Photobiomodulation therapy as an adjunct to resistance exercises on muscle metrics, functional balance, functional capacity, and physical performance among older adults: A systematic scoping review

This scoping review aims to summarize the literature on photobiomodulation (PBM) therapy as an adjunct to resistance exercise among older adults, focusing on its effects on muscle metrics, functional balance, capacity, and physical performance. The participants included were older adults aged ≥ 60 years and either gender. The concept was the application of PBM and resistance exercises with no limits on the context parameters. The databases Medline, Embase, Scopus and Web of Science were searched from inception till February 2024. Methodological quality was assessed using the Cochrane risk of bias 2.0. A total of 10 studies were included in the review. PBM, along with resistance training, was reported in six studies, whereas four reported PBM with isometric fatigue protocol. PBM with a wavelength of 808 nm was given on the belly of the rectus femoris muscle in most of the studies. The common dosimetry parameters used were- power density 35.7 W/cm2, energy density 250 J/cm2, 8 sites, energy per site 7 J, duration of 70 s per site, spot size per diode of 0.028 cm2, and stationary contact of application. The application of PBM therapy alongside resistance exercise has been found to improve muscle metrics, functional capacity, and functional performance and reduce fatigability when compared with the control group in most of the studies. When incorporating PBM therapy as an adjunct to resistance training, it is crucial to consider the dosimetry parameters involved—to achieve the intended therapeutic effect. Adjusting treatment parameters with this information can optimize the effectiveness of PBM therapy and improve treatment outcomes for patients. Supplementary information The online version contains supplementary material available at 10.1007/s10103-024-04177-x.


Introduction
Population ageing is a global phenomenon affecting social demographics [1].As the person ages, there are various evident changes in the musculoskeletal system [2].The changes in the muscle metrics, including the decrease in muscle mass, muscle strength, and physical performance, have been extensively studied in the literature [3].These changes signify a prevalent geriatric disorder with significant healthcare costs [4], identified as sarcopenia by Rosenberg in 1997 [5].Sarcopenia was recognized as a distinct clinical entity in 2016 within the International Classification of Diseases, Tenth Revision, Clinical Modification ICD-10-CM code (M62.84)[6].Managing sarcopenia is crucial, and literature provides evidence for a multi-component program with resistance exercises as the cornerstone [7].Over the past decade, researchers have begun to incorporate photobiomodulation (PBM), a non-invasive therapy that uses light to stimulate cells and tissues, as a therapeutic intervention to explore its positive impact on the musculoskeletal system [8].
Scientific literature has identified several key benefits of PBM therapy, including enhanced performance [8], reduced fatigue [9], increased strength [10] and improved relaxation [11,12].This is largely attributed to the biomodulatory effect of light, which is absorbed by chromophores and transformed into chemical energy that triggers biological responses locally or systemically within the organism [13].
Likewise, mitochondria absorb wavelengths in the red and infrared spectrum and play a crucial role in accelerating the synthesis of adenosine triphosphate (ATP), which is the primary source of cellular energy [14,15].It is hypothesized that optimizing ATP synthesis resources may have a positive impact on functional performance, given that muscle activity requires a significant amount of energy expenditure [8].Resistance exercise training also appears to augment the respiratory capacity and intrinsic function of skeletal muscle [16].This implies that the application of PBM therapy as an adjunct intervention to resistance training could combat sarcopenia.
The selection of appropriate dosimetric parameters by the therapist is a crucial factor in achieving desired outcomes with PBM therapy [17].The literature recommending the dosimetry of PBM therapy to enhance exercise performance is limited to young healthy adults [18].The non-invasive and positive effect of the PBM therapy application among young adults has given clinicians a novel approach to using PBM therapy among older adults [17,18].However, there is ambiguity on treatment parameters, outcome measures, and the benefits of utilizing PBM therapy as a therapeutic adjunct to resistance exercise for muscle metrics in older adults.The review would be a valuable resource for physicians and researchers who intend to utilise PBM in research studies and clinical practice as an adjunct to resistance exercise for managing sarcopenia.Therefore, the hypothesis is that incorporating PBM therapy alongside resistance exercise would further enhance muscle function, physical performance, functional balance, and reduce fatigue in older adults.Hence, the aim of this systematic scoping review was to identify and summarize the currently available literature on dosimetric parameters of PBM, parameters of resistance exercise and the effect of adjunct PBM along with resistance exercise on muscle metrics, functional balance, functional capacity, and physical performance among older adults.Considering the aforementioned contextual background, the objective of this study is to examine the following questions: 1 What are the most commonly used dosimetric parameters (wavelength, dose, number of points, place of application)?
2 What are the most commonly used resistance exercise parameters (frequency, intensity, time/session, total duration, equipment used)? 3 What change in muscle metrics (1-Repition Maximum (1-RM), peak torque, muscle thickness), functional balance, functional capacity, physical performance, and fatigue was observed following PBM and/ or resistance exercise training?

