A systematic literature search was conducted to review the evidence of residual force enhancement (RFE) in vivo human muscle. The search, adhered to the PRISMA statement, of CINAHL, EBSCO, Embase, MEDLINE, and Scopus (inception—July 2017) was conducted. Full-text English articles that assessed at least 1 measure of RFE in vivo voluntarily contracted human skeletal muscle were selected. The methodologies of included articles were assessed against the Downs and Black checklist. Twenty-four studies were included (N = 424). Pooled Downs and Black scores ranked “fair” ([2.26]). RFE was observed in all muscles tested. Joint range of motion varied from 15° to 60°. Contraction intensities ranged from 10% to >95% maximum. Although transient force enhancement during the stretch phase may change with angular velocity, RFE in the subsequent isometric phase is independent of velocity. The magnitude of RFE was influenced by smaller stretch amplitudes and greatest at joint angles indicative of longer muscle lengths. Contraction and activation intensity influenced RFE, particularly during the initial isometric contraction phase of a poststretch isometric contraction. RFE resulted in increased torque production, reduced muscular activation, and enhanced torque production when the neuromuscular system is weakened seen in an aged population.

Hamstring strain injuries are common in sport. Supramaximal eccentric or high-intensity isometric contractions are favored in hamstring strain injury prevention. The effect of combining these contraction modes in such prevention programs as a poststretch isometric contraction is unknown. Poststretch isometric contractions incorporate an active stretch and result in greater final isometric force than isometric contractions at comparable joint angles. This study compared torque and muscle activation levels between maximal voluntary isometric contraction and maximal poststretch isometric contractions of the knee flexors. Participants (n = 9) completed baseline maximal voluntary isometric contraction at 150° knee flexion and maximal poststretch isometric contractions at 120° knee flexion actively stretching at 60°/s to 150° knee flexion for final isometric contraction. Torque of the knee flexors and surface electromyography root mean square (sEMG_RMS) of biceps femoris long head were simultaneously recorded and compared between baseline and poststretch isometric at 150° knee flexion. Torque was 14% greater in the poststretch isometric condition compared with baseline maximal voluntary isometric contraction (42.45 [20.75] N·m, 14% [22.18%], P < .001) without increase in sEMG_RMS of biceps femoris long head (−.03 mV, ±.06, P = .130, d = .93). Poststretch isometric contractions resulted in supramaximal levels of poststretch isometric torque without increased activation of biceps femoris long head.

Experiencing acute (transient) pain and discomfort can be a characteristic of exercise and sport participation. In this context, acute pain or discomfort can be experienced by injured and injury-free athletes alike. Acute pain or discomfort in the strength and conditioning (S&C) environment can be experienced during particular movements or exercises. This may be the result of poor exercise technique and/or soft-tissue dysfunction. This article proposes a possible strategy, which can be used by the S&C coach, to modify training based on the acute (transient) pain or discomfort experienced by athletes during training.

Background: Acceptance is a coping strategy associated with chronic pain management, but its effectiveness is unclear for Chronic Fatigue Syndrome/Myalgic Encephalomyelitis (CFS/ME). Objective: The aim of the study was to investigate the relationship between acceptance, fatigue severity, pain and self-reported physical activity in individuals with CFS/ME. Method: Ten females and seven males diagnosed with CFS/ME (51.9 ± 8.8 years), and gender, age-matched sedentary controls, completed self-reported measures of acceptance, fatigue severity and physical activity. Acceptance was measured using the Chronic Pain Acceptance Questionnaire and Acceptance and Action Questionnaire-II. Fatigue was assessed with the Fatigue Severity Scale, and Physical Activity using the International Physical Activity Questionnaire – Short Form. Self-reported physical activity was calculated using categorical and continuous measures (MET-minutes). Result: CFS/ME and control groups were compared using Independent t-tests and Spearman's Rho correlations. The CFS/ME group reported significantly greater fatigue severity and psychological inflexibility, and lower pain willingness and time spent sitting than controls. However, no between-group differences for activity engagement or physical activity. The CFS/ME group showed a negative relationship between pain willingness and psychological inflexibility, and a positive relationship between walking time and the time since symptom onset, and time since diagnosis. Conclusion: Despite reporting greater fatigue and less acceptance of their illness, CFS/ME patients had comparable levels of physical activity to controls, possibly due to pacing their activity to avoid symptom exacerbation. CFS/ME patients with an older diagnosis walked further than the newly diagnosed, suggesting the development of better coping skills and management strategies over time.

Residual force enhancement (rFE) is observed when isometric force following an active stretch is elevated compared to an isometric contraction at corresponding muscle lengths. Acute rFE has been confirmed in vivo in upper and lower limb muscles. However, it is uncertain whether rFE persists using multiple, consecutive contractions as per a training simulation. Using the knee flexors, 10 recreationally active participants (seven males, three females; age 31.00 years ± 8.43 years) performed baseline isometric contractions at 150° knee flexion (180° representing terminal knee extension) of 50% maximal voluntary activation of semitendinosus. Participants performed post-stretch isometric (PS-ISO) contractions (three sets of 10 repetitions) starting at 90° knee extension with a joint rotation of 60° at 60°·s⁻¹ at 50% maximal voluntary activation of semitendinosus. Baseline isometric torque and muscle activation were compared to PS-ISO torque and muscle activation across all 30 repetitions. Significant rFE was noted in all repetitions (37.8–77.74%), with no difference in torque between repetitions or sets. There was no difference in activation of semitendinosus or biceps femoris long-head between baseline and PS-ISO contractions in all repetitions (ST; baseline ISO = 0.095–1.000 ± 0.036–0.039 Mv, PS-ISO = 0.094–0.098 ± 0.033–0.038 and BFlh; baseline ISO = 0.068–0.075 ± 0.031–0.038 Mv). This is the first investigation to observe rFE during multiple, consecutive submaximal PS-ISO contractions. PS-ISO contractions have the potential to be used as a training stimulus.