A Case for RFD vs Power-Based Training in Physical Therapy Practice
More than 25% of people aged 65 years and older will fall each year. Falls are the most common cause of both traumatic brain injury and fractures in older adults and are the leading cause of unintentional death for this population. Falls ,and a fear of falling, can diminish a person’s ability to lead a full and independent life, or worse, continue living at all. Although 1 in every 4 older adults falls each year, falling is not a part of normal aging. As a medical provider we have a large body of knowledge and skills to reduce these risks and help return the power to the patient/client to reduce their risk of falls.1,2 The key word here is power, which to some may already resonate from your physics 101 course in college. Power is, simply put, the rate at which work is performed. So, does that mean you should do a task at a faster rate to increase your power? Well the answer is: yes and no.
In physical therapy practice, the guiding principle for many years was strength; “You gotta be strong or you’re gonna fall and hurt yourself”. There has been a paradigm shift away from pure strength emphasis to power development training in older adults primarily as a means to reduce or better yet prevent falls.3 This resulted from a review of the causes or precluding factors that may contribute to falls. Power training was introduced as a means to combat falls resulting from a slow transition from sitting to standing. Which again ,if you remember your college physics courses, represents a conversion from potential energy in the form of sitting to kinetic energy upon standing. When this happens, a vector is produced that will resemble a shape like the arc of motion of a bench press or a box squat (e.g., a pseudo J-shaped motion). This arc is essential for allowing not only clearing the surface the individual is resting on, but to also advance the trunk forward just anterior of the resting surface.
So, what could go wrong?
Well, falls result from a prolonged period of time transitioning from sitting to standing as the vector continues to increase in not only its magnitude but also its direction. If and when an individual is unable to accomplish this task in a time frame that allows for a seamless transition from sitting to standing, the magnitude and direction of said vector will take the person to the point of no return, aka on the floor. So, that’s your basic science lesson. Now, how does the rate of force development (RFD) come into play?
Explosive strength is the ability to increase force or torque as quickly as possible during a rapid voluntary contraction from a low or resting level. Rate of force development (RFD) is derived from the force- or torque-time curves recorded during explosive voluntary contractions.4 For this commentary, explosive voluntary contractions will be refer to motion performed during a simple transition from sitting to standing, for the sake of a practical physical therapy intervention and assessment tool. Falls that are often associated with transfers , are often linked to a slow, and often prolonged time frame to achieve the necessary torque to achieve a fluent transition into standing without a loss of balance. This is mainly due to, as compared to pure maximal voluntary contraction (MVC) strength, RFD seems to be (1) better related to most performances of both sport-specific and functional daily tasks, (2) more sensitive to detect acute and chronic changes in neuromuscular function and (3) potentially governed by different physiological mechanisms The ability to properly implement and integrate RFD principles by means of voluntary isometric contractions is extremely important not only for researchers in the field of human and exercise physiology, but also for practitioners in the fields of physical training and rehabilitative sciences.5 Therefore, the ability to rapidly generate force, becomes critical as a means of falls prevention stratization but also as a means to be advance a patient/client through their plan of care to resume and improve upon their functional limitations.
Implications of Getting Stronger
Several lines of evidence suggest that exercise involving explosive-type muscle contractions (i.e., muscle actions performed with maximal intentional RFD) is the most efficient training modality, regardless of the training load used, for inducing maximal gains in RFD and muscle activation at contraction onset. For example, 4 weeks of isometric knee extensor training using a maximal intentional RFD and high peak force level (90 % MVC) produced markedly larger gains in RFD and muscle activation, respectively, than conventional hypertrophy training performed using lower intensities (75 % MVC) and submaximal RFD efforts (insert citation). In addition, robust concurrent increases in RFD and EMG activity have been demonstrated by employing explosive-type resistance exercise in young adults, old to very old individuals, and frail elderly patients recovering from elective hip replacement surgery.4 These findings collectively indicate that explosive-type strength training is not only highly effective in eliciting marked gains in rapid force capacity (RFD and impulse) and increased muscle activation at the onset of muscle contraction, but is also tolerable across a wide range of individual backgrounds from young untrained/trained individuals to inactive frail elderly.4,5 In contrast, it should be recognized that the use of heavy (≥75 % of 1-repetition maximum) training loads also seem effective at eliciting substantial increases in contractile RFD. 5
From a practical, clinical application standpoint, a direct means of calculating a patient’s 1RM for a given task is certainly not impossible, it can however present a challenge for practitioners. Several methods exist for rough estimations for 1RM calculation, each of which present with their own challenges and shortcomings. The common method of 1RM estimation is the load and rep method that roughly correlates to a given rep range performed and the specific 1RM percentage it is associated with. For example, 75% of 1RM would roughly represent a max effort of 9 reps performed, with no repetitions left in reserve. Whereas, 90% of 1RM would represent a 4 repetition set performed with no reps left in reserve. These values are typically representative of a non-isometric based activity, where the joint angle actually changes. However, the general principle can be applied to submaximal isometric performance of a task. Such as, isometric based sit to stand training, manual resistance applied by the physical therapist preventing the patient from leaving the resting surface while the patient generates a submaximal force for no more than 4-6 seconds, as peak force develop waivers quickly. A set of this completed with the patient providing verbal reports of only being able to perform 4 repetitions would represent, in theory, a 90% of 1RM set. Simple handheld dynamotor on either hand of the practitioner, can be utilized to quantify the actual force production by the patient in this circumstance, as traditional force plate technology has not become a staple in most physical therapy clinics, yet.