Methods
The scoping review methodology was chosen as it is the most appropriate method to identify the key characteristics related to the topic of investigation.This review followed the Joanna Briggs Institute (JBI) Scoping Review Methodology [19] and Preferred Reporting Items for Systematic reviews and Meta-Analysis extension for Scoping Reviews (PRISMA-ScR) checklist was used for reporting processes [20].

Eligibility criteria
The selection criteria incorporated the PCC (Participants, Concept, and Context) format [19].The participants included older adults aged ≥ 60 years, either gender.The concept was the implementation of PBM and resistance exercise with muscle metrics (1-RM, peak torque, muscle thickness), functional balance, functional capacity, physical performance, and fatigue as an outcome measure.We set no limits on the context parameters, and all the settings were considered.This scoping review included all the relevant original research literature (published and ongoing), irrespective of the study designs that used PBM and resistance exercise among older adults.It consisted of the implementation of active or sham PBM with resistance training.The studies were excluded if they failed to meet the inclusion criteria and/ or: (a) not in English language (no funding was available to allow for the translation process), (b) no extractable data (studies did not report the dosimetry parameters of PBM and/ or exercise training parameters), (c) full-text not available, (d) if any additional nutritional supplement is given, (e) abstract and conference proceedings, and (f) animal studies.

Information sources and search strategy
In this systematic scoping review, the researchers employed a comprehensive search strategy across four electronic databases: Medline (PubMed), Embase, Scopus and Web of Science from inception till February 2024.The search was carried out after the validation of the keywords by the expert.The keywords related to PBM, older adults, and resistance exercises were used to make search strings in the databases.

Data collection process and data items
Two included studies were randomly selected and shared with all the reviewers, and the data charting sheet was prepared individually and piloted.Further, the data charting sheet was finalized after a consensus discussion with all the authors.The data extracted for PBM therapy parameters included [wavelength (nm), mode, peak power (mW), power density (W/cm 2 ), energy density (J/cm 2 ), energy per diode (J), energy per site (J), number of diodes used (cluster probe), treatment time (per site, per session), number of sites (each leg), spot size (per diode, cm   1 and 2, and 3).

Risk of bias in individual studies
The risk of bias of the eligible studies was evaluated through the Cochrane Collaboration's tool for assessing the risk of bias of randomized trials [21].The classification of this tool includes seven items assessing the risk of bias: selection bias (random sequence generation and allocation concealment), performance bias (blinding of participants and personnel), detection bias (blinding of outcome assessment), attrition bias (incomplete outcome data), reporting bias (selective reporting), and other sources of biases [21].Two reviewers (PK and GN) critically appraised each of the included studies independently for the risk of bias.A third reviewer (SU) was consulted for consensus rating whenever needed.Items were scored as follows: low risk of bias (+), high risk of bias (-), or unclear risk of bias (?) following the recommended algorithms for reaching the risk-of-bias assessment tool for each domain, as outlined in the respective guidelines [21].

Strategy for data synthesis
Data from the included studies were extracted and documented in a standard spreadsheet.Qualitative and quantitative data are displayed in tables and summarized in the text to describe the study results of the scoping review questions.

Study selection
A total of 972 articles were identified during the electronic and hand-searching processes, of which 125 were duplicates.After title and abstract screening, the full texts of 32 articles were considered for full text review.Of these, 22 did not meet the inclusion criteria and finally 10 studies were included.A Preferred Reporting Items for Systematic Review and Meta-analysis statement 2020 (PRISMA 2020) [22] flowchart of the literature search is demonstrated in Fig. 1 and PRISMA 2020 expanded checklist as supplementary material 2.

Descriptive details of the dosimetry of the photobiomodulation therapy used in the studies
The PBM was given as LASER in nine (n = 9) studies [23-29, 31, 32], and only one (n = 1) used a light emitting diode (LED) [30].Continuous mode has been used in all the included studies.The peak power of 100 mW has been used in nine (n = 9) of the studies [23-29, 31, 32], with only one (n = 1) have used 150 mW as peak power [30].

