The American Council on Exercise recently enlisted our team of researchers in the High Altitude Exercise Physiology Program at Western State Colorado University to evaluate the effectiveness of applying the HIIT model to resistance training. We demonstrated that resistance exercise HIIT is more effective and time-efficient when compared to traditional resistance exercise at improving muscular fitness. This article is designed to provide health and exercise professionals with an evidence-based guide on how to translate these recent research findings into everyday practice with their clientele.

The Health Benefits of Muscular Fitness

In the past decade, low muscular fitness has garnered considerable attention as an independent and powerful predictor of cardiovascular disease (CVD) risk and premature mortality. Indeed, it has been reported that increased muscular fitness is associated with a reduced risk of all-cause mortality (Ruiz et al., 2008). Additionally, various muscular fitness parameters (strength, endurance and power) have been found to be associated with common cardiometabolic risk factors, including body mass index, waist circumference, blood lipids and blood pressure (Magnussen et al., 2012). It also has been demonstrated that that there is a strong association between muscular strength and mortality from all causes in various clinical populations, including those with CVD, cancer and arthritis (Volaklis, Halle, and Meisinger, 2015). More recently, elevated levels of both upper- and lower-body muscular strength have been linked to lower risk of mortality (García-Hermoso et al., 2018). Taken together, this body of scientific literature highlights the critical role of muscular fitness in public health.

Resistance-exercise HIIT Explained

Also over the past decade, the concept of aerobic high-intensity interval training (HIIT) has captivated the attention of health and exercise professionals and researchers alike due to its superior ability to improve cardiorespiratory fitness (Weston, Wisløff and Coombes, 2014) and cardiometabolic health (Ramos et al., 2015) for a lesser weekly time commitment relative to the current exercise guideline of moderate-intensity continuous training (MICT) (Garber et al., 2011). Resistance exercise can also be effectively performed as HIIT by alternating brief bouts of higher-intensity sessions with either rest or lower-intensity workloads throughout the exercise training session. This strategy enables less-fit individuals to accumulate periods of exercise that would otherwise not be possible if executed continuously.

However, one drawback to the protocols employed in the majority of previous resistance-training HIIT studies is that they were not actually time-efficient (Giessing et al., 2016; Wingfield et al., 2015). For resistance training HIIT to be a feasible option to improve public health, it must be time-efficient as lack of time has consistently been identified as one of the primary perceived barriers to preventing inactive individuals from becoming and remaining physically active. Accordingly, our experimental design in the ACE-sponsored resistance-exercise HIIT study addressed this issue with a total weekly time commitment of 45 to 60 minutes for the HIIT group, which equated to a 50% shorter time-commitment when compared to the traditional resistance training group. 

Is Resistance-exercise HIIT Safe for Clinical Populations?

Safety is a paramount issue when designing and implementing a resistance-exercise HIIT program. Overall, properly performed exercise is harmless for the majority of individuals. In fact, there is a greater risk associated with remaining physically inactive when compared to commencing with regular exercise training. The absolute risk of sudden death during vigorous-intensity physical activity has been estimated to be one per year for every 15,000 to 18,000 people [American College of Sports Medicine (ACSM), 2018]. In terms of the specific risk associated with aerobic HIIT, it was found that after 129,456 hours of MICT and 46,364 hours of HIIT in 4,846 high-risk participants, there was one fatal and two non-fatal cardiac arrests, respectively (Rognmo et al., 2012). Similarly, another recent study reported that across 12 randomized controlled trials, which compared aerobic MICT with HIIT, only a single adverse cardiac event (orthostatic collapse) occurred in the combined HIIT groups (Hannan et al., 2018). Overall, the consensus to-date in the scientific literature is that the risk of a cardiovascular event with aerobic HIIT is extremely low.

The ACE-sponsored study results corroborated these previous findings as there were no observed or reported safety issues with any study participants in the resistance-exercise HIIT group. The study participants were representative of the typical U.S. adult population: middle-aged (21 to 59 years), not engaged in regular resistance exercise and burdened with various cardiometabolic risk factors (e.g., obesity, high cholesterol, hypertension). Based on our research experiences, we recommend implementing the following strategies to maximize the safety of resistance exercise HIIT: 

• Pre-participation screening and exercise termination criteria: A key to minimizing complications during and after exercise is to identify those individuals who may be at an increased risk of adverse symptoms through appropriate preparticipation screening. Individuals with a clinically relevant cardiovascular disease risk factor or diagnosed CVD will benefit from a medical examination and physician-supervised maximal exercise test prior to participation in vigorous-intensity or near maximal- to maximal-intensity exercise (ACSM, 2018). Despite the absence of untoward events in the present study, given the near-maximal musculoskeletal and cardiovascular strain, careful preparticipation screening prior to the commencement of resistance exercise HIIT for previously inactive and/or risk factor burdened individuals is strongly recommended. These procedures will pinpoint contraindications to resistance-exercise HIIT. Lastly, the presence of chest discomfort, musculoskeletal pain, light-headedness, severe fatigue and shortness of breath are all abnormal responses to exercise and warrant the termination of exercise that day.
• Proper lifting technique, preparation and recovery: Plan on devoting the first several resistance-exercise HIIT sessions to instructing clients on proper lifting technique and breathing patterns. The physiological and psychological demands of resistance exercise HIIT means clients need to be sufficiently prepared for these sessions in advance and understand the importance of recovery upon completion of the training session. Areas that take on increasing importance include proper hydration and nutrition, appropriate exercise attire and a complete warm-up routine. Likewise, following the interval session, clients should perform a complete cool-down and then rehydrate and refuel in a timely manner.
• Adherence to target resistance-exercise HIIT intensity: Confirmation of clients exercising at the correct exercise intensity is always a primary concern; however, this consideration is amplified when performing resistance-exercise HIIT. Initially, it is paramount that you establish the correct individualized intensity for resistance-exercise HIIT. In the ACE-sponsored study, we used with considerable success a five-repetition maximum (5-RM) for each exercise. The protocol for establishing 5-RM can be found in the sidebar below. Regularly monitor resistance-exercise HIIT sessions and verify that the actual exercise intensity is aligned with the target intensity. Finally, the target workload (i.e., 5-RM) for resistance-exercise HIIT should be regularly updated. Initially, it is suggested that the target workload be readjusted every two weeks for the first couple of months of resistance-exercise HIIT. Over the long-term, the target workload can be adjusted less frequently in one- to two-month intervals.