Here’s an example of how one might implement this training modality: a patient is non-weight bearing or perhaps partial weight bearing on the right lower extremity, the left lower extremity is functional but certainly not capable of carrying the load of two limbs. The patient has a key deficit with sit to stand transfers to a rolling walker to initiate standing, activities of daily living (ADL) execution, and gait tasks. The patient lacks specific coordination and force production to execute a single limb sit to stand transfer, regardless of the height of the resting surface. The PT may ask the patient to assume the position as if they were to transfer and cue the patient to “push into the floor as if you’re going to stand up, but I’m going to push against you and keep you in place. But keep pushing asthis can transition into yielding isometric pushes with the PT cueing for the patient to push as hard (or to a certain percent of max effort) as quickly as possible in order to facilitate rapid force production and neuromotor control.
This challenges the power model of training for falls prevention which simply attempts to overcome the arc of motion traveling forward by simply moving faster rather than generating a peak force in a shorter time frame (faster). By achieving a higher force production in that time frame to carry the person safely upward and onto their feet. This not only produces marked increases in strength gains but also develops active neuromotor control of this higher rate of force production.
About the Author
Dr. Chase Edwards, a native of Unicoi, TN, currently serves as an Orthopaedic Resident Physical Therapist at East Tennessee State University at the Mountain Home Veterans Affairs hospital and medical center in Johnson City, TN, where he serves as a physical therapist and faculty member within the ETSU Doctor of Physical Therapy Program. Chase is a recent graduate from Emory & Henry College: School of Health Sciences Doctor of Physical Therapy Program in May 2018. A graduate from the East Tennessee State University Department of Kinesiology and Sport Studies with a Master of Arts in Exercise Physiology and Human performance with a concentration in rehabilitation, and a Bachelor of Science degree in athletic training from Emory & Henry College and continues to maintain his ATC credential. He is currently halfway through his residency training in orthopaedics and will sit for his orthopaedic speciality exam in 2020 and will pursue fellowship level training in manual and orthopedic physical therapy after the completion of residency training. His key interests are bridging the gap between human performance and sport science principles into the rehab setting across all populations, and not just in the high end or purely athletic patient/client population. He also carries an avid interest in mentorship, pain science, strength training, and the advancement of sharing information and knowledge with clients, patients, and colleagues.
The opinions in this commentary are solely those of the author and do not reflect the Department of Veterans Affairs, United States Government, East Tennessee State University, or any combination therein.
Dr. Chase Edwards, PT, DPT, MA, ATC
Orthopaedic Resident Physical Therapist – firstname.lastname@example.org
Fall Risk in Community-Dwelling Elders – PTNow . (2019). Ptnow.org. Retrieved 24 March 2019, from https://www.ptnow.org/clinical-summaries-detail/fall-risk-in-communitydwelling-elders
Falls . (2011). American Physical Therapy Association. Retrieved 24 March 2019, from https://www.moveforwardpt.com/symptomsconditionsdetail.aspx?cid=85726fb6-14c4-4c16-9a4c-3736dceac9f0
Cadore, Eduardo Lusa, Leocadio Rodríguez-Mañas, Alan Sinclair, and Mikel Izquierdo. 2013. “Effects Of Different Exercise Interventions On Risk Of Falls, Gait Ability, And Balance In Physically Frail Older Adults: A Systematic Review”. Rejuvenation Research 16 (2): 105-114. Mary Ann Liebert Inc. doi:10.1089/rej.2012.1397.
Aagaard P, et al. 2019. “Increased Rate Of Force Development And Neural Drive Of Human Skeletal Muscle Following Resistance Training. – Pubmed – NCBI “. Ncbi.Nlm.Nih.Gov. Accessed March 24 2019. https://www.ncbi.nlm.nih.gov/pubmed/12235031/.
Maffiuletti, Nicola A., Per Aagaard, Anthony J. Blazevich, Jonathan Folland, Neale Tillin, and Jacques Duchateau. 2016. “Rate Of Force Development: Physiological And Methodological Considerations”. European Journal Of Applied Physiology 116 (6): 1091-1116. Springer Nature. doi:10.1007/s00421-016-3346-6.