Human strength and power

Strength and power defined

Factors affecting strength

1. Neural control: This includes motor unit recruitment and motor unit firing rate. Training can improve motor unit synchronization and overall activation.

   • Motor unit recruitment determines how many muscle fibers are engaged during a contraction.
   • Improved firing rates increase force output and can increase the speed of muscle contractions.
   • Size principle of motor unit recruitment: Smaller Type I units are activated first, then larger Type II.

2. Muscle cross-sectional area (CSA):

   • A larger CSA is associated with greater force production. Resistance training can increase CSA through muscle hypertrophy, which increases the ability to generate force.
   • Both myofibrillar hypertrophy (increase in contractile proteins) and sarcoplasmic hypertrophy (increase in muscle glycogen storage) contribute to CSA.

3. Muscle architecture:

   • Muscles with greater pennation angles (e.g., gastrocnemius) tend to generate higher force but contract more slowly. This relates to fiber alignment and how efficiently force is transmitted.
   • Longitudinal fibers, on the other hand, favor speed and range of motion over maximal force.

4. Muscle length and joint angle:

   • Muscles produce the most force at optimal lengths, where cross-bridge overlap is maximal.
   • Joint angles change muscle length and leverage, influencing the length-tension relationship and force production across the range of motion.

5. Force-velocity relationship:

   • The force a muscle can produce decreases as contraction velocity increases. One reason is reduced time for cross-bridge formation at higher speeds.
   • Eccentric contractions (lengthening of the muscle under tension) can produce greater force than concentric contractions.
   • This relationship also shapes the power curve: power output peaks at moderate loads, where force and velocity are both relatively high.

6. Joint angular velocity:

   • Joint angular velocity is the rate of change of a joint’s angular position, typically measured in radians per second (rad/s).
   • High angular velocities are important in sports that require rapid joint movement, such as throwing or kicking.
   • Resistance training can emphasize joint angular velocity by using lighter loads moved at higher speeds, which can enhance power development.

7. Muscle actions in strength and power:

   • Concentric muscle action: The muscle shortens as it produces force to overcome resistance. Example: lifting the barbell during a biceps curl.
   • Eccentric muscle action: The muscle lengthens while resisting a load. Example: lowering the barbell during a biceps curl.
   • Isometric muscle action: The muscle generates force without changing its length. Example: holding a barbell in a fixed position.
   • These muscle actions are all used in strength and power training, but they emphasize different aspects of force production and control.

8. Strength-to-mass ratio:

   • This ratio is important in sports that require acceleration, such as sprinting and jumping. Smaller athletes often have a higher ratio, which can support faster acceleration relative to body weight.

9. Acceleration and power:

   • Power is the product of force and velocity. Athletes who can produce high force quickly (explosive power) tend to perform well in dynamic movements like jumping and sprinting.

Equations in strength and power

1. Work: Force × Displacement

   • Force is measured in newtons (N) and displacement in meters (m). Work is expressed in joules (J).
   • Written as:

   $$\text{Work}=\text{Force}\times \text{Displacement}$$

2. Power: Work ÷ Time

   • Power is measured in watts (W) and represents the rate of doing work.
   • Written as:

   $$\text{Power}=\frac{\text{Work}}{\text{Time}}$$

3. Angular work and power:

   • Rotational work is calculated as:

     $$\text{Rotational Work}=\text{Torque}\times \text{Angular2˘004displacement}$$

     • Work is measured in joules (J).
   • Rotational power is calculated as:

     $$\text{Rotational2˘004Power}=\frac{\text{Work}}{\text{Time}}$$

These equations are critical for understanding and optimizing mechanical output in resistance training.

Factors for conversion of common measurements

This table highlights key conversions for understanding and applying biomechanical equations, especially when measuring work, power, and force in resistance training scenarios.

Source and biomechanics of resistance training

Understanding the types of resistance helps you design effective programs:

1. Gravity: Resistance due to gravitational force depends on the load’s mass and its distance from the axis of rotation. Exercises like squats and presses use gravity to develop strength.
2. Inertia: When you accelerate a load, resistance increases, which is especially relevant in explosive movements.
3. Friction and fluid resistance: These appear in specialized activities such as sled pushing or swimming. Friction increases resistance at the point of contact, while fluid resistance is proportional to movement velocity.
4. Elasticity: Bands and other elastic components provide resistance that increases as they stretch. However, they don’t provide the same consistency as free weights.
5. Chains and accommodating resistance:\u202fChains and bands change resistance across the range of motion. The goal is to better match the strength curve by increasing load where the lifter is mechanically stronger (e.g., near the top of a squat or bench press). These tools are used to develop explosive strength and improve force production through sticking points.

Applications of biomechanics to resistance training

Free weights vs. machines:

• Free weights: Engage stabilizing muscles, mimic real-world movements, and are versatile for multi-joint exercises.
• Machines: Often safer for beginners, isolate specific muscles, and allow a controlled range of motion.

Torque and range of motion:

• Resistance torque changes throughout an exercise depending on joint angles and body positioning. For example, changing the position of a weight during a squat changes the force required at different phases of the movement.

Cam-based machines:

• These machines vary resistance throughout the range of motion to better match the muscle strength curve, helping maintain more consistent tension.

Joint biomechanics in resistance training

Resistance training can strain joints, so proper mechanics are important for injury prevention.

Back:

• Maintaining a neutral spine reduces stress on intervertebral discs. Excessive rounding or arching during exercises like deadlifts increases injury risk.
• Intra-abdominal pressure (IAP): Engaging core muscles (e.g., Valsalva maneuver) can stabilize the spine, but it must be used cautiously to avoid excessive pressure.

Shoulders:

• High mobility makes the shoulder joint more prone to overuse injuries. Balancing pushing and pulling movements helps prevent imbalances that can lead to impingement or rotator cuff damage.

Knees:

• Proper alignment during squats and lunges reduces stress on the patellar tendon and surrounding ligaments. Wraps may provide stability but shouldn’t replace proper technique. Valgus collapse during these movements can increase strain on the ACL, especially when combined with poor foot or hip mechanics.

Elbows and wrists:

• Overhead lifts and grip-intensive exercises can strain these joints. Maintaining proper wrist alignment and managing volume in high-stress exercises helps reduce overuse injuries.