Components of exercise training sessions delivered in the studies
We have described the results of the exercise training sessions in the format of the FITT (frequency, intensity, time, and type) for easier understanding of medical professionals and rehabilitation experts.The resistance/ strength training is the most common type of training used in six (n = 6) studies [24,[26][27][28][29]31], with four (n = 4) of the studies have used the fatigue protocol [23,25,30,32].The frequency per week ranges from 2 to 3 times per week, with five (n = 5) studies delivered it 2 times per week [24,26,28,29,31].The intensity of the resistance training was decided using the one-Repetition Maximum (1-RM).The intensity used is moderate to high intensity, ranging from 50% of 1-RM to 80% of 1-RM.Three (n = 3) studies have used 60-80% of 1-RM intensity [24,26,29].One study has not reported the intensity [31], with one each has used 50% [28], and 60% [27] of 1-RM as intensity.The exercise sets and number of repetitions range from 2 to 3 sets with 8-12 repetitions.The progression of the training was done based on increasing the percentage of 1-RM (5-10%) [26][27][28][29] or the number of sets and reps per set.The total time per session of resistance training has been mentioned only in one (n = 1) study of 30 min [26], rest studies have mentioned the 2-3 min of rest period provided between sets [24,[27][28][29]31].Among the muscle groups used for resistance training, the knee extensor (quadriceps) muscle group is the commonest one [23,24,[26][27][28][29]31].The total duration of the exercise session ranges from 8 to 12 weeks.Three (n = 3) studies have delivered the exercise training for 8 weeks [24,26,29], one (n = 1) each for 1 week [23], 4 weeks [32], 10 weeks [28] and 12 weeks [31], with three (n = 3) have not reported the number of weeks exercise delivered for [25,27,30].To find out one of the important things while delivering the exercise session to older adults, which is supervision, the exercise session delivered was supervised in eight (n = 8) studies, while two (n = 2) have not reported [26,31].The description of the components of resistance exercises can be found in Table 2.

Effect of photobiomodulation as an adjunct to the resistance training on muscle metrics
Seven (n = 7) of the ten studies included in the review have assessed the 1-RM.Out of these seven studies, two (n = 2) studies have used 1-RM to decide the intensity of fatigue protocol and not as an outcome measure.Three studies have two groups [26,28,31], one active PBM group and other placebo PBM group.There found to be increase in the 1-RM in all the studies in both the groups, with no significant difference between the groups.The two (n = 2) studies [24,29] having three groups (active PBM, placebo PBM, and control group) have found that when compared to the control group, there was a significant increase in the 1-RM in active and placebo PBM groups.However, no difference was found between the PBM groups.The second component of muscle metric, peak torque (nm), has been used as the outcome measure in three (n = 3) studies [24,27,31].An increase in the peak torque within group in both the active and placebo PBM therapy was reported.However, between group comparisons have found no additional benefit of adding PBM.The third component of muscle metrics, muscle thickness, has been used as an outcome measure in two (n = 2) studies [28,31].Muscle thickness is an important outcome measure, especially in research related to older adults who have a poverty of muscle, sarcopenia.
Both the studies which have utilized muscle thickness as an outcome measure have measured the thickness of a large lower limb muscle group, with the commonest muscle being the vastus lateralis and rectus femoris of knee extensor groups.The results of the study suggest that resistance exercises alone can improve muscle thickness, with one study found that the addition of PBM has an additional effect in improving muscle thickness [28].

Effect of photobiomodulation as an adjunct to the resistance training on functional balance
Three (n = 3) studies out of the seven studies have considered balance as an outcome measure [26,28,31].The three studies utilised different tests to assess the balance with one (n = 1) have used a functional test like timed-upgo and chair rise to standing [31], one (n = 1) used fall risk test and functional stability test [26], one (n = 1) have used analysis of center of pressure to calculate the following balance parameters: area of center of pressure (A-COP), velocity anteroposterior (Vel AP), velocity mediolateral (Vel ML), frequency anteroposterior (Freq AP), and frequency mediolateral (Freq ML) [28].Since all the studies have used different tests to assess the balance, a definite conclusion cannot be made.However, one of the studies found improvement in the balance of the participants in the active PBM group, which received both resistance and active LASER [26], with two (n = 2) stating similar effects between the active and placebo groups [28,31].

