Strength training, also known as resistance training or weight training, is a critical part of any fitness routine. At its most basic, resistance training is a series of movements that require a muscle to lift, push, or pull until fatigued. These movements not only benefit gains in the gym and training for competitive sport, but they also provide indisputable benefits to overall mental and physical health.
What is strength training?
Strength training builds muscular fitness by working muscles against a form of resistance.  This resistance is achieved through free weights, weight machines, resistance bands, and/or bodyweight exercises.
A weight training routine should include exercises that target all major muscle groups: chest, back, shoulders, biceps, triceps, abdomen, quadriceps, hamstrings, and glutes. Total body training refers to targeting all muscle groups during the same training session, whereas split-body training focuses on a set of muscles in one session and the remaining muscle groups in another (e.g. upper versus lower body sessions). Both total and split-body training provide equal health benefits. 
Strength training techniques
There are three types of strength training techniques that build muscular fitness.  All are important and contribute to optimal health, but the best combination and training frequency will vary depending on your unique goals.
This type of training focuses on the muscle's ability to generate force. The stronger the muscle, the more force it generates, and the more weight it can move. Lifting heavy weights at low repetitions—the number of times each exercise is performed—increases muscular strength and builds muscle mass.
Muscular power refers to a muscle's ability to generate explosive force and move weight with speed. Lifting light-to-moderate weights for three to six repetitions while focusing on explosive force and speed within the movement improves muscular power.  Movements like jumping, sprinting, Olympic lifts, and everyday activities like climbing the stairs and standing from a seated position all rely on muscular power.
Endurance refers to the ability to stay active for prolonged periods. So training for muscular endurance refers to working that muscle for a sustained duration. Typically, this results from lifting light weights or using body weight with little to no rest for 15-20 repetitions.  Muscular endurance improves endurance-based aerobic activities like long-distance running, swimming, and rowing, in addition to aiding in good posture.
The science-backed benefits of resistance training
The benefits of strength training go beyond improvements in overall fitness. Regular resistance exercise also supports a healthy heart, strong bones, a sharp mind, and efficient metabolic function, as well as reduces the risk of numerous chronic diseases.
1. Improves body composition
Strength training reduces body fat while simultaneously building lean muscle mass. [3,4,5,6] A recent study even showed that those who consistently engage in resistance training at least two days a week are 30% less likely to gain excess body fat over time. 
Elevated levels of body fat significantly increase the risk of chronic diseases including, metabolic syndrome, cardiovascular disease, diabetes, and certain cancers, whereas lean mass is known to play a central role in reducing the risk of these chronic conditions. [8,9]
2. Helps manage blood glucose
Higher levels of muscle mass are associated with better blood sugar control (indicated by having lower levels of HbA1c) and a reduced likelihood of developing type 2 diabetes. [10,11,12]
Glucose is the primary source of energy for muscles—the more muscles are required to work, the more glucose the body will pull from the bloodstream and send to the muscle cells for fuel. This lowers blood sugar during training. But, muscle mass is also more metabolically active than fat cells at rest and can uptake more glucose in response to insulin even when you're not actively exercising.
Insulin is a hormone the pancreas releases when blood glucose is elevated. And muscle mass is directly associated with insulin sensitivity, the efficiency at which cells and muscles uptake glucose from the bloodstream.  Insulin sensitivity is essential for achieving and maintaining optimal blood sugar control.
3. Reduces levels of chronic inflammation
The presence of internal inflammation for prolonged periods can accelerate the aging process and is associated with conditions such as metabolic syndrome, cardiovascular disease, and diabetes. Inflammation can be measured through the blood biomarker hsCRP.
Studies show that those who participate in resistance training consistently have significantly lower levels of hsCRP compared to those who do not. [13,14] When muscle cells are strong and are regularly engaged, they release compounds that help to control and manage the body's long-term inflammatory response.
It’s important to note that exercises like strength training can lead to acute, or short-term, inflammation that is then resolved with proper rest and recovery. Read more about that relationship here.
4. Increases strength, power, and endurance
To build up a muscle's strength, power, and endurance, the muscle must first be damaged. And the most effective way to do this is through strength training. Strength training creates tiny tears in muscle fibers and breaks apart muscle cells. When the body repairs that damage, the muscles then grow back stronger. Repeatedly training the muscle in this capacity—coupled with rest periods and adequate intake of dietary carbohydrates and protein—results in functional and structural adaptations that produce increases in strength, power, and endurance.[2,15]
5. Enhances flexibility
Maximizing gains in strength, power, and endurance requires a certain level of flexibility. Surprisingly enough, strength training can be as effective as stretching for increasing flexibility and range of motion. [16,17] One possible explanation? Muscle weakness can actually result in reduced flexibility and range of motion.
6. Sharpens brain health
Resistance training sharpens brain health by promoting the growth of new brain cells, increasing blood flow to the brain, maintaining synaptic plasticity, which is crucial for memory and protecting existing brain cells from inflammation induced damage.
At least two weight training sessions per week may significantly improve general cognitive function, executive function, and working memory—particularly in older adults. [18,19] As cognition declines with age, those who incorporated resistance training three times a week for at least 45-60 minutes showed even greater improvements in markers of cognitive function. [20,21]
7. Maintains bone and muscle health
Bone and muscle mass naturally decline with age, but engaging in strength straining can help reduce the risk of falls, the subsequent risk of hospitalization or early death from falls, and fractures. In addition, healthy bone and muscle mass preserve a person's ability to perform day-to-day activities with ease. 
Just as strength training stimulates muscle growth by first breaking down the muscle and sending a signal to the body to rebuild it, weight-bearing exercises place stress bones. That stress stimulates the production of new bone cells, strengthening existing bone mass and density. 
8. Promotes heart health
Pairing aerobic activities that get the heart and lungs pumping with muscle-building activities supports optimal heart health. Adding as few as one 60-minute resistance training session has shown to significantly decrease the risk of cardiovascular disease, improve cholesterol levels, lower blood pressure, reduce BMI, and increase longevity. [6, 24-27]
Maximizing strength training benefits
The American College of Sports Medicine recommends adults include at least two non-consecutive days a week of strength training along with a minimum of 150-minutes of aerobic activity a week.  Check out this article to learn how to integrate both activities into your routine.
And more strength training isn’t always better. Two to three sessions per week may be sufficient for most people. Excessive soreness, poor sleep, lack of energy, and plateaus in training are all signs you’re putting too much stress on your body, and your fitness routine may be doing more harm than good. This overtraining can lead to muscle damage rather than muscle growth (hypertrophy).
Incorporating adequate rest between training sessions, fueling the body properly, and hydrating effectively all contribute to getting the most from each training session. This guide to gaining muscle elaborates on some of these key factors, and the biomarkers associated with building muscle.
Don't know where to start?
Strength training is beneficial for everyone, regardless of age, sex, or prior experience. However, it’s a personalized practice, and your ideal training plan will depend on your unique goals and baseline fitness level. A fitness professional or personalized health analytics system like InsideTracker can help you determine where to start and best track and evaluate the results of the program you implement. Because even the most basic strength routine—no fancy equipment or gym needed—can pay dividends to overall health.
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