Adding leg training did not cost any arm growth
In short
One hundred five untrained participants completed 18 supervised sessions over six weeks of low-load elbow flexion (2 sets at 30% 1RM to failure); one group added four sets of knee extensions per leg (20-30RM). Anterior upper-arm muscle thickness increased 0.18 cm with arm training alone and 0.19 cm with legs added, and 1RM gains were 0.32 kg and 0.25 kg respectively. Under uncontrolled eating conditions, activating substantially more muscle mass did not compromise growth elsewhere.
The worry that leg day siphons off the 'growth resources' your upper body needs is one of the longest-lived theories in the gym. This trial, published in the Journal of Strength and Conditioning Research, measured it head on. After six weeks, the difference in arm growth was 0.01 cm.
How was the trial set up?
105 untrained participants were split three ways: unilateral elbow flexion to failure only (n = 36), the same arm training plus four sets of knee extensions per leg (n = 33), and a non-exercising control (n = 36). Training groups completed 18 supervised sessions over six weeks, with the arm work done as 2 sets at 30% of 1RM to failure and the leg work at 20-30RM.
How close were the results?
Anterior upper-arm muscle thickness was measured by ultrasound and elbow-flexor 1RM tested before and after. Thickness increased 0.18 cm with arm training alone and 0.19 cm with leg work added; 1RM rose 0.32 kg and 0.25 kg. Bayesian analysis found no support for the competition hypothesis — if anything, the group doing extra leg work grew fractionally more.
Why don't muscles compete?
The trial did not probe mechanisms, but the result fits a picture in which growth signalling is local to each trained muscle rather than a whole-body resource allocation problem — a theme covered further in mTORC1 is not a volume knob. The authors leave one door open: whether much higher training volumes or restricted nutrient intake could create competition remains untested.
What this changes for the Big 3
Muscle Index is built from the sum of squat, bench press, and deadlift 1RM — two of the three lifts are legs and posterior chain. Skipping leg training to protect your bench loses its justification here: the arms grew just as well alongside leg work, and two-thirds of the score never moves without your legs anyway. For how to divide your training budget, see volume versus intensity.
Participants were untrained, the exercises were two low-load single-joint movements, and the block lasted six weeks. The authors themselves note that highly trained lifters on high-volume programs, or people in a restricted-nutrition phase such as a hard diet, may not get the same answer.
Frequently asked questions
Does leg training interfere with upper-body muscle growth?
Not in this trial. Across 105 untrained participants training for six weeks, arm muscle thickness grew 0.18 cm with arm training alone and 0.19 cm when four sets of leg work were added — effectively identical.
What is the theory that muscles compete for growth?
The idea that recruiting more total muscle mass spreads a finite pool of recovery and muscle-building resources thinner, shrinking growth per region. This trial found no evidence of such competition under normal eating conditions.
Could competition appear under other conditions?
The authors flag two untested scenarios: much higher training volumes and limited nutrient intake. This trial used two low-load exercises and let participants eat freely, so those cases remain open questions.
Does this apply to experienced lifters?
The participants were 105 untrained individuals. The study offers no direct evidence about trained lifters running high-volume programs.
Source: PubMed