If you consistently lift weights, there are 3 main types of adaptations your body goes through: Neuromuscular, Muscular & Connective Tissue adaptations.
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First, we have neuromuscular adaptations. Your body recognizes movement patterns and becomes more efficient as you perform certain movements consistently. Say, for example, you often practice the squat. Eventually, your nervous system adapts in a way that it can recruit more of your lower body muscle fibers and activate these fibers more frequently.
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Rapid gains in neuromuscular efficiency is oftentimes the reason why beginner trainees can gain strength relatively quickly without gaining much muscle [1]. But there's only a limited amount of strength you can gain without gaining actual muscle size. This brings us to the 2nd adaptation: muscle growth. As you consistently overload your muscles, eventually they'll increase in size [2].
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The last one is connective tissue (tissue that attaches muscle to bone) adaptations. Your tendons and ligaments strengthen if you lift weights properly. Overall bone health seems to increase as well [3]. This helps explain why there's good research showing that training with weights can relieve low-back pain and arthritis-related discomfort [4].
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All in all, lifting weights gives you more than just "muscle gains." You can also expect general improvements in your everyday movement [5].
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References:
1. Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training - PubMed
2. The role of resistance exercise intensity on muscle fibre adaptations - PubMed
3. Exercise and bone mass in adults - PubMed
4. Resistance training is medicine: effects of strength training on health - PubMed
5. Physical activity and public health: updated recommendation for adults from the American College of Sports Medicine and the American Heart Association - PubMed