Effect of photobiomodulation as an adjunct to the resistance training on functional capacity and physical performance.
Two (n = 2) studies have assessed the functional capacity of older adults using the six-minute walk test (6-MWT).One study with three groups (active PBM/ placebo PBM/ control group) suggested that between group comparison, there was no significant differences in the functional capacity [24].In the second study with active and placebo PBM groups, there found to be an increase in the functional capacity by 40.3% in the active and 26.6% in the placebo group with a p-value of 0.001 intragroup comparison, however, this difference between the groups was not statistically significant (p-value 0.175) [26].The physical performance of older adults has been assessed in two (n = 2) studies using the Short Physical Performance Battery (SPPB).Intragroup comparison has suggested an increase in both the groups, with 0.91% and 0.65% increase in SPPB score in active and placebo groups, respectively [26].However, the between group comparison was not significant, with a p-value of 0.534.Similarly, in the second study there found to be no additional effect of using PBM on the physical performance of the older adults [27].

Effect of photobiomodulation on the fatigability
Four (n = 4) studies have assessed the aspect of fatigability among older adults having received the PBM [23,25,30,32].Of the three studies, one (n = 1) used LED [30] and three (n = 3) have used LASER [23,25,32].The participants received the phototherapy before the fatigue protocol in three (n = 3) studies [23,25,30], and simultaneously along with the protocol in one (n = 1) study [32].The findings support the prior application of PBM in reducing fatigability [23,25,30,32].The study participants were older women in three (n = 3) studies [23,25,30] and one (n = 1) study had both the genders [32].There found an increased number of repetitions of knee flexion-extension [23] and no change in pre-post grip strength (p-value 0.063) following the fatigue protocol in the active PBM therapy group when compared to the placebo group [30].The study utilized the Electromyographic Fatigue Index (EFI) have reported a significant increase in EFI in the active LASER group (p-value 0.011) compared to the placebo LASER group [25].Lastly, the study conducted on either gender reported a pre-post decrease of 13.13% in the Fatigue Severity Scale (FSS) in the active PBM group as compared to a change of 8.1% in the placebo PBM group (p-value < 0.05) [32].
The outline of the main findings of the included studies with active PBM therapy group details, placebo PBM therapy group details for outcomes muscle metrics, functional balance, functional capacity, physical performance, and fatigability are provided in Table 3.

Risk of bias assessment
The risk of bias analysis demonstrated a low risk of bias for random sequence generation, with most studies (90%, n = 9) providing clear information.The analysis for allocation concealment suggests n = 5, 50% of the study with low risk, with the remaining n = 5 divided as an unclear risk 30%, n = 3; high risk n = 2, 20%.The analysis revealed no lack of performance bias, however there found to be a detection bias in (40%, n = 4) studies.The details of the risk of bias assessment of all included studies are summarized in Figs. 2 and 3.

