Caffeine Supplementation Increases Muscle Strength, but Not Endurance, While Both Caffeine and Its Expectation Elevate Blood Lactate: A Balanced-Placebo Design Study.

Caffeine supplementation increased peak torque during static and 60°/s contractions but not at higher velocities.

• Peak torque increased by 4% at static and 60°/s conditions (p ≤ 0.003; d = 0.3 for both speeds)
• Peak torque did not significantly increase at 180°/s or 300°/s velocities
• Caffeine dose was 5 mg/kg body weight
• Sample consisted of resistance-trained men aged 18–30 years (n = 16)

Caffeine supplementation increased maximum work across all isokinetic speeds tested.

• Maximum work increased by 4% across all speeds (p < 0.001; d = 0.2)
• Speeds tested were 0, 60, 180, and 300°/s in the knee extensors
• Effect size was small (d = 0.2)

Caffeine supplementation did not significantly increase muscle endurance performance.

• Muscle endurance showed a non-significant 3% increase with caffeine (p = 0.184; d = 0.1)
• Muscle endurance was tested as three sets of maximal repetitions using 60% of 1RM
• Despite the non-significant group-level result, 50% of participants were classified as responders (Δ > 0)
• Improvements in responders ranged from 1–16%

Caffeine supplementation significantly elevated capillary blood lactate levels after muscle endurance testing.

• Blood lactate increased by 1.2 mmol/L with caffeine (p < 0.001; d = 0.7)
• Effect size was moderate (d = 0.7)
• Blood lactate was measured via capillary sampling after muscle endurance testing

Caffeine expectation (placebo belief) did not improve muscle strength or muscle endurance but significantly increased blood lactate levels.

• No significant placebo effect was observed for peak torque or muscle endurance performance
• Blood lactate increased by 0.7 mmol/L under caffeine expectation conditions (p = 0.003; d = 0.4)
• The balanced-placebo design included four conditions: C/C (informed caffeine/used caffeine), P/C (informed placebo/used caffeine), C/P (informed caffeine/used placebo), and P/P (informed placebo/used placebo)

Individual responses to caffeine for both strength and endurance outcomes were markedly heterogeneous.

• Between 50% and 88% of individuals were classified as responders (Δ > 0) across strength and endurance measures
• Improvements in responders ranged from 1% to 16%
• The authors noted this variability and suggested fiber-type predominance as a potential explanatory mechanism

The study used a randomized, double-blind, balanced-placebo design with four counterbalanced experimental sessions.

• Participants were resistance-trained men aged 18–30 years (n = 16)
• Two assessment/familiarization visits preceded four experimental sessions
• One repetition maximum (1RM) was assessed during familiarization
• Caffeine dose was 5 mg/kg body weight