2025-01-31

The Science Behind Measuring Your Contact Strength

Climbing

Training

Strength

RFD

In climbing, it’s not just about how much strength you have, but how quickly you can apply it. That critical split second when grabbing a hold in a fast, dynamic, or unstable move is what we know as Contact Strength or RFD (Rate of Force Development). If you’ve ever wondered why a hold “slips away” at a deadpoint, even when you feel strong, the answer likely lies in your RFD.

In this post, we’ll break down what RFD is, why it’s so crucial for your climbing performance, and how we can measure it accurately to optimize your training.

What is contact strength and why is it so important?

The ability to “hold onto holds” refers to the skill of applying the necessary force to grips when we contact them from unstable equilibrium positions. This strength is vital in moves like deadpoints, where technique and scapular stability allow us to reach advantageous positions, but the RFD of the finger flexors is the physical capacity that lets us execute the move and is directly measurable.

RFD is crucial because:

  • It’s the most important physical performance factor in bouldering.
  • While it doesn’t correlate as well with general sport climbing performance, it is very relevant in elite populations (men >8a, according to IRCRA).
  • Therefore, measuring RFD makes a lot of sense for boulderers of any level and for high-level sport climbers.

RFD is defined as the derivative of the applied force, that is, how quickly we can apply force. Graphically, it’s represented by the slope of the force-time curve; a steeper slope indicates greater force production in less time.

Key Factors Determining Your Finger RFD

The ability to develop force quickly depends on several physiological and biomechanical factors:

  1. Muscle Contractile Capacity (Strength and Neural Adaptations):

    • The speed of force generation is related to muscle fiber composition. Those with more fast-twitch fibers (Type II) have an inherent advantage in explosive strength.
    • At the nervous system level, neural adaptations are crucial, including the speed of motor neuron recruitment and the firing rate of motor units. RFD is a highly neural capacity, linked to the speed of motor neuron recruitment before they fire.

  2. Muscle-Tendon Complex Stiffness:

    • The stiffness of the muscle and tendon is fundamental for efficient force transmission. A stiffer tendon allows faster transmission of the force generated by the muscle to the skeletal system.

How do we measure RFD and at what point on the curve?

Since the slope of the force-time curve is not constant, consensus is needed to establish at what point RFD is measured. There are mainly two approaches:

  1. Absolute (Mean) RFD:

    • Measured at fixed, standardized time points from the start of force application. Calculated as the slope between two points on the force-time curve. For example, RFD at 100 milliseconds (RFD 100ms) is calculated as the force applied divided by 0.1 seconds.
    • The most common time scales are 0–50 ms, 0–100 ms, and 0–200 ms.

  2. Relative RFD:

    • Measures RFD relative to the peak force reached or the time needed to reach a percentage of body weight.
    • RFD at 95% of Peak Force: RFD is measured when 95% of maximum force is reached, regardless of time. 95% is used because the force-time curve tends to flatten near the peak.
    • Time to Reach 50% of Body Weight (T50%): This indicator has proven to be the most reliable and valid for predicting climbing ability in advanced climbers (7a+ to 8a) and should be used to assess RFD in climbers.

Which measure is best?

  • For elite climbers (>8a) and boulderers, RFD 200ms is the most recommended measure, as it best differentiates between levels and is the most reliable. RFD at 95% is also very differentiating in high-level athletes.
  • For lower-level populations, T50% (time to reach 50% of body weight) is a valid and reliable indicator of contact strength.
  • The best part is that a single RFD test can provide values for RFD 200ms, RFD 95%, and T50%, so you don’t need multiple procedures.

Key Considerations for RFD Assessment

To ensure your RFD assessment is accurate, reliable, and safe, keep the following in mind:

  1. Surface Stability: Fundamental! Tests must be performed on a completely stable surface. Instability significantly reduces total force and power you can express (between 12% and 80% drop in power), as extra force is needed to stabilize joints. Instability increases joint stiffness, interfering with precise measurement.
  2. Reliability: RFD measurement is inherently less reliable than peak force, as it depends heavily on neural factors that introduce more variability. However, reliability improves with athletic level (good in advanced, excellent in elite). To increase it, it’s recommended to perform the measurement at least 2 or 3 times per assessment and keep the highest value produced, as well as standardize the protocol.
  3. Measurement Start: The effective start of force application is set when an increase of 4 Newtons (or 0.3 kg with a Tindeq) is recorded between two consecutive measurements.
  4. Edge Size: 23 mm edges are recommended for measurements, for consistency with scientific literature and safety. 10 mm edges are more prone to finger slips and are more demanding on the skin.
  5. HIMA (Passive) Contraction: As with maximal strength, RFD should be measured passively (HIMA) because it’s more specific to climbing. This means placing the fingers on the hold without applying pressure (or with a maximum of 2–3 kg to avoid slipping) and then applying force as quickly and strongly as possible, abruptly flexing the knees and activating the scapular muscles, without moving the elbow or shoulder.
  6. Verbal Instructions: Precise instruction is key: “Drop as much weight as possible onto the edge as quickly as you can.” This is preferable to “Apply as much force as possible in the shortest time,” as the latter can induce pulling and reduce the peak force reached. The test is recommended with the arm in a neutral position, perpendicular to the board, for greater comfort.
  7. RFD and Peak Force: Separate Tests: The forces reached in an RFD test are generally lower (about 95% on average) than those in a peak force test without time restriction. Therefore, you cannot use the same test to obtain both values. It’s recommended to assess peak force first on the same edge (which will serve as activation) and then RFD, considering the RFD measure valid if the peak reached in RFD is above 90% of the peak force without time limitation.

Requirements and Safety of the Test

  • Relevance: RFD is highly relevant for bouldering, competitions, and high-level sport climbers.
  • Validity: RFD 200ms and RFD 95% are valid for elite/boulderer populations; T50% may be valid for lower levels.
  • Specificity: Although specific, it doesn’t fully replicate real climbing, as the maximum force peak is rarely reached on a hold during movement.
  • Safety: Rapid and maximal force application presents risks, especially for adolescents during peak height velocity (PHV). Caution is advised. For people with a short pinky, syndactyly taping (pinky to ring finger) should be used.
  • Familiarization: The RFD test requires learning and practice. Untrained subjects may give unreliable values initially.

Conclusion

RFD is a fundamental capacity that can make the difference in your climbing performance, especially if you’re a boulderer or high-level sport climber. By understanding how it’s measured and following a standardized and safe assessment protocol, you’ll obtain precise data that will allow you to train smarter. So next time you face an explosive move, remember that the speed at which you grab that hold is just as important as the strength you have!

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