등척성 중간허벅지 당기기(IMTP): 테스트 프로토콜 & 연구

The isometric mid-thigh pull (IMTP) is the most widely validated maximal isometric strength test in sport science. Performed on a force plate with the athlete pulling maximally against a fixed immovable bar positioned at mid-thigh height, it provides a comprehensive assessment of whole-body force production capabilities through a range of force-time variables.

Originally developed as a more standardized alternative to the isometric squat and isometric deadlift, the IMTP has become a cornerstone test in elite sport monitoring programs globally. It is used by national sporting institutes, professional sports teams, and research programs to quantify maximal strength, rate of force development, and neuromuscular fatigue. This review covers the complete evidence base: from test standardization through normative data to practical monitoring applications.

Why Isometric Testing?

Isometric (fixed-length) testing offers several advantages over dynamic strength tests for athlete monitoring:

• High reproducibility: Without the movement variability of dynamic tests, isometric tests show excellent reliability (ICC = 0.95-0.99) across sessions
• Safety: No movement means no failure position risk — athletes can exert maximal effort without spotters or equipment failures
• Speed: A complete IMTP assessment (3 trials) takes under 10 minutes including warm-up
• Rich data: A single force plate provides not just peak force but the entire force-time curve, enabling computation of RFD, impulse, and time-to-peak force

Test Position Rationale

The mid-thigh pull position (approximately 120-125° knee angle, 140-150° hip angle) was selected as a compromise between multiple considerations: it allows near-maximal force production in the hip and knee extensors, corresponds to the power position in Olympic weightlifting (where athletes demonstrate the highest peak force in dynamic movements), and provides sufficient stability for reliable force measurement.

Equipment Requirements

• Bi-axial force plate (minimum 1000Hz sampling rate)
• Fixed bar at adjustable height (typically a squat rack with safety pins)
• Adjustable platform or steps to raise athlete to correct joint angles
• Straps (optional, allows focus on leg drive rather than grip)

Position Setup

1. Set bar height at mid-thigh level — specifically at the crease between thigh and hip when standing. Use goniometer to confirm knee angle of 120-125° and hip angle of 140-150° when in pulling position.
2. Athlete adopts a double overhand grip (hook grip recommended for comfort during maximal efforts)
3. Feet flat on force plate, hip-width apart
4. Arms straight — no bend. This is critical: if arms are bent, they will contribute force that was not produced by the lower body, artificially inflating peak force.
5. Neutral lumbar spine — not flexed or hyperextended

Warm-Up Protocol

1. Three submaximal warm-up pulls at 50%, 75%, and 90% of perceived maximum
2. 2-minute rest between each
3. Full recovery (3 min) before maximal testing trials

Maximal Testing Protocol

1. Standardized verbal cue: "Push as hard as you can, as fast as you can" — research shows this combined cue produces higher peak force and RFD than separate instructions
2. 3-second countdown: "3-2-1-PUSH"
3. Hold for 5 seconds at maximum effort
4. 3 trials minimum, 3-minute rest between trials
5. Accept trial if coefficient of variation between trials is < 5%

Body Weight Measurement

Stand quietly on the force plate for 1 second before each trial to measure body weight (used for normalization). Peak force is commonly reported both in Newtons and relative to bodyweight (N/kg).

Peak Force (PF)

The highest force value recorded during the pull, typically occurring 0.3-0.8 seconds after the onset of force. Reported in Newtons or relative to bodyweight (N/kg). Peak force is the most commonly reported IMTP variable and shows the highest reliability.

Rate of Force Development (RFD)

The slope of the force-time curve, calculated as ΔFORCE/ΔTIME over specific time windows. The most commonly reported windows are:

• RFD0-50ms: Force generated in the first 50ms — reflects explosive strength
• RFD0-100ms: 0-100ms window
• RFD0-200ms: 0-200ms window
• Peak RFD: The maximum instantaneous slope of the force-time curve

RFD is more variable than peak force (CV = 10-15% vs. 3-5%) and reflects neural drive and muscle architecture more than maximal strength per se. Athletes with high peak force but low early RFD are strength-powerful but not explosive — they produce great force given enough time but are slow to develop force.