Discussion
This review has scoped the parameters of PBM, and resistance exercise used among older adults.Additionally, the review has descriptively summarized the change in muscle metrics, functional balance, functional capacity, physical performance, and fatigability observed following the application of PBM and/ or resistance exercise.
The review found that LASER is the most commonly used tool to deliver phototherapy compared to LED.There exists a variation in the studies regarding the time of application of PBM, either before the resistance training or after.The review found that six (n = 6) studies out of the included ten studies applied the PBM before the training.In 2016, Vanin and colleagues conducted a study aimed at identifying the most effective timing for administering phototherapy irradiation in conjunction with strength training.The study found that administering active phototherapy prior to resistance training, followed by a placebo treatment afterwards, yielded positive results [10].Administering PBM prior to resistance exercise is found to reduce post-training fatigue.This review found a range of LASER wavelength from 808 nm to 850 nm used, which is similar to the findings suggesting the optimum wavelength near to 810-840 nm, since in these regions the surface chromophores have weak absorption, and therefore there is maximum penetration of light into the skin, generating an optimal window of penetration and absorption by organic molecules [33].The majority of studies (n = 8, 80%) included in this review utilized a wavelength of 808 nm.For young healthy adults, recent evidence has recommended a wavelength of 640 nm (red) to 950 nm (infrared) [17,18].There found to be an added benefit of employing PBM therapy alongside resistance exercise compared to the control group.However, the outcomes did not show significant variance between the active LASER and placebo LASER groups.A recent systematic review and meta-analysis identified a wavelength range of 655 nm to 905 nm for healthy individuals [34].This indicates that PBM therapy applications could be tailored to different age groups to address their unique physiological needs.The 808 to 850 nm range is commonly used for its ability to penetrate deeper into tissues.This near-infrared wavelength effectively targets muscles, joints, and deeper cellular structures, stimulating mitochondrial activity, boosting ATP production, and supporting cellular repair and regeneration.This makes it particularly beneficial for older adults.On the other hand, younger adults, including athletes, benefit from a wider range of wavelengths, typically from 655 to 905 nm.This spectrum includes both red light (around 655 nm) and nearinfrared light (up to 905 nm), each offering its own advantages.Therefore, the specific physiological challenges faced by each age group underpin the scientific rationale for these different wavelength preferences.
Utilizing resistance training as a mode of exercise intervention is a common practice among older adults seeking to enhance their muscle metrics [7].Interestingly, the studies included in this review reveal similarities, with resistance exercise being employed along with PBM therapy as the most common mode of exercise intervention.The delivery of the resistance exercises in the included studies has followed the frequency, intensity, time, and type framework.The components of resistance exercise intervention used in the studies included in this review are in line with the resistance exercise parameters used in the management of sarcopenic older adults [7,35].The studies in the review have used the leg extension machine system to perform the resistance training of the major muscle group of the lower limb, quadriceps.However, these extension machines have a demerit of measuring a single joint movement, which does not reflect the muscle power required while doing functional activities like a chair rise which involve multi-joint leg extension [36,37].
The rising population of older adults globally is a serious concern with a parallel increase in the prevalence of a common geriatric syndrome, sarcopenia [38].The decrease in muscle strength is a prominent feature among older adults.This review found that the PBM have a significant effect on improving the 1-RM among older adults when compared to the control group.However, there found to be no significant difference in muscle strength between the active PBM and placebo PBM.Similar findings have been reported in the study conducted on post-menopausal women which shows that PBM was not able to induce additional benefits to resistance training to improve muscle strength [39].In this review, we found that there was an increase in the peak torque pre-post when compared within the group, with no significant difference between the active and placebo groups.The study conducted on young adult males found an increase in the peak torque if the PBM is applied before the resistance training [10].A study on male Wistar rats by Albuquerque-Pontes et al., 2015 has reported that the irradiation of healthy muscles by phototherapy leads to increased cytochrome c-oxidase activity.The study utilized different doses (660 nm, 830 nm, 905 nm) to irradiate the tibialis anterior muscle and reported benefits with PBM therapy doses of 660 nm with 1 J, 808 nm with 3 J, and 905 with 1 J, protecting against skeletal muscle tissue damage and enhancing performance [40].This review also reports the most common wavelength of 808 nm (near infrared) used for older adults.This may partially explain the mechanism, however human studies on cytochrome activity are limited.
The impact of PBM therapy on the functional balance of older adults is not yet fully understood.Only three studies have used balance as an outcome measure, which limits our understanding of its effects.The two studies reported no additional benefit of PBM therapy along with resistance exercise as compared to resistance exercise alone.One study reported an increase in stability and a decrease in fall risk among active LASER group participants.Additionally, the wide range of tests used to assess balance further complicates drawing clear conclusions about the benefits of PBM therapy.Improvement in strength and physical performance could be the main reason alongside PBM may have an influence on the sensory inputs specifically proprioception thereby improving functional balance, however these mechanisms need further confirmatory studies.
Among older adults, the decline in functional capacity and physical performance is a serious concern, often resulting from muscle atrophy, reduced mitochondrial function, and increased inflammation.The application of PBM therapy could have a positive effect, considering that PBM improves microcirculation, enhancing oxygen and nutrient delivery to muscles during and after exercise [41].The combined effects of increased ATP production, enhanced muscle protein synthesis, and improved circulation could result in significant improvement in functional capacity and physical performance among older adults.This review found only two (n = 2) studies have used functional capacity and physical performance as one of the outcome measures, indicating a new area that would require further research to get more clarity on the use of the application of PBM therapy as an adjunct to resistance exercise.The studies included in this review reported no additional benefits of adding the PBM therapy along with resistance exercise on functional capacity and physical performance.Similar results have been reported in a systematic review and meta-analysis assessing the effect of PBM therapy in healthy individuals [34].Another study done on young healthy women reported no enhancement in muscle performance following PBM therapy [42].
The studies in this review which have explored the effect on the level of fatigue, three of the studies were conducted on healthy older women, while one on either gender.The findings of the study included in this review provide evidence in support of the application of PBM to reduce the level of fatigue among older adults.A recently published systematic review with meta-analysis also supports the evidence about the PBM reducing fatigue [34].Delayed onset of fatigue following the application of PBM therapy could be through several potential mechanisms.PBM therapy enhances the mitochondrial function by stimulating the cytochrome c oxidase, a key enzyme in the mitochondrial electron transport chain.This stimulation boosts ATP synthesis, providing cells with more energy to function effectively [41].Additionally, PBM therapy reduces oxidative stress and inflammation, which are common contributors to fatigue in older adults, thus alleviating muscle soreness and overall muscle function, thereby delaying the onset of fatigue [43,44].A study conducted on young women has reported reduced levels of fatigue following the application of PBM [45].Another study conducted on young women found no significant difference between active LLLT and placebo group on muscle fatigue, although a tendency was observed in the active laser group toward showing lower electromyography fatigue of the biceps brachii muscle [46].Another study conducted on male athletes has reported the ergogenic effect of the PBM evident from the delayed onset of fatigue [47], although similar mechanisms are documented, it may not be comparable with older adults since athletes are trained for high performance.
When incorporating PBM therapy as an adjunct to resistance training, it is crucial to consider the dosimetry parameters involved in order to achieve the intended therapeutic effect.These parameters determine the amount of light energy delivered to the target tissue and significantly impact the clinical outcomes of PBM therapy.To better understand PBM therapy dosimetry, healthcare professionals can refer to Table 4 for a comprehensive summary of the various parameters involved in targeting specific outcome measures.Adjusting treatment parameters with this information can optimize the effectiveness of PBM therapy and improve treatment outcomes for patients.
Strengths and limitations: First, a comprehensive systematic search strategy was performed in four electronic databases, to identify a broad range of studies related to the topic.Second, no time restriction in the search strategy strengthens the search for literature published on the topic.Third, we followed acknowledged method recommendations for scoping reviews and did duplicate study selection and data extraction to raise validity.Also, to the best of our knowledge, this is the first study systematically scoping the literature regarding the application of PBM therapy and its dosimetry, components of resistance exercises, and the effect of PBM as an adjunct to resistance exercise.Lastly, we have used the risk of bias 2.0 tool to add value to the evidence provided in this review.This study has a few limitations as well.First and foremost, majority of the studies were conducted among older women, which limits the generalisation of the review findings and thus requires caution in interpreting the results.Second, the findings of the review lack objective metaanalysis evidence, as the purpose of this systematic scoping review was to find out and explore the existing studies' details on the use of PBM therapy as an adjunct to resistance exercises among older adults.Third, a review has considered only the published articles in electronic databases.Fourth, only full-text articles were considered as abstracts, and proceedings were excluded.Fifth, we did not use search strategies with terms other than English, and we may have missed eligible studies in the other languages we intended to include, which could have enriched the review.
Future recommendations: Conducting future meta-analyses could provide a more robust quantitative understanding of the efficacy of photobiomodulation therapy as an adjunct to resistance exercise.Furthermore, to improve the quality and applicability of results, future research should focus on increasing the number of studies, standardizing protocols and including rigorous quantitative analysis of the data.Including studies in multiple languages and considering currently unextractable data could also enrich the understanding of the impact of PBM.

