to this.
Research pertaining to the BLD has consistently shown a BLD during ballistic contractions, with the majority of literature displaying a BI ranging from -11 to -35%. Only research using pre-pubertal children has shown a BLF (Veligeskas and Bogdanis). However, if athlete populations were recruited for these studies it could be argued that results would be more varied. For example, a BLF has been shown in sport performers who are accustomed to bilateral tasks (Howard and Enoka, 1991), though this was in relation to isometric contractions. Therefore, future research should aim to test athlete population groups from a range of sporting backgrounds under various contraction models.
It is also worth noting that the standing broad jump (SBJ) has not been utilised within the literature, possibly due to jump distance being the only metric that can be measured. Additionally, the SBJ is far more difficult to standardise, compared to other ballistic tasks. For example, arm swing during the SBJ can alter jump distance by up to 21.2% (Ashby and Heegaard, 2002). Also, if hands are placed on the hips for the SBJ, this is also far more restrictive in terms of movement due to the requirements to produce force both horizontally and vertically (Ashby and Heegaard, 2002).
Based on the available literature, the magnitude of the BLD is greatest during lower body dynamic contractions, in comparison to upper body. The literature also suggests that presence of the BLD increases during high velocity contractions. Furthermore. isometric contractions display a much smaller BLD compared to dynamic contractions, possibly due to differences in postural stabilization requirements and ability to use counterbalances. The BLD is also consistently observed in ballistic movements, however the mechanisms may differ in comparison to the other contraction types and may relate to F-v mechanisms (Samozino et al. 2014). In order to better understand the presence of the BLD in relation to contraction types, future research needs to ensure that the BI is reported, to enable comparisons to be made. In addition, the association of the BLD in relation to sport-specific movements needs to be further explored.
5.0 Directions for future research
5.1 Sport performance
Due to the paucity of appropriate data, future research should aim to include athlete population groups, in order to determine the relationship between the BLD and sport performance. This would provide coaches and practitioners with a greater insight into whether they should predominately incorporate unilateral or bilateral training when programming for athletes. Based on the principle of specificity, it would seem prudent to suggest that individuals participating in bilateral dominant sports, such as powerlifting should predominately train using bilateral exercises such as the back squat. Therefore, sports that are unilateral in nature or require a unilateral weight distribution, such as football, may emphasise exercises that are unilateral (Santana, 2001), such as the rear foot elevated split squat. However further clarity is needed to identify optimal training methods in relation to the BLD phenomenon.
To date, the only investigation that has attempted to make an association between these factors was conducted by Bračič et al. (2010), who investigated the relationship between the CMJ and sprint-start performance in elite-level sprinters. The results indicated that lower BLD scores were correlated with greater peak force production of the rear leg during the sprint start, as well as a higher total force impulse on the starting blocks. While the results of this research fail to account for other factors that may have influenced sprint start performance, the data does indicate that sprinters who have lower BLD are better able to produce greater power and accelerate more effectively, thus potentially improving sprint performance over 60 and 100 meters. Based on the research findings of Bračič et al. (2010), it would be prudent to suggest that implementing training strategies to minimise the BLD would subsequently improve sport performance, particularly in sprinters or sports that require good acceleration and block push-off, such as bobsleigh. Research has shown that placing an emphasis on bilateral training can minimise BLD, whilst unilateral training increases it (Weir et al. 1995; Häkkinen et al. 1996; Taniguchi 1997, 1998; Kuruganti et al. 2005; Janzen et al. 2006; Beurskens et al. 2015). However, the magnitude of this change due to training remains unclear. In order to extend this literature, future research is needed to examine the effects of the BLD in relation to a variety of athlete populations, in order to determine the importance of the phenomenon in relation to sport-specificity.
5.2 Injury
To the authors’ knowledge, the relationship between injury and/or injury prediction and BLD has not been researched. It is commonly recognised that one of the most significant factors that contributes to the prevention of injury is strength (Lauersen, Bertelsen and Andersen, 2013). Research conducted by Hernandez et al. (2003) showed a larger BLD in older adults, compared to younger ones, which was considered to be a direct result in strength differences. Although Hernandez et al., (2003) did not directly research the BLD in relation to injury, they suggested that a large BLD in older adults could increase injury risk during everyday activities, as a result of reduced strength. However, everyday activities are not typically maximal in nature and the presence of the BLD is a product of maximal force production. Despite no research identifying an association between injury and the BLD, the potential risk should not be overlooked.
The available literature has demonstrated that unilateral training increases the BLD, whilst bilateral training reduces it in as little as 6-12 weeks (Häkkinen et al., 1996; Taniguchi, 1997). Therefore, if the objective is to minimise the BLD and prevent injury, as suggested by Hernandez et al., (2003), then bilateral training is preferential. In contrast to this, Howe et al., (2014) examined the beneficial effects of unilateral training, based on the presence of the asymmetries and the BLD. It was suggested that single leg exercises increase the recruitment of muscles that provide joint stability, that may also control undesired compensatory movements. It was also stated that unilateral work can help to correct muscular imbalances and asymmetries between limbs. However, the BLD in relation to asymmetries has not been researched and therefore needs to be investigated. Ultimately, future research should aim to explore the BLD in relation to injury risk and prediction in athletic population groups, to gain a more profound insight into the phenomenon and whether or not it is beneficial to minimise or facilitate its presence.
Lastly, no study to date has analysed the longitudinal changes in BLD (e.g. athletes over the course of a season). Therefore, future research should aim to analyse these changes over various time points (pre, mid and post-season), as oppose to a singular time-points. This would allow for comparisons between BLD, performance and injury over long periods. It should not be assumed that the BLD will remain consistent over time and therefore may subsequently impact the programming of training sessions for athletes.
6.0 Conclusion
The vast majority of literature has demonstrated a BLD across a variety of contraction types and is therefore not contraction specific, though findings present some inconsistencies. It may also be assumed that athletes performing in sports that are bilateral in nature (e.g. powerlifting, weightlifting and skiing) are more likely to display a BLF, while unilateral sport performers (e.g. football, rugby, sprinting) are more likely to display a deficit (Howard and Enoka, 1991). Despite the lack of literature surrounding the topic area, the BLD may have significant practical implications with regards to improving sport-specific performance (Bračič et al., 2010) and minimising the risk of injury (Hernandez et al.,2003). The present systematic review identifies the complexity of the BLD and suggests further research to be conducted in order to identify its relationships with measures of physical/sports performance and injury.
Funding
No sources of funding were provided to aid the preparation