In young men who have not trained before, 8 weeks of weight training at moderate intensity increases power in the legs but not in the arms, no matter how the sets are arranged.
Mechanism
Synthesis from 1 study
Lifting weights makes your legs stronger much more than your arms, and since power comes from how hard you push, not just how fast you move, your legs get more powerful but your arms don't — even with the same training, because your arms just don't get strong enough to make a difference in power...
Most probable mechanism
When untrained men lift heavy weights, their legs get much stronger, which lets them push harder and faster during movements like squats, increasing power — but their arms don't get strong enough to do the same, so arm power doesn't improve even with the same training. This happens because the legs naturally respond to this training by getting significantly stronger, while the arms don't, and power depends more on how hard you can push than how fast you move.
Resistance training at 70–75% 1RM increases maximal strength (1RM) in lower-body muscles, such as the quadriceps and glutes, through neural adaptations including increased motor unit recruitment and corticospinal excitability, as well as muscle hypertrophy from mechanical tension and mTOR activation.
In contrast, upper-body muscles (e.g., pectoralis major, biceps brachii) show similar hypertrophy and neural adaptations but do not achieve the same magnitude of 1RM strength gains, likely due to lower baseline muscle mass, smaller motor unit pools, or reduced neural drive capacity.
The increase in lower-body maximal strength shifts the force-velocity relationship upward, allowing greater force production at the same contraction velocity during submaximal power tasks like the back squat at 75% 1RM, directly increasing power output.
Upper-body power output does not increase because the smaller gains in maximal strength are insufficient to shift the force-velocity curve meaningfully, and contraction velocity during bench press power testing remains unchanged despite training.
Evidence from Studies
Supporting (1)
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Effect of resistance training programs differing in set structure on muscular hypertrophy and performance in untrained young men
Contradicting (0)
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