Conclusion
The findings of this systematic scoping review underscore the potential of photobiomodulation (PBM) as an adjunct to resistance exercise in enhancing muscle metrics, functional balance, capacity, and performance among older adults.While PBM, particularly delivered via LASER with wavelengths ranging from 808 to 850 nm, shows promise in reducing fatigue and supporting muscle function, the evidence does not consistently demonstrate significant additional benefits over resistance exercise alone.The variability in dosimetric parameters and study designs highlights the need for standardized protocols to optimize the application of PBM.Future research should focus on establishing clear guidelines for PBM dosimetry and exploring its long-term effects on sarcopenia management in older populations.This review provides a valuable foundation for clinicians and researchers aiming to integrate PBM into therapeutic regimes for ageing individuals, promoting better health outcomes through combined physical and phototherapeutic interventions.

Fig.
Fig. 1 Preferred Reporting Items for Systematic Review and Meta-analysis statement 2020 (PRISMA 2020) flow diagram

Fig. 2 Fig. 3
Fig. 2 Risk of bias graph The search strategy was: (older adults OR older person OR aged) AND (photobiomodulation OR low-level laser therapy OR phototherapy OR LASER therapy) AND (resistance exercise OR strengthening OR exercise).The detailed search strategy is attached as supplementary material 1. 2

Table 1
Photobiomodulation therapy parameters

Table 3
Outline of the main findings of the included studies

Table 4
Summary of the PBM therapy dosimetry for each outcome measure