Human strength and power

Strength and power defined

Factors affecting strength

1. Neural control: Includes the recruitment and firing rate of motor units. Training increases motor unit synchronization and activation.

   • Motor unit recruitment determines how many fibers are engaged during a contraction.
   • Improved firing rates increase the force output and 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):

   • Larger CSA correlates with greater force production. Muscle hypertrophy through resistance training 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) generate higher force but slower contractions. This is due to the alignment of fibers, which affects how efficiently force is transmitted.
   • Longitudinal fibers, on the other hand, favor speed and range of motion over force.

4. Muscle length and joint angle:

   • Muscles produce the most force at optimal lengths where cross-bridge overlap is maximal.
   • Joint angles influence the length-tension relationship, affecting force production throughout the range of motion.

5. Force-velocity relationship:

   • The force generated by muscles decreases as the velocity of contraction increases. This is due to the reduced time for cross-bridge formation at high speeds.
   • Eccentric contractions (lengthening of the muscle under tension) can produce greater force than concentric contractions.
   • This relationship also influences the power curve: power output peaks at moderate loads where force and velocity are both optimized.

6. Joint angular velocity:

   • Joint angular velocity refers to the rate of change of angular position of a joint, typically measured in radians per second (rad/s).
   • High angular velocities are crucial in sports requiring rapid joint movement, such as throwing or kicking.
   • Resistance training exercises can be modified to focus on joint angular velocity by incorporating lighter loads moved at higher speeds, enhancing 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 critical in strength and power training, each emphasizing different aspects of force production and control.

8. Strength-to-mass ratio:

   • This ratio is critical in sports requiring acceleration, such as sprinting and jumping. Smaller athletes often have a higher ratio, enabling faster acceleration relative to their body weight.

9. Acceleration and power:

   • Power is the product of force and velocity. Athletes with the ability to produce high force quickly (explosive power) excel in dynamic movements like jumping and sprinting.

Equations in strength and power

1. Work: Force × Displacement

   • Where 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), representing the rate of doing work.
   • Written as:

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

3. Angular work and power:

   • Rotational work is calculated as:

     $$\text{Rotational Work}=\text{Torque}\times \text{Angular\hspace{0.33em}displacement}$$

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

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

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 in designing effective programs:

1. Gravity: Resistance due to gravitational force depends on the load’s mass and distance from the axis of rotation. Exercises like squats and presses exploit gravity to develop strength.
2. Inertia: Accelerating a load increases resistance, particularly in explosive movements.
3. Friction and fluid resistance: Found 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 elastic components provide resistance that increases with stretch length. However, they lack the consistency of free weights.
5. Chains and accommodating resistance: Chains and bands modify resistance throughout the range of motion, matching the strength curve by increasing load where the lifter is mechanically stronger (e.g., top of a squat or bench press). These 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: Safer for beginners, isolate specific muscles, and allow controlled range of motion.

Torque and range of motion:

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

Cam-based machines:

• These machines vary resistance throughout the range of motion to match the strength curve of muscles, providing consistent tension.

Joint biomechanics in resistance training

Resistance training can strain joints, making proper mechanics crucial for injury prevention.

Back:

• Maintaining a neutral spine minimizes 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) stabilizes the spine but must be used cautiously to avoid excessive pressure.

Shoulders:

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

Knees:

• Proper alignment during squats and lunges reduces stress on the patellar tendon and surrounding ligaments. Using wraps may provide stability but should not 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 may strain these joints. Maintaining proper wrist alignment and reducing volume in high-stress exercises prevents overuse injuries.