Gravity is a measure of how fast an object accelerates toward the center of the planet. Gravity on Mars is much lower on Mars than on Earth ( (3.721 m/s2 vs Earth’s gravity 9.807 m/s2). The effects of low gravity on humans are two-fold:

- direct biological effects of low gravity
- indirect effects of low gravity on human activities

### Direct Biological Effects
Generally, the moderately low gravity on Mars will result in changes in a number of important functions: 

- muscle bulk
- bone thickness and density
- inner ear function
- blood pressure in different areas of the body
- pressure within the sinus cavities of the head 
- pressure within the eyes
- medical care

#### Muscle tone
Humans are able to stand upright because of a complicated symphony of leg and core muscle groups. Mars’ low gravity eats at this functionality. Muscle bulk loss may be countered by additional exercise.

#### Bone thickness and density
Like muscle bulk, human bones are use-it-or-lose-it. While not as dramatic as the bone loss experienced by astronauts in microgravity, astronauts on Mars will experience some bone loss. Exercise is the best remedy, especially exercise in artificial gravity.

#### Inner Ear Function
The inner ear provides humans with our sense of movement and the position of our body relative to the ground. Astronauts new to Mars will be particularly vulnerable to vertigo (sense of spinning movement when still) and fear of falling. Positional exercises help relocate the inner ear bones and relieve these sensations.

#### Regulating Pressure in the Body
Human bodies are adapted to counteract the effects of gravity on our water-based physiology. We are like a stacked column of connected water balloons. Without active work, gravity causes the upper balloons to lose fluid and pressure in favor of the lower ones. On Earth, humans do this active management of fluid automatically. 

On Mars the effects of gravity on our bodies is moderated. The automatic active measures human bodies perform to drive fluid “up the column” will be more than needed, resulting in cold feet (less blood available in lower extremities), stuffed up heads (sinus cavities inflamed), and higher than desired pressure in the eyes. High blood pressure is a particular concern. Strokes are a risk. 

Fortunately, human physiology adapts to the new level of gravity with some help of behavioral adjustments and medications. Behavioral adjustments might include Swinging Yoga where the astronaut dangles upside down from their feet and ankles and performs short stretch and strength routines throughout their day.

#### Medical care
Crushing injuries and other traumatic injuries are difficult to manage in microgravity because blood doesn’t drain away from the injury site in the way our bodies expect. Mars’ lower gravity does not cause as severe issues as microgravity, but for certain circumstances, time in artificial gravity is helpful.

During pregnancy, there is concern that lower gravity may affect the womb environment and fetal development, making early spontaneous termination of pregnancy more likely. More information will be gathered as the colony becomes established. Gestating patients may spend time in artificial gravity environments to help development to stay on course.

### Indirect Effects of Low Gravity on Activities
Low gravity makes a lot of activities easier, such as lifting, tossing upward, bouncing and sliding. However, while the weight of objects (including human bodies) is lower on Mars because of the low gravity, the mass of objects is the same. In other words, if a human is shot, the mass and velocity of the bullet will do the same damage. 
This has implications for capabilities and risks. 

### Capabilities
Humans are able to lift much higher mass objects on Mars, but the work required to accelerate or decelerate the object horizontally will be the same. Striking a golf ball feels like the same effort (though the golf swing may feel strange due to the low weight of the club), but the ball goes a greater distance because gravity will pull it back to the ground more slowly. 

If an astronaut is trying to push a high mass object horizontally, it feels somewhat harder to get traction to push effectively as traction is a function of gravity pressing the pusher into the ground. On the plus side, the object causes less friction itself. It is harder to get a good purchase on the ground and easier to jump upward.

For this reason, the natural human instinct to take advantage of the low gravity to do enthusiastic rock-hopping will tend to put more stress than astronauts expect on their muscles and tendons.

### Driving and Vehicles
Similarly, the lower gravity reduces friction, making every outing in a quad bike or rover feel as if you are driving on ice. Because weight is lower, but mass is the same, hitting a bump will throw you much higher into the air … but hitting a wall will do just as much damage as on earth.

For wheeled loads, the weight of the object accelerating and decelerating horizontally feels higher than the weight of the object when lifted. Crushing injuries and damage to equipment are risks as astronauts tend to underestimate the force needed to slow or stop a heavy load moving horizontally. Injuries from misjudged jumps are also a risk. When driving a vehicle over irregular terrain, astronauts underestimate the ease with which vehicles become air-borne, resulting in injuries and damage.