Resistance-exercise HIIT: Evidence-based Program Components

A resistance-exercise HIIT program consists of six separate components that you need to consider when designing programs for clients:

• HIIT sessions per week: This component refers to how many resistance-exercise HIIT sessions will be performed each week. Our research demonstrated that two to three resistance-exercise HIIT sessions per week is well-tolerated, effective and safe.
• Intensity: This component refers to the intensity of each resistance exercise. We used an intensity of 5-RM. Further research is required to determine whether greater or lesser intensities would elicit similar findings.
• Number of sets: This component refers to the number of sets performed during each resistance-exercise HIIT session. Our participants performed a single set of exercises during our study. It is probable that additional sets would equate to greater muscular fitness benefits. However, additional sets would also diminish the time-efficiency of a single-set, resistance-exercise HIIT program.     
• Number of repetitions: This component refers to number of repetitions performed for each exercise in the resistance-exercise HIIT session. Our participants performed five repetitions at a 5-RM intensity. In terms of time allocation, five repetitions took approximately 10 to 15 seconds to perform. Future research is required to identify whether fewer or morerepetitions provoke different muscular fitness outcomes.
• Recovery between exercises: This component refers to the duration of time (in seconds/minutes) between each exercise. On average, our participants had 60 seconds of recovery between each resistance exercise.
• Type of recovery: This component refers to the type of recovery performed between each resistance exercise. Our study employed a practical active recovery that consisted of movement from one piece of equipment to the next between each exercise.

As a health and exercise professional, you can apply the evidence-based recommendations presented in Table 1 to design training programs for clients who are ready to commence with resistance-exercise HIIT.

Table 1.  Evidence-based Resistance-exercise HIIT recommendations

Note: HIIT = High-intensity interval training; 5-RM = Five-repetition maximum

Resistance-exercise HIIT: Progression, Maintenance and Programming Tips

Progression

• Based on our experience, two nonconsecutive resistance-exercise HIIT sessions per week is a proper starting point. Once your client has successfully maintained this frequency for approximately one month, it is appropriate to add an additional resistance-exercise HIIT session to the weekly program.
• As resistance-exercise HIIT progresses from two to three sessions per week, these workouts should be performed on nonconsecutive days, thus permitting 48 hours of recovery between sessions. 
• As training progresses, the time-efficiency of resistance-exercise HIIT can be enhanced by decreasing the time between exercise stations. In fact, many of our participants completed their workouts (including warm-up and cool-down) in 12 minutes, which equates to slightly more than 30 minutes per week devoted to resistance-exercise HIIT.  

Maintenance

• Frequently, health and exercise professionals and clients alike fall victim to the mindset that more is better. After progression has been made to target resistance-exercise HIIT parameters (e.g., number of sessions per week), the reality is that these training parameters will then be maintained long-term. The temptation to increase various parameters of the resistance-exercise HIIT program should be avoided, as it will likely lead to overtraining, untoward events or both.
• Our participants performed the following standardized resistance-training exercises: back extension, biceps curl, chest press, lat pull-down, leg curl, leg extension, leg press, seated row, shoulder press and triceps extension. You may elect to incorporate new exercises into the program to maintain novelty and stimulation for clients. However, the top-end recommendations listed in Table 1 for each parameter should continue to be observed.     

Programming Tips

• Our resistance-exercise HIIT group performed a three-minute warm-up consisting of light calisthenics and a three-minute cool-down of walking around a 200-meter indoor track. This volume of warm-up and cool-down has been used previously in aerobic reduced-exertion HIIT and contributes to the overall time-efficiency of the program (Vollaard and Metcalfe, 2017). 
• Periodic reassessment of 5-RM for each resistance exercise is necessary to account for favorable adaptations in muscular fitness and ensure a precise resistance-exercise HIIT workload.   
• We permitted our participants to vary the order of their resistance exercises between sessions to accommodate which machines were available and ensure the HIIT workouts were time-efficient. Changing the order of exercise, however, did not impact training responsiveness.

Practical Applications for Health and Exercise Professionals

Low muscular fitness has recently garnered considerable attention as an independent and powerful predictor of chronic disease risk and premature mortality. As the study cited here demonstrates, applying the HIIT model to resistance training is safe, time-efficient and more effective relative to traditional resistance training. As a health and exercise professional, you are in an ideal position to apply these research findings, guidelines and programming tips to your work with clients and positively impact both their overall strength and well-being.