Effects of unilateral vs. bilateral resistance training on lower body muscle strength and sport-specific performance in adolescent rugby union players

Abstract

Youth rugby players require multiple physical fitness including muscle strength, endurance, agility and acceleration. Resistance training (RT) which is often included as a part of rugby players’ training, helps improve young people’s strength and sport-specific performance. Traditionally, bilateral (BL) RT is more frequently implemented than unilateral (UL) RT. It has been argued that bilateral resistance training (BLRT) is superior to UL as it allows a greater absolute force production. However, this argument can be challenged by the ‘bilateral deficit (BLD)’ phenomenon, where the maximal voluntary strength of both limbs contracting simultaneously is less than the sum of the maximal voluntary strength of each limb contracting independently. Furthermore, many sporting movements are performed unilaterally or with weight transferred to one leg at a time, such as running, jumping, kicking and changing direction. Due to the greater specificity to sporting movements, unilateral resistance training (ULRT) may therefore be more desirable for enhancing some aspects of physical performance compared with BL. However, at present very few studies exist comparing the effectiveness of UL or BL training on adolescent population. Thus, the aim of this thesis was to investigate the effects of UL and BL RT on strength and athletic performance in adolescent rugby players. Study one investigated the between-day reliability and sensitivity of single leg press (SLP), leg press (LP), back squat (BS) and rear leg elevated split squat (RLESS). Study two and three investigated the effectiveness of a short-term RT using different UL and BLRT on lower body strength, countermovement jump (CMJ) height, linear sprint and change of direction performance. In study one, fifteen participants performed the tests on two occasions separated by 48 hours following three familiarisation sessions. This study found that after three familiarisation sessions, 5-repetition maximum (RM) SLP, LP, BS, RLESS indicated good or acceptable reliability with intra-class correlation coefficient ≥ 0.97 and % coefficient of variation ranges from 2.9- 5.9%. This study also assessed measurement sensitivity by comparing the smallest worthwhile change (SWC) and typical error (TE). The results indicated that 5-RM SLP and LP tests had good sensitivity (SWC > TE), 5-RM BS has acceptable sensitivity (SWC ~ TE), while the sensitivity for 5-RM RLESS test was rated marginal (SWC < TE). Study two employed a randomised block parallel pre-post measure design and reported that after 5 weeks SLP or LP training added on their habitual training, both training groups significantly increased 5-RM LP (UL =8.9%, d = 0.5; BL =10.9%, d =0.6) and 5-RM SLP (UL = 20.2 %, d = 0.8; BL = 12.4%, d = 0.45). There was no significant difference between the size of improvement in UL and BL groups for 5-RM LP assessments but 5-RM SLP increased significantly more in the UL group (p < .05). No significant training effects were found for CMJ height or linear sprint performance. With the similar study design, it was demonstrated that UL RLESS and single leg hip thrust (SLHT) was as effective as BL BS and hip thrust (HT) after 6 weeks training. Both training groups significantly improved 5-RM BS (UL = 11.8%, d = 1.5 , BL = 14.3%, d = 1.5), RLESS (UL = 19.1%, d = 2.5 , BL = 13.6%, d = 1.4), HT (UL = 14.5%, d = 1.1 , BL = 16.6%, d = 1.0), SLHT (UL = 23.6%, d = 1.5 , BL = 14.7%, d = 0.8), CMJ height (UL = 3.9%, d = 0.4 , BL = 3.5%, d = 0.5), 10-m sprint time (UL = -1.9%, d = 0.6 , BL = -2.1%, d = 0.7), and 30-m sprint time (UL = 1.8%, d = 0.6 , BL = 1.6%, d = 0.5). There was no significant difference between the size of improvement in UL and BL groups. However, the results indicated that the effect size of UL training for UL strength improvement was considerably greater compared with BL training. No effect of training protocols were found for pro-agility performance, for either the UL or the BL group. This thesis found 5-RM ULLP, 5-RM LP and 5-RM BS assessments to be reliable and sensitive while the 5-RM RLESS was reliable, however less sensitive for adolescent rugby players. Five-weeks of SLP training was as effective as BL LP training in improving BL strength and more effective in improving UL strength in adolescent rugby players. However, the strength improvement did not transfer to athletic performance improvements including CMJ and linear sprint in either group. Groups trained with ULRT (RLESS+SLHT) and BLRT (BS+HT) both significantly improved lower body UL and BL strength, power, and linear sprint performance to a similar extent. This thesis indicated that lower body ULRT may be considered as an effective alternative to BLRT for improving strength and athletic performance in adolescent rugby players. Furthermore, ULRT cuts down external loads may benefit for those who suffer from back injuries. This thesis has provided robust rationale for the inclusion of ULRT for adolescent rugby players. Future work should focus on comparing other UL and BL exercise beyond LP, BS and HT