Impulse

Impulse = Force × Time = area under the force-time curve. Computed over specific time windows (50, 100, 150, 200, 250, 300ms). Impulse over time windows relevant to the athlete's sport (e.g., 200ms = typical sprint ground contact time) provides more actionable information than peak force alone.

Time to Peak Force

The time elapsed from onset of force to peak force. Shorter time-to-peak force indicates greater explosive capabilities. Research shows that well-trained athletes achieve peak force in 0.3-0.5 seconds, while less trained individuals often take 0.6-1.0 seconds.

Peak Force Normative Data (Relative to Bodyweight)

Haff et al. (2015) and subsequent reviews provide the following ranges from competitive athletes:

• Weightlifters (national level): 3.0-4.0 N/kg
• Powerlifters (competitive): 2.8-3.8 N/kg
• Rugby players (professional): 2.6-3.4 N/kg
• American football (D1 college): 2.4-3.2 N/kg
• Soccer players (professional): 2.2-3.0 N/kg
• General trained males: 2.0-3.0 N/kg
• General trained females: 1.6-2.4 N/kg

RFD Normative Data (0-200ms)

• Elite strength-power athletes: 8,000-16,000 N/s
• Team sport athletes: 6,000-10,000 N/s
• Recreationally trained: 3,000-6,000 N/s

Interpretation Notes

IMTP values are highly influenced by training history, sport, and position. A 2.0 N/kg peak force may represent outstanding strength for a distance runner but poor strength for a weightlifter. Always interpret values in the context of sport-specific benchmarks and within-athlete longitudinal trends rather than cross-sectional population norms.

Reliability

The IMTP consistently demonstrates excellent within-session and between-session reliability when standardized protocols are followed:

• Peak force ICC: 0.95-0.99
• RFD (0-200ms) ICC: 0.82-0.95
• Impulse (0-200ms) ICC: 0.88-0.97

Key factors that improve reliability: standardized warm-up, standardized verbal instructions, consistent bar height and joint angles, minimum 3 trials per session, exclusion of trials with CV > 5%.

Validity

IMTP peak force correlates significantly with:

• 1RM squat (r = 0.75-0.88)
• 1RM deadlift (r = 0.80-0.92)
• CMJ height (r = 0.60-0.80)
• Sprint 10m (r = -0.65 to -0.78)
• Sprint 30m (r = -0.55 to -0.72)

Crucially, IMTP RFD and impulse variables show stronger correlations with dynamic performance measures (sprint, jump) than peak force alone — validating the multi-variable approach to IMTP assessment.

Fatigue Monitoring

The IMTP is sensitive to neuromuscular fatigue. Studies show that peak force decreases 5-10% and RFD decreases 10-20% following heavy training sessions or competitions. This makes the IMTP valuable for tracking recovery in situations where a complete return to baseline is required (e.g., before major competition).

However, the IMTP's requirement for force plate access and 10-minute testing time makes it impractical for daily monitoring. It is best used as a weekly or bi-weekly anchor measure, complemented by daily CMJ monitoring which can be done with simpler equipment.

Longitudinal Strength Development Tracking

The IMTP provides a reliable benchmark for tracking strength development over training blocks. Unlike 1RM testing (which fluctuates based on neural factors, skill, and fatigue), the IMTP shows lower variability and can be assessed more frequently without the fatigue cost of a maximal dynamic lift. Test every 4-6 weeks during training periods and at the beginning and end of each training block.

Rehabilitation Return-to-Sport Criteria

The IMTP is increasingly used as a return-to-sport criterion after lower limb injuries. Limb symmetry index (LSI) — comparing the affected to unaffected limb in single-leg isometric testing — is a more sensitive measure of asymmetry than bilateral IMTP. Research suggests LSI > 90% on both peak force and RFD before return to full training.

Talent Identification

IMTP data, particularly relative peak force and early RFD, correlates with athletic performance across multiple sports. Some national talent ID programs include the IMTP as part of their assessment battery, particularly for strength-speed sports (Olympic lifting, sprinting, throwing events).