Specificity and Transfer of Lower-Body Strength: Influence of Bilateral or Unilateral Lower-Body Resistance Training.
Level 2 - randomized trial
Randomized controlled trial in trained athletes
PubMed 30688873 · doi:10.1519/JSC.0000000000002923
What was done
Developmental rugby players (n = 33, training age 5.4 ± 2.9 years, baseline 1RM 90° squat 178 ± 27 kg) were randomized into three groups: bilateral training (back squat, n = 13), unilateral training (step-up, n = 10), or a comparison group (n = 10). Training lasted 8 weeks with 2 volume-load matched sessions per week (6–8 sets × 4–8 reps at 45–88% 1RM). Maximum strength was evaluated using 1RM back squat and 1RM step-up tests, analyzed via effect sizes (ES) with 90% confidence limits (CL).
What was found
Both interventions improved performance in their trained exercise: BIL back squat ES = 0.67 ± 0.48; UNI step-up ES = 0.74 ± 0.38. Both also transferred strength to the untrained exercise: BIL step-up ES = 0.27 ± 0.39; UNI back squat ES = 0.42 ± 0.39. Between-group differences in squat adaptation were unclear (BIL ES = -0.34 ± 0.55), whereas the unilateral group had a clear advantage in step-up strength adaptation (ES = 0.41 ± 0.36).
Why it matters
Unilateral resistance training yields strength adaptations comparable to traditional bilateral squats and transfers meaningfully to bilateral movement, providing a viable alternative when bilateral loading is restricted or contraindicated.
Limits
The total sample size was small (33 participants split across three groups), limiting statistical precision as reflected by wide 90% confidence limits. The cohort was restricted to well-trained male rugby players, limiting direct generalizability to untrained, female, or older populations. Athletic performance metrics such as sprinting or jumping transfer were not reported.